National Radio Company
Desktop HF Amplifier
|" 20 watts in and 1500 watts out...."||
The New Manual
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OPERATION AND SERVICE
MANUAL FOR THE
The NCL-2000 is a legal limit HF amplifier for use on amateur bands 80 thru 10 meters. The amplifier is exciter driven into a tapped grid drive resistive input network that requires as little as 20 watts to provide full legal output using two 8122 Tetrode Tubes.
This manual has been compiled from the original manual and assorted documents available on the web. The original service bulletins were included along with modifications and updates by Carl Motsinger N9EFJ, with additional suggestions that pertain to keeping the amplifier running. This document may be freely copied as long as it is distributed free of charge and not as an addition to other material that is available for sale.
HERES WHAT I THINK
THIS AMPLIFIER WAS FIRST INTRODUCED IN 1963-64 AND SOON BECAME
A WELL SOUGHT AFTER DESKTOP AMPLIFIER. TODAY, TUBES ARE VERY
EXPENSIVE AND EVEN HARD TO FIND. A NEW PAIR RUNS IN EXCESS OF
$700.00. USED TUBES CAN BE BOUGHT ON EBAY BUT I HAVE FOUND
THAT THOSE TUBES ARE SELDOM USABLE. THIS AMP SELF DESTRUCTS
VERY EASILY WHEN USED ON AM. THAT IS PARTLY DUE TO THE
KNOWLEDGE THAT TWO 400 WATT TUBES SHOULD LOAF ALONG AT A
KILOWATT. WELL THEY DONT WHEN THE OPERATOR HAS THE MIC GAIN
WIDE OPEN AND TRANSMITS FULL SPECTRUM AUDIO INCLUDED THE
KIDS AND THE AIR CONDITIONER SETTING NEXT TO HIS BIG MUG OF
I HAVE USED MINE EVERY DAY FOR 2 YEARS AND IT IS STILL GOING STRONG. I HAVE MADE A FEW MODS AND IMPROVEMENTS IN THAT TIME AND HAVE DOCUMENTED THEM IN THIS MANUAL. THIS BABY NEEDS TENDER LOVING CARE FOR EVERY WAKING MOMENT. IF YOU CAN DO THAT IT WILL LAST A LONG TIME ..Carl / N9EFJ
TABLE OF CONTENTS
SECTION 1 GENERAL DESCRIPTION . 3
SECTION 2 INSTALLATION ... .4
2.2 Unpacking and Tube installation
2.3 Power Connection
2.4 Antenna Connection
2.5 ALC Connection
2.6 Exciter Requirements
2.7 Exciter Connections
2.8 RF Exciter Connection (Transceiver)
2.9 RF Exciter Connection (Separate Transmitter and Receiver)
with external antenna transfer relay
2.10 RF Exciter Connection (Separate Transmitter and Receiver)
without external antenna transfer relay
2.11 Relay Connection
2.12 Ground Connection
SECTION 3 OPERATION ... ..8
3.1 Explanation of Controls
3.2 Preliminary Adjustments
3.3 CW Tune-up Procedure
3.4 SSB Tune-up Procedure
3.5 AM Tune-up Procedure
SECTION 4 THEORY OF OPERATION . .. .12
4.1 Class of Operation
4.2 Grid Bias Supply
4.3 Input Circuit
4.5 Output Circuit
4.7 Primary Power Circuits
4.8 CW-SSB Mode Selection
4.9 Antenna and Transmit Standby Switching
SECTION 5 SERVICE INSTRUCTIONS 15
5.1 Tube Replacement
5.2 Filter Capacitor Board Removal
5.3 Cabinet Removal
5.4 Front Panel and Side Gusset Removal
5.5 Regulator Transistor Change
5.6 Air System Cleaning
5.7 Grid Resistor Tap Change
SECTION 6 TROUBLESHOOTING .17
SECTION 7 PARTS LIST ..19
SECTION 8 MODIFICATIONS ..20
SECTION 9 FINDING REPLACEMENT PARTS ... 21
The NCL-2000 is a 1000 watt average, 2000 watt PEP, linear amplifier for the 80 through 10 meter amateur bands, housed in a desktop cabinet with self-contained power supply. It utilizes two type 8122 miniature ceramic tetrodes (in parallel) in its output circuit, which provides 800 watts of available plate dissipation to allow operation at the legal input with high efficiency and excellent linearity. The output circuit components, power supply and other components in the NCL-2000 are rated to allow operation at 1000 steady carrier input for AM, CW, FM and RTTY service.
The NCL-2000 utilizes a passive untuned grid circuit which allows the amplifier to be driven to full output from any exciter providing from 20 to 200 wars of power output. In addition the use of a 100 watt non-inductive swamping resistor in the grid circuit provides the operator with the ability to load and match his exciter into the amplifier grid circuit without placing a signal from either the exciter or the amplifier on the air. The grid swamping resistor functions as a dummy load in this instance, and relative power output from the exciter is indicated as final grid current on the NCL-2000 multimeter.
The NCL-2000 incorporates a built-in antenna transfer relay with all contacts and coaxial jacks necessary to allow inter-connection with either transceivers such as the NCX-3, or separate transmitter-receiver combinations, without necessity for additional controlled relays. Instantaneous selection of either high power (using the amplifier) or low power straight through operation of the exciter is possible by simply turning the PLATE switch of the NCL-200 ON or OFF.
Two screw type connections are available at the rear of the amplifier to allow control of its internal antenna transfer relay by an external switch. The internal antenna transfer relay of the NCL-2000 utilizes a separate pair of contacts to automatically bias off the output tubes when the amplifier is in the standby mode to prevent tube noise from appearing in the receiver and to reduce standby dissipation in the amplifier.
ALC circuitry is incorporated in the NCL-2000 with an output jack to allow ALC control of an exciter incorporating ALC input. The ALC voltage from the amplifier is developed when ever grid current exceeds the point at which limiting or "flat topping" of the amplifier occurs.
The NCL-2000 incorporates two D'Arsonval precision meters - a plate current meter with a 0 to 1000 ma scale and a multimeter with readings selected by a front panel MULTIMETER switch. Plate voltage, screen current, grid current and exciter relative output (Grid Current) may be selected by the multimeter switch. Front panel switches are provided for primary power, plate voltage and choice of either SSB or the CW mode. Pilot lamps are provided to indicate primary power ON, plate power ON and operation of the one minute time delay relay indicated by the READY indicator. In addition to this a protective relay, a 1000 ma plate overload relay is incorporated in the NCL-2000 as well as primary fusing. Two separate safety interlocks are built-in; a lid interlock which breaks power to the plate relay and a spring actuated mechanical shorting bar which discharges residual plate voltage to ground when the lid is raised. The mechanical shorting bar also serves as added protection against the remote possibility of plate interlock failure or an open bleeder resistor. Should either failure occur, no damage will be done to the amplifier by the action of the mechanical shorting bar since the overload relay contacts will immediately open thereby removing AC from the plate transformer input.
The NCL-2000 may be operated from either a 230 volt 3 wire single phase supply or from a 115 volt 2 wire line. All power supply components are rated for 50 cycle operation but in such instances the cooling fan must be operated from a separate 60 cycle source for a full power rating.
2.1.1 The voltages used in the NCL-2000 are lethal. Never defeat the electrical or mechanical safety interlocks. Never work inside the equipment with primary power connected. This paragraph refers to the normal user. (During troubleshooting there are different circumstances that a properly trained technician can override.)
2.1.2 The output tubes used in the NCL-2000 require a constant supply of air from the cooling fan at all times. To prevent overheating and subsequent damage, do not place objects on top of the NCL-2000 cabinet. Do not accidentally block the air intake underneath the cabinet.
2.1.3 Be extremely careful when inserting or removing the 8122 output tubes. Carefully note the way the base and socket are keyed and insert the tubes gently into the sockets without using force.
2.1.4 Never operate the NCL-2000 without an antenna or adequate dummy load connected. Light bulbs will not present the proper load impedance. Damage to the output circuit is certain to occur if the NCL-2000 is run unloaded. Do not operate the NCL-2000 into an antenna system with a high VSWR.
2.1.5 Persistently blown input fused or constant overload relay tripping indicates a malfunction or faulty installation. Cease operation and determine the difficulty before permanent damage occurs.
2.1.6 Make certain that the primary wiring to the station is adequate to handle the
requirements of the NCL-2000. Thirty amp 115 volts AC service or 3 wire 20 ampere 230
volts AC service is necessary to comply with fire laws and insurance requirements in most
parts of the
2.1.7 Refer again to 2.1.1 above
2.2 UNPACKING AND TUBE INSTALLATION:
Carefully unpack the NCL-2000, its plugs, tubes and chimneys, setting aside all packing material for possible future use. Lift the lid by turning the fasteners counter-clockwise one quarter turn, and inspect for visible shorts or other damage which may have occurred during shipping. If damage is evident, immediately notify and file claim with the carrier. Place the tube chimneys, narrow end up, under the tube clamps, carefully insert the tubes to prevent damage to their pins, and tighten the tube clamps. Also check to see that the time delay (which looks like a miniature tube) located just behind the front panel is seated in its socket.
2.3 POWER CONNECTION
The NCL-2000 is designed for operation from either 115 volt 2 wire or 230 volt 3 wire single phase service. Under peak power input conditions, the NCL-2000 may draw in excess of 10 amperes from a 230 volt service or in excess of 20 amperes from a 115 volt service. This latter power requirement will exceed the capabilities of most home 115 volt circuits. We therefore strongly recommend operation of the NCL-2000 from a 230 volt, 20 ampere supply circuit. Such a circuit will, in addition generally have superior regulation to a 115 volt circuit. In the event that a 230 volt circuit cannot be supplied, a special 115 volt circuit rated at 30 amperes should be provided. Operation on the 230 volt 2 wire service available in many countries will necessitate the use of a 2 wire to 3 wire 1:1 conversion transformer. Fifty cycle operation is permissible provided that the cooling fan is operated from a separate 60 cycle supply. If such a supply is unavailable, the NCL-2000 may be operated at reduced input.
The NCL-2000 is shipped with the primary power connections wired for 230 volt operation and a power cord for 230 volt operation is supplied. No power plug is supplied because the enormous variety of 230 volt sockets makes it impossible to supply a single plug to meet all possibilities. Procure a suitable 230 volt plug and wire the red and black wires of the cable to the hot sides of the plug and the white wire to the neutral.
If 115 volt operation is contemplated, a separate power cord and plug should be obtained. Number 8/2 cable is recommended to minimize line drop under the 25 ampere peak load of the NCL-2000. To install the 115 volt cable, remove the four self tapping screws of the primary power junction compartment cover, the cover itself, and the 230 volt cable. Prepare the new cable by connecting the plug, stripping the sheath of the outer end back five inches and stripping and tinning the conductors approximately one inch. Then insert the new cable far enough so that it bottoms in the compartment, dressing the conductors to the right and below the barrier strip. Remove the existing jumper and wire new jumpers and the new cable as shown in Figure 1 or as shown in the diagram on the compartment cover.
2.4 ANTENNA CONNECTION:
The antenna to be used with the NCL-2000 should present a 50 ohm impedance and a VSWR of 2:1 or less. It should be fed with 50 ohm cable (RG-8A/U size minimum), and provided with a PL-259 UHF series coaxial plug. Connect it to the coax jack marked "ANT".
2.5 ALC CONNECTION:
The NCL-2000 contains an ALC circuit that provides a negative voltage at the rear panel phono jack whenever the NCL-2000 is overdriven or "flat-topped". This may be used with exciters incorporating an ALC input. Connect the ALC output to the exciter ALC input, using a shielded cable with suitable connectors. See paragraph 4.4 for description of this circuit. Use of the ALC connection is optional.
2.6 EXCITER REQUIREMENTS:
The NCL-2000 is shipped with the output tube grids tapped down on the grid swamping resistor network for use with exciters providing 80 to 200 watts of peak output. If the exciter peak output is less than 80 watts, see paragraph 5.7 which describes the alteration of the tap on the grid swamping resistor network for use with low power driver sources.
2.7 EXCITER CONNECTIONS:
The NCL-2000 incorporates all necessary antenna change-over relay terminals to allow its use with either a transceiver or transmitter/receiver combinations. When used with a transceiver or with a transmitter/receiver combination with their own separate antenna change-over relay, high power or low power "straight thru operation may be selected by simply turning the amplifier plate voltage ON or OFF. The NCL-2000 requires no separate antenna change-over relay when used with transmitter/receiver combinations but in this case the NCL-2000 must be used at all times, since no straight thru connection is available.
2.8 RF EXCITER CONNECTION (Transceiver)
(See Figure 2)
The NCL-2000 has been designed to provide for automatic connection of the antenna directly to the exciter during receive periods or when the NCL-2000 plate supply is off. This eliminates the usual necessity for a separate "extra" antenna change-over relay and further provides straight thru operation of the transceiver without the linear when desired. A transceiver such as the NCX-3 should have its antenna input /output connected directly to the NCL-2000 coaxial jack marked "XCVR" with a 50 ohm cable and PL-259 connectors. The input swamping resistor of the linear presents a constant 50 ohm resistive load to the exciter so that the length of coaxial cable connecting the exciter to the NCL-2000 is not critical and no special matching devices or cable lengths are necessary.
2.9 RF EXCITER CONNECTION
(Separate Transmitter and Receiver with External Antenna Transfer Relay)
(See Figure 3)
When a separate transmitter and receiver are used with their own separate antenna transfer relay, a direct connection may be made from the antenna connection of the separate relay to the NCL-2000 coaxial jack labeled "XCVR" and operation will be identical to that described in paragraph 2.8. * Note that no provisions are provided for muting the receiver on transmit.
2.10 RF EXCITER CONNECTION
(Separate Transmitter and Receiver Without External Antenna Transfer Relay)
(See Figure 4)
The NCL-2000 has been equipped with a separate receiver output (RCVR) and transmitter input (XTMR). The receiver output is connected through the antenna switching relay of the NCL-2000 so that the antenna will be connected to the receiver during receiving periods and disconnected from the receiver during transmitting periods. The transmitter input is directly connected to the input circuit of the NCL-2000 at all times. When this connection is used, the linear plate power must always be ON to provide proper operation. In the event that the linear plate power is removed, the receiver will perform normally but the transmitter output will not be connected to the antenna line. Therefore it is not possible to operate the separate transmitter and receiver without the linear under these conditions.* Note that no provisions are provided for muting the receiver on transmit.
2.11 RELAY CONNECTION
Automatic transmit/receive operation of the NCL-2000 is afforded by energizing the NCL-2000 antenna transfer really during transmitting periods by connecting together the relay terminals on the rear apron of the linear chassis. Most transceivers, transmitters and receivers have such switching contacts available. The NCL-2000 relay terminals should be connected to the normally open (on receive) terminals available on the exciter. Since the current through the transfer relay is small, "zip cord" or hook-up wire may be used for this connection.
2.12 GROUND CONNECTION:
It is strongly recommended that the chassis of the NCL-2000 be grounded to a cold water pipe of a good earth ground, as w3ell as to the other equipment in the station. A stud is provided on the rear apron of the chassis for this ground connection. If the ground is inadequate, the chassis of the linear may be "hot" with RF, which can cause operating difficulties and danger to the operator. If this occurs check the ground connection and change the ground location until satisfactory operation is obtained.
SECTION 3 OPERATION
3.1 EXPLANATION OF CONTROLS:
The main power switch, labeled ON -OFF, energizes filaments, the grid bias supply, the blower and the 60-second time delay that prevents application of grid drive and plate and screen voltage until the tubes are fully warmed up. When the main switch is pushed, the green ON lamp will light, and approximately one minute later the ivory READY lamp will light indicating the tubes are warmed up and plate power may be applied.
The plate power switch is a center neutral, momentary - each - side - of - center, switch, labeled PLATE-OFF, which operates the latching type plate relay. After the READY lamp is on, plate power may be applied by pushing the switch up and releasing it. The red PLATE light indicates that the plate and screen supplies are operating.
The CW-SSB switch changes the plate voltage under load from about 1800 volts in the CW position to 2500 volts in the SSB position. The screen voltage also changes from 290 to 400 volts. This allows selection of 1000 watt operation for tune-up, CW and RTTY or 2000 watt PEP operation for SSB.
The MULTIETER switch allows measurement bf the following parameters as read on the multimeter: Plate voltage 0-5000 volts, screen current 0-50 ma., grid current 0-50 ma., and exciter tune (relative output) 0-50 ma. When the MULTIMETER switch is in the EXCITER TUNE position, and the amplifier plate voltage is off, power from the exciter is applied to the 100 watt input swamping resistor in the NCL-2000 which then acts as a dummy load for optimum tune-up of the exciter. Relative exciter output power is indicated as final grid current on the multimeter and the exciter is adjusted for maximum multimeter indication. This unusual feature of the NCL2000 allows optimum independent adjustment of the exciter without the usual necessity for a separate dummy load or the alternative procedure of attempting to simultaneously adjust both the amplifier and the exciter.
3.2 PRELIMINARY ADJUSTMENT:
DO NOT ATTEMPT TO TURN ON THE NCL-2000 OR TO OPERATE ITS SWITCHES
OR CONTROLS UNTIL YOU READ AND COMPLETELY UNDERSTAND THE FOLL0WING PARAGRAPHS. EXPENSIVE DAMAGE MAY RESULT IF THE NCL-2000 IS IMPROPERLY CONNECTED TO OTHER STATION EQUIPMENT OR IMPROPERLY OPERATED.
Carefully double-check to make certain that the power cable is wired correctly for your power source, that the proper coaxial cable connections are made between the NCL-2000 and your antenna and exciter and that the proper connections are made between the NCL-2000 relay terminals and the controlling terminals on the exciter. If a high power dummy load is available, it is strongly suggested that initial adjustment and tune up be made with the dummy load instead of the antenna. Tune-up of the NCL-2000 quickly becomes second nature and may be totally accomplished when changing bands within 15 or 30 seconds after a little practice, but until complete familiarity is achieved it is wise to go only one step at a time, removing drive and/or amplifier plate voltage after each step so that you have time to relax and review previous steps without worry.
3.21. PRELIMINARY CONTROL SETTINGS. CHECKS AND ADJUSTMENTS:
1. Turn on exciter and allow it to warm in standby or receive mode.
2. Set NCL-2000 controls as follows:
a. POWER: OFF
b. PLATE: OFF
C. CW-SSB Switch: CW
d. BAND: to desired band
e. PA TUNE and LOAD: to approximate setting in Figure #5
f. MULTIMETER: PLATE VOLTAGE
3. Insert the NCL-2000 power plug into power source and press power switch to ON. The green ON lamp will light, as well as the two rear-illuminated meters. Check to see whether the blower is operating. Note: The blower operates at all times, not just when plate power is applied.
4. After approximately one minute, the ivory READY lamp will light, indicating that the time delay relay is operating and plate voltage may now be applied. The READY lamp remains lit once the time delay relay has closed.
5. Press the PLATE-OFF switch to PLATE. The red PLATE lamp will light and the multimeter should read 2000 volts.
6. Press the CW-SSB switch to SSB. The multimeter should now read 3000 volts. No plate current will be indicated on the plate meter since the output tubes are cut off when the exciter is in the receive mode.
7. Press the PLATE-OFF switch to OFF, and put exciter in transmit mode. Adjust exciter for zero output in transmit mode, preferably by depressing push-to-talk switch with microphone gain backed down, or with the NCX-3, by placing the function switch in the CW position and depressing the key with the carrier completely balanced out.
8. Return exciter to receive mode, and press the NCL-2000 PLATE-OFF switch to PLATE with the CW-SSB switch still in the SSB position. Put exciter in transmit mode with zero output as above. You should hear the NCL-2000 antenna transfer relay click, and idling plate current should now be indicated on the plate meter.
9. Adjust the rear panel BIAS ADJUST control for 225-250 ma. idling plate current.
10. Return exciter to receive mode, press CW-SSB switch to CW, and press PLATE-OFF switch to OFF. Check for proper straight through operation of the driver transceiver or transmitter-receiver. This completes the preliminary checks and adjustment of the NCL-2000.
DO NOT OPERATE THE NCL-2000 WITH BADLY MISMATCHED ANTENNAS OR WITH NO ANTENNA CONNECTED, SINCE EXPENSIVE DAMAGE TO THE NCL-2000 TANK CIRCUIT WILL OCCUR. IT IS NOT ADVISABLE TO SWITCH THE MULTIMETER TO EXCITER TUNE WHILE THE UNIT HAS BOTH PLATE VOLTAGE AND FULL DRIVE APPLIED SINCE A MOMENTARY NO-LOAD CONDITION OCCURS WHEN THE ANTENNA RELAY THROWS, WHICH MAY CAUSE A MOMENTARY ARCOVER IN THE PLATE CIRCUIT
3.3 CW TUNE-UP PROCEDURE:
1. Set Controls as in paragraph 10 above.
2. The exciter, whether transceiver or transmitter-receiver combination, should now operate conventionally as a low power station when either the NCL-2000 primary power or plate power switch is off, except when the MULTIMETER switch is in the EXCITER TUNE position or when no separate antenna transfer relay is used with a transmitter-receiver combination as noted in paragraph 2.10.
3. Place MULTIMETER switch in EXCITER TUNE position (PLATE-OFF switch still OFF).
Place the exciter in the transmit mode. You should hear the NCL--2000 antenna transfer relay click, indicating that the exciter output has been switched from the antenna to the 100 watt input resistor in the NCL-2000 which will now act as a dummy load providing optimum exciter adjustment.
4. Place the exciter in the transmit mode and adjust it for normal CW loading and output as indicated on the NCL-2000 multimeter, which is reading 0-50 ma. grid current in the EXCITER TUNE position. At least 15 ma. should be indicated. If not, check for proper exciter operation or, if necessary, adjust the tap on the NCL2000 input resistor per paragraph 5.7.
5. Adjust the exciter so that CW output may be varied continuously from zero to full output by insertion of more or less carrier in the CW mode in equipment such as the NCX-3, or by whatever means is available. It may be found easiest in some equipment to detune an. early transmitter stage.
6. Note carefully that the NCL-2000 screen current will be your most valuable tuning and loading indication during the following steps. The object of the tune-up procedure, as you will see, will be to strike the proper balance between plate current and screen current for maximum output. Excessive screen current (above 25 ma.) indicates insufficient loading, and in addition, the screen current will act as an extremely accurate indication of plate circuit tuning by reading maximum at resonance. The NCL-2000 is so designed that maximum efficient output may be accurately determined in every instance by proper adjustment of plate and screen current and "output" meters are not necessary. In the following CW tune-up procedure, the objective is to achieve a plate current at resonance of 550 to 650 ma. with 10-25 ma. of screen current.
AT NO TIME SHOULD SCREEN CURRENT BE ALLOWED TO EXCEED 40 MA.
7. Reduce exciter output to zero in the transmit mode, move the MULTIMETER switch to SCREEN CURRENT, and press the plate switch of the NCL-2000 to ON. Increase drive to the NCL-2000 until the plate meter reads approximately 200 ma. and then quickly adjust the PA TUNE control for a plate current dip. Observe the screen current. If there is no indication, increase drive from the exciter until screen current is 10 to 25 ma. If, at this point, plate current is below the target of 550 to 650 ma., slightly increase the setting of the PA LOAD control. This will cause plate current to rise and screen current to fall. Again, adjust PA TUNE for plate current dip or maximum screen current and increase drive until the screen current is 10 to 25 ma. Continue this procedure until the desired 550 to 650 ma. plate current and 10 to 25 ma. screen current is obtained. If the plate current exceeds 550 ma. or if too little screen current is apparent at 550 ma. plate current, reduce the PA LOAD setting and dip the PA TUNE again. Then, adjust the PA TUNE control for maximum screen current, the truest indication of resonance and maximum output. The final condition should be plate current of 550 ma. (times 1800 volts equals 990 watts) with a screen current of 10-25 ma. at the screen current peak. If the NCL-2000 is properly tuned and loaded, additional drive will not now increase the plate and screen current appreciably above the proper operating conditions. This completes the tune-up of the NCL-2000 for CW operation.
It may be noted that the 8122 tubes in some NCL-2000 amplifiers are driven so easily that with a resonant plate current of 550 ma., the amplifier is still not fully loaded. This condition will be apparent in that the grid current will be much less than 15 ma., with loading and drive adjusted for 550 ma. plate current, and it will be possible to drive hard enough to exceed the nominal 25 ma. maximum screen current. In such instances, the NCL-2000 should be loaded more heavily, so that screen and grid currents are both about 15 ma., with the screen current peaked and the plate reading slightly higher than 550 ma. This will assure peak plate efficiency and proper ALC action, when the amplifier is placed in the SSB mode.
3.4 SSB TUNE-UP PROCEDURE:
1. Follow CW tune-up steps 1 through 7 above.
2. Press PLATE SWITCH to OFF and place CW-SSB switch in SSB position.
3. Adjust exciter for SSB operation.
4. Press NCL-2000 PLATE switch to PLATE, which increases plate and screen voltages to allow 2000 watt PEP operation. After tune-up in CW and switching to SSB, it is advisable to briefly recheck the screen current with carrier inserted to double-check for proper loading. If
the NCL-2000 is loaded heavily enough, there will be only a slightly increase in screen current.
5. Place the MULTIMETER Switch in GRID CURRENT position.
6. While speaking into the microphone, advance exciter audio gain until the NCL2000 grid current just flicks off zero occasionally or until plate current kicks up to 400 ma on speech peaks. Plate voltage is 2500 volts under full instantaneous peak current of 800 ma., or 2000 watts peak input. Remember that the inertia of the Multimeter in the GRID CURRENT position requires strict adherence to the instructions concerning grid current indication during SSB operation. When the GRID CURRENT meter just occasionally flickers off zero on voice peaks, the NCL-2000 is being properly operated. Should the NCL-2000 be driven so hard that there is a constant grid current indication, no matter how small, actual peak grid current will be in excess of 15 ma. "flat topping" and consequent splatter will occur.
YOUR NCL-2000 IS A MAXIMUM KILOWATT IN EVERY
SENSE OF THE WORD. UNDER THE ABOVE CONDITIONS YOUR POWER IS A "DC" KILOWATT,
2000 WATTS PEAK. AN INCREASE IN MICROPHONE GAIN THAT RESULTS IN
RESULT IN FLAT-TOPPING AND SPLATTER AS WELL AS
ILLEGAL EXCESSIVE POWER INPUT.
7. If your exciter contains an ALC input and it has been connected to the NCL-2000, microphone gain should be advanced until:
a. the exciter ALC indication is correct, or
b. speech peaks reach 400 ma. on the NCL-2000 plate current meter, or
c. an increase in exciter microphone gain does not result in increased NCL-2000 plate current.
3.5 AM TUNE-UP PROCEDURE:
For AM operation, adjust the NCL-2000 for SSB operation as in steps 1 through 4 above, and increase exciter carrier output until 400 ma. of plate current is indicated by the NCL-2000 plate milliampeter. Increase the exciter audio gain until the NCL-2000 plate current flickers slightly upward on voice peaks, and then reduce exciter audio gain to just below this point.
TABLE OF NCL-2000 CONTROL SETTINGS FOR 50 OHM LOAD
FREQUENCY P. A. TUNE P. A. LOAD
Approximate Actual Approximate Actual
The above tables are based on average NCL-2000 control settings when operated into a purely resistive 50-ohm load. They will, of course, vary with many installations and it is suggested that you note in the appropriate column the final control settings required for use with your own antenna to facilitate faster initial adjustment when changing bands.
SECTION 4 THEORY OF OPERATION
4.1 CLASS OF OPERATION:
Modern linear amplifiers are typically operated in Class B, Class ABI, or Class AB2 service.
The Class B amplifier has the distinct advantage of drawing negligible current and dissipating no power when no drive is present. It also has the advantage of relatively high efficiency, particularly when operated with grid current flow and with grids driven well into the positive region. The Class B amplifier does, however, have the disadvantage of being relatively non-linear because of non-linearity of the amplifier tube characteristics in the cut-off region. This will, of course, vary with the type of tube selected for the amplifier. A second (less well-known) disadvantage of the Class B amplifier appears when excessive drive is available and the linear is heavily loaded. Under these conditions, increasing input will drive the grid, screen, and plate currents to higher and higher levels and the amplifier stage can be easily damaged.
Class ABI operation overcomes these disadvantages. By allowing idling current, the effects of non-linearity near cut-off are avoided, thus causing a reduction in amplifier distortion. By definition, the Class ABI amplifier does not draw grid current and the grid bias supply can have relatively poor regulation (or high internal impedance) . In the event of over-drive, the amplifier tubes will draw grid current and make the bias voltage more negative. The result is effective limiting of plate and screen currents to safe operating levels even under conditions of severe over-drive. Unfortunately, these advantages of AB, operation are only gained with a loss of amplifier efficiency.
The Class AB amplifier operates with the same idling current as the AB1 amplifier, and therefore offers the same low distortion and excellent linearity, providing a well-regulated bias supply and sufficient driver power are available. The well-regulated bias supply is necessary to prevent change in bias voltage as the grids are driven positive. The ability to drive the grids positive offers the important advantage of higher efficiency than the Class ABI amplifier. However, the presence of the necessary well-regulated bias supply presents the same disadvantage apparent in the Class B amplifier - increasing input may result in dangerously high grid, screen and plate currents.
The NCL-2000 operates in grid-current-limited Class AB2 - a National development (patent pending) which combines the advantages of Class AB2 operation in terms of high efficiency and linearity with the effective limiting of plate and screen currents typical of the Class AB, amplifier. This is accomplished by the use of an electronically series regulated grid bias supply designed to hold constant grid bias with normal driving levels, but which allows grid bias to abruptly increase when grid current exceeds the pre-determined value of 15 ma.
The NCL-2000 bias supply has a constant 15 ma.
bleed. When grid current flows, it causes the regulator output current to decrease toward
zero; and when the grid current reaches 15 ma., the amount of the bleed, there is no
longer any current flow in the series regulator and the circuit loses regulation. As more
drive is applied beyond this point, grid voltage will become more negative and plate
current is limited just as in
4.2 GRID BIAS SUPPLY:
The grid bias supply is a conventional full wave center-tapped circuit using rectifiers CR1 and CR2 and filtering network R36 and C6. Output of this supply is fed to the collector of the series regulator transistor QL Operating bias, in turn, is obtained from the emitter of the regulating transistor. The 15 ma. bleed is obtained through R40, R41 and R42. A sample of the output voltage is obtained from the bleed network and applied to the base of the control transistor Q2. The collector of the control transistor is fed through resistor R38 from the -80 volt supply and the emitter of the control transistor is returned to ground through zener diode CR9. This zener diode is kept in its regulating range by current through resistor R39 from the negative supply. When the grids of the final amplifier draw current, the bias voltage will tend to go more negative. This will result in a negative change at the base of the control transistor and will result in more collector current flow through Q2. This will make the base of the regulator transistor go more positive, which results in less current flow through the regulator in such a manner as to cancel any change of bias to the final until grid current exceed 15 ma. and the regulator can no longer function. Adjustment of R41, the bias potentiometer, will result in a variation of bias from -25 to -45 volts to set proper idling current.
4.3 INPUT CIRCUIT:
Input power from the exciter is applied directly to a 50-ohm non-inductive metal film swamping resistor (R1 through R14). This swamping resistor is capable of absorbing more than 100 watts continuously for short periods of exciter tune-up. Nearly all modern exciters develop in excess of 80 watts PEP output which will develop excessive driving voltage across the resistor. Therefore, the NCL-2000 has been designed so that only one-half of the voltage across the resistor is applied to the final amplifier grids. When a low power 20 watt to 80 watt peak output exciter is used to drive the NCL-2000, an internal re-connection may be made to provide the full driving voltage of the exciter to the final grids (See paragraph 5.7 under Service Instructions). The low resistive impedance of the NCL-2000 input circuit presents a constant load to the exciter even under peak signal conditions, so that operation in Class AB service does not result in non-linearity due to poor exciter regulation. Also, this low resistive grid-to-ground impedance eliminates the need for neutralization and results in an amplifier that is extremely stable under all operating conditions. The use of this grounded-cathode Tetrode circuit with a swamped grid input, instead of the more common grounded grid circuit, prevents tuning of the final from affecting tuning of the exciter, allows lower driving power, and provides maximum useful power, since the exciter power does not have to be added to the amplifier power when input power measurements are made to assure legal operation.
4.4 A LC:
The RF input signal is coupled to the 8122 grids through capacitors C21 and C30. Bias is supplied to the grids through L5 and metering resistor R15. Excessive drive applied to the final tubes (modulation peaks producing more than 15 ma. of grid current) will cause an audio voltage to appear on the bias circuit. This voltage is coupled through C19 to voltage doublers rectifiers CR10 and CR11 to develop negative DC voltage for use as ALC information. This may be fed back by means of output jack J5 to exciters provided with ALC input circuits capable of accepting negative ALC voltage.
4.5 OUTPUT CIRCUIT:
The plate circuit of the 8122 final tubes is a conventional pi-network. High voltage is supplied through plate choke L2 and the output circuit is isolated from this plate voltage by blocking capacitor C48. The PA tune capacitor C42 is a double section unit. The smaller section of this capacitor is in the pi-network at all times and the remaining section is used only on the 80 meter band to provide full coverage. In addition, a 100 pf. capacitor C45 is switched into the input to provide additional tuning capacity on 80 and 40 meters. The pi-network coil is tapped by means of the bandswitch for operation on 80 through 10 meters. The PA load capacitor C43 is used on all bands, and the additional load capacitor C47 is used on 80 meters together with C46 which is also used on 40 meters to allow matching loads from 40 to 60 ohms.
Plate voltage is developed by a full wave voltage doubler using rectifiers CR7 and CR8. Filtering is accomplished by a bank of electrolytic capacitors C32 through C39. Screen voltage is developed by a full wave bridge circuit operating from another winding on the plate transformer, and screen filtering is accomplished by choke L2 and capacitor C31. Resistors R18, R19, R24 and R43 provide for short circuit surge limiting in the plate and screen circuits of the final amplifier.
The negative return of the high voltage supply is connected through a 1 ampere overload relay and a 1 ohm metering resistor to ground. A 0 to 1 volt DC meter connected in parallel with the 1 ohm metering resistor provides continuous plate current measurement. A separate I volt Multimeter is connected through the MULTIMETER switch to the multiplier resistors R20 through R23 for 0 to 5 KV plate voltage measurement, to resistor R16 for screen current measurement or to resistor R15 for grid current measurement and exciter tuning indication.
4.7 PRIMARY POWER CIRCUITS:
Primary power may be provided from either a 115-volt, 2-wire, or 230-volt, 3-wire line. In either connection, 115 volts is supplied through the main power switch S1 to the blower and the primary of the bias transformer which also provides low voltage A.C. for the final amplifier filaments, the ON lamp, and to the one minute time delay relay and the control circuits. After the time delay relay has closed (lighting the READY lamp), and if the cabinet interlock in the plate relay ground return is closed, the plate power relay maybe energized by pushing plate power switch S2. The plate relay in turn will apply A.C. to the plate/screen transformer and to the plate lamp. One contact of this plate relay acts as a holding contact and will keep the plate relay closed until the coil circuit is interrupted by pushing the plate power switch off. It may also be interrupted by a momentary trip of the plate overload relay, by an interruption of the cabinet interlock, or by interruption of the primary power switch. An important additional interlock, provided in the high voltage circuit, is a spring-actuated mechanical shorting bar which grounds the high voltage as soon as the cabinet lid is opened.
4.8 CW-SSB MODE SELECTION:
In single sideband operation, peak plate current is the controlling parameter with a given plate voltage. The amplifier must be capable of accepting proper peak input at high efficiency if maximum output is to be obtained without flat topping. The NCL-2000 is designed so that at the peak input of 2000 watts (2500 v. at 800 ma.), output efficiency is 60% minimum and peak output is as high as 1400 watts. In CW operation it is necessary to operate the NCL-2000 at only 1000 watts input. If this is done by operation at 2500 v. at 400 ma., the efficiency will drop and the power output will be less than optimum. It is possible to provide optimum efficiency for 1000 watt CW operation by matching the load impedance of the tubes to the load impedance of the amplifier. This may be done by altering the plate voltage and plate current of the final amplifier tubes. The NCL-2000 includes a front panel switch marked CW-SSB. When thrown to the CW position, the plate and screen voltage on the 8122 output tubes is reduced from 2500 v. and 400 v. to 1800 v. and 290 v. respectively. The amplifier can now be loaded to 1000 watts (550 ma. at 1800 v.) and the load impedance will be properly matched because of the lower plate voltage and current. The efficiency will now be back to 60% minimum and output will be in excess of 600 watts. There is an added advantage in this switching arrangement. The NCL-2000 is so designed that if it is properly loaded to 1000 watts in the CW position, it is only necessary to switch to SSB and the amplifier is correctly loaded and tuned for 2000 watt PEP SSB operation without the necessity of tuning up at an illegal key-down 2000 watts.
4.9 ANTENNA AND TRANSMIT - STANDBY SWITCHING:
An internal antenna transfer relay is incorporated in the NCL-2000 to switch the antenna from the linear output to the transceiver or separate receiver during receiving periods. When the plate relay is energized, power may be supplied to this antenna relay by shorting the "relay" terminals at the rear of the chassis with switching contacts on the exciter. During transmitting periods, the "relay" terminals are shorted, the antenna relay will be energized, the exciter input will be transferred to the linear input, and the antenna to the linear output.
If the plate power relay is open, whether as a result of intentional switching or if primary power is not applied to the amplifier, the antenna relay is de-energized and the exciter is automatically connected directly to the antenna.
4.10 CUT-OFF BIAS:
Idling current in the final amplifier tubes is approximately 250 ma. for the pair, which results in over 600 watts of plate dissipation. It is undesirable to draw this power during receiving periods and, in addition, current flow in the final amplifier tubes will result in noise generation which may be heard in the receiver. For this reason, the return connections of the zener diode and the 15 ma. bleed resistors of the bias regulator are returned to ground through an additional contact on the antenna relay. During transmitting periods, the bias supply operates as described previously. During receiving periods, the ground connection is broken and the bias regulator cannot function. This will cause the full bias supply output, approximately -80 volts, to be applied to the final grids, thus cutting off the tubes and eliminating the idling current.
SECTION 5 SERVICE INSTRUCTIONS
5.1 TUBE REPLACEMENT:
The NCL-2000 (like all linears with paralleled output tubes) requires matched tubes for efficient operation.
Should one of the output tubes fail, it is necessary to match its replacement to the remaining good tube. This may be done by measuring the grid bias voltage of the good tube at rated idling current. Remove the bad tube and connect the negative lead of a high impedance voltmeter with a 50-volt scale to the tip jack on the rear apron of the chassis: The other lead should be grounded. With the NCL-2000 adjusted for no-signal idling and with the CW-SSB switch on SSB, adjust the bias control for 125 ma. idling current as indicated by the plate milliameter. Read the voltage at the tip jack. This test should be performed as quickly as possible since the air supply of the tube is reduced drastically with the other tube removed. The National Radio Company Service Department must be advised of this voltage measurement, together with the Multimeter plate voltage reading taken at the same time, in order to supply a properly matched replacement tube. Should both tubes fail, the replacement pair will be automatically matched correctly by National.
To change the tubes, loosen the wing nuts on the tube clamps. Carefully remove the tubes and replace with the new ones, then tighten the wing nuts securely.
The 8122 output tubes used in the NCL-2000 are guaranteed against failure for a period of 90 days from date of purchase. Tubes will be replaced under the terms of the warranty provided they are returned to National for examination and such examination indicates that they are indeed defective from causes other than abuse, mechanical damage, or from failure resulting from excessive temperature due to mistuning or operation out of rating.
5.2 FILTER CAPACITOR BOARD REMOVAL:
The filter capacitor board is held in place by the four sheet metal screws near the corner of the board. It is not necessary to disconnect wires to the board in order to remove it - simply remove the four sheet metal screws and swing the board free for examination.
5.3 CABINET REMOVAL:
The cabinet is held in place by self-tapping screws around the opening in the rear of the cabinet and by four screws in the bottom of the cabinet. To remove the cabinet, open the lid and remove the high voltage interlock rod. Remove all of the retaining screws and pull the chassis and front panel forward out of the cabinet. It may be convenient to simultaneously exert pressure on the chassis through the opening at the rear of the cabinet.
5.4 FRONT PANEL AND SIDE GUSSET REMOVAL:
The side gussets are held in place by the four screws at the top and bottom of the front panel and by the sheet metal screws holding the gussets to the side of the chassis. To remove the front panel, remove the cabinet, the side gussets, the interlock socket on the top lip of the front panel, the knobs, the palnuts on the MULTIMETER switch, BAND switch and PA LOAD shafts, and the three screws near the PA TUNE shaft. Pull the front panel forward and remove the meter mounting plate and the three pilot light sockets, noting their relative positions so that they may be replaced correctly.
5.5 REGULATOR TRANSISTOR CHANGE:
The regulator transistor cases are insulated from ground by a very thin mica washer. CAUTIONI - Care must be exercised to avoid destroying this insulation. To change a regulator transistor, remove the cabinet and the right side gusset, unsolder the wires to the base and emitter pins and remove the screws that hold the transistor to the chassis. This will also free the collector (case) connection. Pull the transistor out of the chassis and replace with a new one by reversing the above procedure. Before replacing transistor connections, check the collector-to-chassis resistance to see that the insulation is intact.
5.6 AIR SYSTEM CLEANING:
It is recommended that the air system be cleaned every three months and more frequently in dusty locations. Remove the cabinet and the cover of the pressurized compartment beneath the tubes. Clean all areas of dust accumulation with a small brush or with a household vacuum cleaner; also clean the inside of the blower squirrel cage and the tube plate radiators. An air jet, if available, will speed air system cleaning.
5.7 GRID RESISTOR TAP CHANGE:
If less than 80 watts PEP of exciter power is available, it is necessary to change the tap on locate the tinned wire running from a feed-through bushing on the pressurized compartment to the swamping resistor bracket nearest the side of the chassis. Move this wire to the bracket nearer the center of the chassis. This bracket also has the wire from the coaxial jack marked "XMTR" connected to it.
VOLTAGE AND RESISTANCE CHECKS
Note: Resistance measurements are made with all power disconnected, and with the negative ohmmeter lead grounded. Since many of the measurements are made across semiconductors, the readings may vary moderately from those given here. Voltage measurements are made with the MULTIMETER switch in EXCITER TUNE position with the relay terminals shorted. The bias should be set to -35 volts at the rear-apron BIAS CHECK jack, and line voltage is 117 volts.
MEASUREMENT POINT RESISTANCE TO GROUND VOLTAGE TO GROUND
SECTION 6 TROUBLE SHOOTING
- CAUTION -
VOLTAGES USED IN THE NCL-2000 ARE LETHAL. MAKE CERTAIN THAT ALL INTERLOCKS ARE OPERATIVE AND THAT PRIMARY POWER HAS BEEN REMOVED BEFORE MAKING INTERNAL ADJUSTMENTS.
It is assumed that, if possible, the Linear has been correctly tuned and loaded before any operation checks are made.
SYMPTOM PROBABLE CAUSE AND CURE
80 (3.7 to 4.0 mhz)
40 (7.0 to 7.3 mhz)
20 (14.0 to 14.35 mhz)
15 (21.0 to 21.45 mhz)
10 (28.0 to 29.7 mhz) meter bands plus overlap at band edges.
2. POWER INPUT (SSB): 1000 watts average, 2000 watts PEP.
POWER INPUT (AM): 1000 watts.
2A. POWER OUTPUT (SSB): 1300 watts minimum on all bands.
POWER OUTPUT (CW, RTTY): 1000 watts.
POWER OUTPUT (AM): 300 watts minimum on all bands.
3. OUTPUT IMPEDANCE MATCHING RANGE: 40-60 ohms minimum.
4. INPUT IMPEDANCE: 50 ohms nominal, unbalanced.
5. DRIVE REQUIREMENTS: 20 to 200 watts PEP, adjustable.
6. COMPONENT RATINGS: All components specified to best commercial; all power components rated at 1000 watt average input ICAS.
7. POWER SUPPLY:
Bias voltage electronically regulated
Plate supply utilizes solid state rectifiers in full wave voltage doubler configuration for better ripple & regulation, rated to deliver 2500 watts at 800 ma. keydown.
8. DISTORTION PRODUCTS AT FULL RATED OUTPUT: 30-45 db signal to distortion ratio.
9. NOISE: NLT 40 db down.
10. ALC: Rear panel output for use with equipment incorporating such provision.
13. ALTITUDE: To 8000 ft.
14. TUNE-UP PROVISIONS:
SSW/CW switch permits tune-up at 1000 watts to comply with FCC.
Built-in dummy load for exciter permits tune-up into amplifier grid circuit with amplifier plate voltage removed and with relative exciter power indicated on amplifier multi-meter.
15. SAFETY PROVISIONS:
Primary power fuses
One minute time delay
Plate current overload (1.2 amp) relay
Hinged cover utilizes Dzus fasteners with plate power interlock
Automatic shorting bar connects plate power to ground (when cover is raised) in case of interlock or bleeder failure.
16. INTERNAL RELAYS:
Plate power time delay
Plate current overload
Precision plate milliamp meter; rear illuminated, 0-1000 ma.
Precision multi-meter; rear illuminated;
0-5000 plate volts
0-50 screen milliamps
0-50 grid milliamps
Exciter relative power output.
18. FRONT PANEL CONTROLS:
Power On/Off, SSB/CW
Plate power On/Off
Power pilot light
Amplifier ready light
Plate power light
19. REAR PANEL:
Control relay terminals
Grid Bias test jack
Primary power input.
20. TUBE AND SEMI-CONDUCTOR COMPLEMENT:
Two RCA 8122 output tubes; rated 400 watts each
13 semiconductors for rectification and regulation.
21. SIZE AND WEIGHT:
7 5/8 inches High
16 ½ inches Wide
12 ¾ Deep.
22. PRIMARY POWER REQUIREMENTS:
115 VAC 60 cycle single phase
230 VAC 60 cycle single phase
3 wire neutral ground.
Current 15 amperes maximum as 230 V.
May be operated with 50 cycle source at reduced input or with 60 cycle source for the fan.
Tubes, transistors and labor warranted for three months, all other components for 12 months after date of purchase.
NCL-2000 Instruction Manual Addenda
Remember that the inertia of the Multimeter in the Grid Current position requires strict adherence to the instructions concerning grid current indication during SSB operation. When the Grid Current meter just occasionally flickers off zero on the voice peaks, the NCl-2000 is being properly operated. Should the NCL-2000 be driven so hard that there is a constant grid current indication, no matter how small, actual peak grid current will be in excess of 15 ma. and flat-topping" with consequent splatter will occur.
It is not advisable to switch the Multimeter to Exciter Tune while the unit has both plate voltage and full drive applied since a momentary no-load condition occurs when the antenna relay energizes, which may cause a momentary arc-over in the plate circuit.
After tune-up in CW and switching to SSB, it is advisable to briefly recheck the screen current with carrier inserted to double check for proper loading. If the NCL-2000 is loaded heavily enough, there will be only a slight increase in the screen circuit.
The NCL-2000 transmit-receive switching has been modified in units with a "B" succeeding the serial number, to give correct ALC operation. Instead of opening the ground return to the grid bias regulator's zener diode and control transistor as was done previously, the ground return of the screen supply is now opened during receive. This cuts the 8122's fully off, since they will draw no plate current with the screen voltage at zero. The modification has the effect of removing the switching transient from the grid bias line that was causing a large ALC voltage to be developed at the instant of receive to transmit switching.
The following changes should be noted: The content of paragraph 4.10 should be changed to reflect the modification and the contacts of K1 that ground the junction of CR9, C17, C12 and R42 should be inserted between the junction of CR3 and CR6 and the junction of R16, C31, R35 and the contact of S4B. C17 and C12 are deleted, and the junction that now consists of CR9 and R42 is permanently grounded. It is suggested that these changed be noted on the schematic.
Locate R15 below the input resistance network. From the top of R15, draw a line to meet the vertical line approximately 1/2 inch to the right.
Customer Service Bulletin
ALC TX/RX Switching
The NCL-2000 TX/RX switching has been modified to correct the ALC delay sometimes encountered at the beginning of a transmission when using the NCL-2000 ALC circuitry with exciters having an ALC input. Instead of opening the ground return of the grid bias regulators zener diode and control transistor as was previously done to cut off the 8122 tubes, the ground return of the screen supply is now open during standby. This modification removes a switching transient from the grid bias line which caused a large ALC voltage to be developed as the instant of receive to transmit switching.
The following wiring change should be made in the NCL-2000. DO NOT disturb any of the other wiring in the NCL-200 while making this change.
STEP 1. Disconnect the
STEP 2. Connect a jumper between the junction of CR9 and R42 and the adjacent ground lug.
STEP 3. Disconnect rectifiers CR3 and CR6 from the lug nearest the front panel; reconnect CR3 and CR6 to the adjacent empty lug.
STEP 4. Splice a 9 inch length of
STEP 5. Connect the other end of the wire spliced on in Step #4 to the lug mentioned in Step #3 where CR3 and CR6 are newly connected.
STEP 6. Disconnect the BLUE wire connected to the ground lug near the relay terminal strip J6. Splice a 17 inch length of BLUE wire onto the BLUE wire and route the new wire along the harness, past the PA LOAD capacitor, C6 and S4. Connect it to the lug where CR3 and CR6 used to be, where the black wire and one end of R35 are connected.
STEP 7. Solder all the new connections.
National Radio Customers Service Bulletins are provided free to all registered owners of our equipment. Information contained herein is gathered from our factory and field service organizations as well as from National owners. We DO NOT recommend the automatic incorporation of modifications except when the trouble description is identical to your problem or when suggested for preventive maintenance.
It has been found that in some installations, switching transients exceed the PIV ratings of the screen diodes. It is therefore recommended that .01uf, 1KV special AC rating disc capacitors (National Part #A51457) be placed across each screen diode. Designate these C53, 54, 55 and 56. These have been installed in production on Series 80 and higher. These parts are $.50 each and are available through our service department.
On all NCL-200 schematics the screen diodes are shown incorrectly. The diodes are arranged in a normal full wave bridge fashion with all cathodes pointing to the choke.
On the original schematic you should draw a line from the top of R15 to the bias checkpoint directly to the right.
On series 80 schematics reverse the polarity of CR10 and be sure the lower terminal of the relay control panel is shown as connected to chassis ground.
For improved loading on 80 meters on series 71 sets, change C47 to 300 pf at 5Kv.
You must always use tubes that are matched as a pair. It is vital to the correct operation of the NCL-2000. It is possible to use cathode resistors to maintain a match between a pair of unmatched tubes. It is possible to use cathode resistors to match irregular tubes. Apply cathode resistors of carbon composition. Place a low ohm resistor on each of the cathode pins. There are three. Now you can monitor the voltage (and current) by keying the amp with full voltages applied but do not apply any drive.
SYMPTOM PROBABLE CAUSE AND CURE
All commonly available resistors and capacitors are fully identified on the schematic and are not included in this parts list. See the end of this list for information on acquiring new parts for this 40 year piece of gear. If you have more pertinent information on where new parts or equivalent parts can be obtained, please email the data thru the website at www.ncl2k.n9efj.net. I am sure all NCL-2000 users would appreciate the info.
PART DESIGNATOR DESCRIPTION NATIONAL PART #
BL1 Blower B51481
C1 through C5 Capacitor, Ceramic; .01 uf (Special AC line rating) A51457
C6 Capacitor, Electrolytic; 150 uf, 150 Volts DC C19411-5
C31 Capacitor, Electrolytic; 80 uf, 450 Volts DC C19411-4
C32 through C39 Capacitor, Electrolytic; 80 uf, 450 Volts DC (Special) C51447
C42 Capacitor, PA Tune A51400 Pg 19
C43 Capacitor, PA Load C51441
C44 Capacitor, Ceramic; .001 pf 6Kv A50849-9
C45 Capacitor, Ceramic Transmitting; 100 pf 5Kv A51403-1
C46 Capacitor, Ceramic Transmitting; 750 pf 5Kv A51403-2
C47 Capacitor, Ceramic Transmitting; 300 pf 5Kv A51403-4
C48 Capacitor, Ceramic Transmitting; 1000 pf 5Kv A51403-3
CR1, CR2 Rectifier; Silicon;400 volt PIV 500 ma A51407-3
CR3 thru CR6 Rectifier, Silicon;1000 volt PIV 500 ma A51407-1
CR7,CR8 Rectifier, silicon; Stack; 5000 PIV 1 amp A51431
CR9 Diode ; Zener 18 volts 1 W Type 1N1777 1N1777
F1, F2 Fuse; 12 amps 250 volts A51444
J1 thru J4 Connector UHF SO-239 A51479
K1 Relay, Antenna; DPDT and SPST-NO contacts
Contact rating 10 amps at 30 mhz, 12 VAC coil B51416-1
2 amps. Relay pulls in at 1 amp coil current. B51416-3
K3 Relay, Plate Power; SPST NO contacts; rated at 10
12 VAC coil B51416-2
K4 Relay, Time Delay; SPST NO contacts; 12 v heater A51411
L1 Choke, Swinging 6 H at 35 ma; 3 H at 70 ma; 1500 V
Insulation; 150 ohms DC resistance B51428
L2 Choke RF Plate; 165 uh 800 ma DC B51448
L3 Choke, RF; 2.5 mh; National Radio Catalog #R100S-2.5 D50530-12
L4 Final; Tank B51473-3
L5 Choke, RF; 750 uh; National Radio Catalog #R33-750 C50527-5
M1 Meter, Plate Current; 0-1 VDC; 1000 ohms per volt C51442-1
M2 Meter, Multimeter; 0-1 VDC; 1000 ohms per volt C51442-2
Q1, Q2 Transistor, PNP Type 2N1159 2N1159
R1 thru R14 Resistor, Swamping; 180 Ohm 7 watt 10%; metal film type A41402-1
R15, R16 Resistor, 20.5 ohms ½ watt 1% A51402-1
R17 Resistor, 1 ohm 2 watt 1% A51402-5
R20, R21, R22 Resistor, 1.6 mohm 1 watt 1% A51402-3
R25 thru R32 Resistor, 50 kohm 5 watt 10% A51402-2
R33, R34 Resistor, 75 kohm 55 watt 10% A51412
R35 Resistor, Bleeder; 11.5 kohm 15 watt 10% B17436-18
R42 Potentiometer, Bias adjust; 1.2 kohm ½ watt B51378
R43 Resistor, 15 ohm 10 watt 10% (Special surge rating) A51332
S1 Switch, Rocker; SPST 6 amps B51424-1
S2 Switch, Rocker; Special switching configuration 6 amps B51424-3
S3 Switch, Rocker; DPDT 20 amps B51424-2
S4 Switch, Meter B51476
S5 Switch, Bandswitch B51446
T1 Transformer, Power D51455
T2 Transformer, Filament C51440
V1, V2 Tube, 8122 Ceramic Tetrode 8122
Z1, Z2 Chokes, Parasitic 2 turns #16 wire on 33 ohm 2W Resistor B51430
Misc Front Panel Assembly E51498-6
Misc Cabinet E51496-5
Misc Cover, Cabinet E51495-4
Misc Terminal Board, High Voltage C51434-2
Misc Power Cord A51482
Misc Knobs, 4 A51200
Misc Socket Tube x 2, Elevenar A51406
Misc Feedtrough Capacitors x10 SA481
This modification resulted from excessive heat on one tube on key-up without drive. The amp on low was drawing 125 ma. This was with the cathode connected to ground as originally configured. During testing of the tubes I bought on eBay, I noticed one tube was getting HOT! Tuning was erratic and the overload relay had tripped once. No doubt the tubes were not NOS matched as the eBay seller had stated in the auction. BUYER BEWARE OF NOS TUBES. Several other good or NOS tubes failed later as I tried to collect a spare set of tubes. Thankfully I finally got 4 more tubes that are OK and will work as spares. But I was lucky. So I set out to equalize the current draw thru the tubes and here is the result:
Matched tubes is an absolute must with this amplifier. Unless you purchase new tubes you will have to match the tubes with cathode resistors or any means to control and set the standby current of each tube. There are 3 cathode pins on each tube. I placed 3.9 ohm carbon resistors on each cathode pin. Then I keyed the amp with plate and screen voltage applied but no drive. I measured the voltage at each cathode. Then by trial and error I removed a resistor and watched the voltage on the tube with lower voltage. Removing one resistor brought the lower voltage tube up to the level of the other. I had both tubes drawing almost identical voltage.
You might wonder that both tubes are not drawing the same current. The cathode of one tube has a 1.3 ohm resistance to ground and the other has 1.9 ohms. Doing the math we get .06 volts across 1.3 ohms. Thats 46 milliamps and 32 milliamps on the other with .06 volts across 1.9 ohms. But I did end up with almost equal current drawn thru each tube. It was simple! Those resistors I used had a 20% tolerance. That led me to try each resistor in the group and it worked. They were not all 3.9 ohms as we might suspect. Twenty percent either way and trial and error got it right.
I had a suggestion from a friend that I could match the tubes with variable bias controls for each tube. I opted out for fear of the pots failing and loosing the control the amp needs. There is plenty of room in the compartment to implement a dual bias control. You would need to duplicate the input circuit to the tubes. Maybe that would work for you. I feel safer with the cathode raised slightly above ground.
You can try a modification I got from the web if you think borrowing HV voltage for the screen supply is more efficient or trustworthy. This mod is simply a circuit similar to the diagram in any ARRL handbook. It is called the economy power supply. It uses large resistor/s to drop the HV down to a 400 volt Zener. Tap there to get screen voltage. I like the regulated 400 volts and I would certainly use the circuit if I lost only the screen winding in the power transformer.
OBTAINING NEW OR EQUIVALENT PARTS
BL1, the blower as a unit, is unavailable. I
could not find a listing on it anywhere. Fortunately
you will more than likely just need the motor which is still available. You can get a
replacement motor from Grainger. It is not and exact fit because the motor is taller than
the old one. That means you have to increase the height of the strap that mounts on the
top of the motor. Not impossible but it isnt easy. I would make a new metal strap
for the top mounting hole in a U shape. Cut the flat part off the old motor
mount leaving only the vertical pieces. Mount
the new metal by bolting it to the old vertical pieces you have left. The motor is a
All relays use 12 volt AC coils. They cost more but most manufacturers have a model for your needs.
During my initial encounter with a NCL-2000, the overload relay tripped every time it keyed up. The relay has a 2.2 ohm resistor across the coil. The resistor was blown and that pulled in the relay (removing AC voltage from the relay farther down the 13.6 volt AC rail) with any key up. One too many glitches and the HV low end got into the meter coil damaging it beyond repair. It this case the plate current meter is connected to the hot side of the relay coil. There is a 1 ohm resistor shunting the meter. Too much voltage ( from a parasitic glitch?) came down the negative HV line, popped the 2.2 and the 1 ohm resistor, and then found the path of least resistance thru the meter. So long meter, by bye ..
The only relay I couldnt replace exactly is the overload relay. The relay has to draw ~1 amp to work properly, thereby protecting the amplifier. I am sure a replacement can be found but I did not make the extended effort to locate one. I just used one from a junker so it worked OK.
The plate power relay can be replaced by any 3PDT 12 volt AC relay. I got one at a local electronics house. All the main relay manufacturers have a model.
The antenna change-over relay is another 12 volt AC coil relay. Ive read it is recommended that this relay be replaced even though it works. Thats up to you ..
If you get a NCL-2000 needing work, there is a 90% chance you will need a plate current meter. Most replacement meters will fit. I have used two different styles and there was ample room to get them in. Try looking for the GC brand. There is a foreign model, SUN I think, that will work or you can order one thru the neighborhood electronics store or from the web. Your problem is getting it read 1 volt DC. I should say task because most of us know how to drop the voltage or make meter shunts. All that information is in any ARRL handbook for any year. I have made a scale for the GC meter and it is included elsewhere in this manual or available from the website http://ncl-2000.n9efj.net.
HV INLINE CHOKE R43:
The manufacturer used a wirewound resistor here to limit any HV / Parasitic glitch. Being wirewound the coil functions as a choke and I suppose it has to be inline with the HV for blocking purposes. I guess I was the only one that had trouble with this device. I burnt out at least 6 of these resistor/chokes trying to tame the beast. I finally took some Nichrome wire from a hair dryer and put it in line. That allowed me to finish taming the amp and get it settled down without blowing the $1 cement type power resistors. After all was well, I put the correct resistor/choke back inline and its been OK since. Those resistors I burnt up were destroyed by a spike and the voltage ran the length of the devices and completely burnt the wire into inside the 10 watt resistor. In the end having matched tubes was the answer to taming the beast.
The transformer supply plate and screen voltage. The most common problem is a short between the two windings. The answer is PETER DAHL. You may be able to get yours rewound. Here is the link
This is the most reoccurring problem you will have. My answer is to make a roller inductor from the tank coil. I know I can fix one up proper with a right angle drive or a motor driven unit. Im gonna tackle that problem this winter. The switch is similar to several others I have seen and replaced in the past. You should try to get an equivalent switch if you see one at a hamfest. That is if you plan on keeping the amp.
I have had to repair the bandswitch twice. Both problem came up when I tried new experimental antennas. Hopefully you will use resonant antennas and not be troubled by bandswitch problem.
The other day I had a glitch trying to work 20 meters. I kept getting an arc over and that trouble was the front wafer. There was some carbon from a previous problem that provided a path to the mounting screw on the front wafer. This is where the caps are added to the network on 80 and 40 meters. I simply cut the wire going to the tank coil. To fix this Ill have to replace the bandswitch. I have an extra bandswitch but I plan to do the roller fix first.
OK now it gets expensive to own a NCL-2000. I told you not everyone can own an NCL-2000. New Burle tubes are gonna set you back over $750 for a matched pair. I intend to get a pair of new ones before they are all gone.
I have bought at least a dozen tubes on eBay. I dont recommended getting tubes that way. All but two shorted out on the initial key-down. I was lucky with the present pair Im using. I bought a couple of pairs from buyers who said they were pulled tubes from a working amp. Well they didnt work in my amp except for that one pair and I had to match them with cathode resistors. I would advise you to ask lots of questions from the sellers if you plan to bid on some 8122s on eBay. You might ask the seller, if he claims the tubes are new, to take a really close-up picture of the tube/s. A close-up picture may reveal scratches on the plate where the tubes have been installed. Those tubes are difficult to install and remove so the evidence will show.
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