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• Repair Summary
• RCA 8122 Beam Power Tubes

NCX-1000 Transceiver 

1. Power Supply
   
   Replace all the electrolytic capacitors. Measure 280vdc at the NCX-5 connector. If  it is higher than 290vdc , I suggest changing R9 from 660 ohms to 1K ohms. AC mains are much higher these days and there is no reason to push tube life.
   
   
2. VFO Shaft Sloppy
   
   The fiber retaining ring for the clutch spring had broken and was pushing the shaft bushing out the front.
   
   I had to disassemble the gears from the bottom, removing the VFO cover.
   I replaced the washer with a nylon washer, sanded down to the right thickness.
   I thoroughly cleaned all the gears/discs/pinions and made sure all had 2 notches spring loading when re-assembled.
   I re-lubricated with sparing, light lithium grease only.
   
   
3. VFO Display Wheel Dead Zone
   
   It turns out that the BAND control shaft CAM was ramming the digital display assembly too far to the right and binding the wheels. The digital display wheels must flop freely on all bands. I used the set screw and rotated the cam until all wheels flop freely on all bands. The dead zone is now gone.
   
   
4. VFO End Points
   
   There is a coil and an trimmer to set end points. This is a real pain unless you have the DD-103 hooked up and the temperature compensating trimmer hoses up the works every time you make a change so end point have to be re-done.
   
   
5. VFO Drift
   
   There is a documented trimmer to find a drift setting you like. I set mine for longer term stability vs. short term. It runs up 550Hz, back a bit, then rock stable. Any changes of the temperature compensation trimmer affect the low end point - of course.
   
   
6. VFO Linearity
   
   The manual sort of documents how to bend the special stator tabs to improve end-to-end linearity. I gave up after 4 hours, not worth it. I got 4, 2,9, 3.8, 3.7 and 3.5 withing 200 Hz - not bad for its day.
   
   
7. VFO Mechanical Instability Whilst Tuning or Beating Cabinet With Rubber Mallet or Baseball Bat
   
   I carefully cleaned the VFO tuning capacitor with special attention to the 3 WIPER fingers. I used DeOxit and sprayed away all excess. I soldered a bus wire through all 3 WIPER finger terminals and soldered the bus wire to the capacitor frame. Rivets can be troublesome in their old age, very stable now.
   
   
8. Local Oscillator Trimmers
   
   After the VFO is the best it can be on 75 meters, go to each band and adjust the appropriate trimmer for the same dial setting. I could only get within 500Hz but that just fine. If you don't have a digital display, you can attempt to follow the instructions in the manual.
   
   
9. Transceiver Vernier (RIT) Instability
   
   I added a ground wire from the RIT potentiometer to a new GND lug on the chassis to eliminate the poor path through the front panel.
   
   
10. S-Meter Zero Drifts
    
    It takes about 5 minutes for the S meter to drift down to the proper ZERO setting. I attempted no fix for this - maybe a componenet change somewhere.
    
    
11. 40+ Years of  Grime and the Magic Feather
    
    Clean with a neutral cleaner. Alcohol can damage geezer painted items. A mid sized pheasant or guinea fowl feather makes an amazing digital display wheel and grass cleaner - no disassembly required. They are curved just right and have little dusting feathers at the ends.
    
    
12. Aluminum Knob Inserts Randomly Fall Out
    
    Clean and reattach with clear silicon adhesive. I had one that cracked down the side that epoxy fixed pretty well.
    
    
13. Calibrator
    
    This radio really needs a crystal calibrator due to the VFO linearity irregularities. I am going to explore a modern solid state version I can plug into the socket.
    
    
14. Crackle Noises in AF During Warm Up
    
    I suspected the multi section (40uf, 40uf, 20uf) can capacitor. I have one ordered from www.tubesandmore.com and will report on the final outcome. The new capacitor solved the problem. It measured ok but must have been breaking down from time to time. 
    
    

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1. Does not Transmit and Receive on the Same Frequency

The original design has insufficient VFO buffering to handle all V3 tube characteristics changes. The only fix is to swap V3 until you find a tube that works. It took 4 tubes to get the XMT-REC delta to < 10Hz. It ranged up to 110Hz and National thought 50Hz was in spec! If anyone has a better idea, please post it. The voltage regulation has NOTHING to do with this frequency delta - it's the huge load change on V3. I even tried a high quality 3 legged voltage regulator to no avail. Use only NOS tubes for this exercise. 

2. Annoying 1,800 Hz CHIRP-YELP in the Speaker on SSB PTT/MOX and First Key Down on CW

On AM and VOX mode the problem does not exist.
Some units do not have the problem.
I suspect that the TR relay drop out speed and physical contact spacing just happens to be fast enough to mute some units.

The goofy design pulling in the relay for RECIEVE provides a sure thing, long time magnetization behavior change factor.

The chirp (ugly 1800Hz triangle wave) is being generated by the LOCAL OSC tube, primarily on 75 meters when it is not oscillating - no crystal or trimmer between plate and grid, etc. The chirp gets fed into the V13 mixer grid when the TR BIAS changes all XMT tube bias points. The chirp is only approx 25 msecs but damn annoying, especially if the AF gain is up on weak signals.

If the AF stages were muted properly, no one would care about these transitional glitches.

The PTT/MOX design sequence is all wrong and relies on the DROP_OUT speed of the relay which gets magnetized over time since it must be pulled in for receive mode – this is odd in itself.

The unfortunate sequence that allows the annoying chirp to sneak in is as follows:

1.    PTT/MOX/FIRST-KEY closure to ground
2.    TR BIAS (135) gets grounded.
3.    This changes all XMT tube bias points for XMT mode. This is where chirps and other weird transitional crap happens.
4.    V3’s screen voltage drops and a BIAS CONTROL voltage divider develops -40vdc REC BIAS.
5.    This -40vdc is sent to the PPT DIODE to drop out the TR RLEAY and mutes the PROD DET and AF at the same time – TOO LATE BINKY!

The possible fix scenarios were.

1.    Redesign the whole XMT transition process. This would be pretty invasive.
2.    Change the design to work like VOX mode (use the TR relay for bias changes, muting etc.) This too would require cuts and hacks.
3.    Install a reed relay to disconnect the speaker.
4.    Design a clever solid state switch to overlay the current design and always mute the PROD DET and AF before all the goofy XMT bias transitions happen. This might not require any wire cutting or hacks. When the TR relay opens and mute happens again - WHO CARES, AF land is already muted.
5.    Take the NCX-5 to desert and make it pay the price of its dishonor ability with FMJ 5.56 NATO rounds. The chirp was starting annoy me big time.
6. Figure out a way to dampen/choke the LOCAL OSC and the SSB CAR OSC to snuff out all chirp-like crud. I spent too much time on this option and gave up like I suspect National did in the 60's.

The Fix – Option 4 Chosen

With inspiration from WA6TJN and a cool little MOSFET switch designed by K7JEB, a reasonable solution is at hand. I have tested it in all modes for days now. NCX-5 operation is a real pleasure now. The only XMT transition noise is a small thunk - no longer annoying.

The following picture shows the prototype board with an overkill 12amp MOSFET and junk box resistors.

The schematic is quite simple. The parts list is small (1 MOSFET, diodes and 4 1/8watt resistors, 1” by 1” Radio Shack perf board, Velco. It connects to 4 points with no wire cutting: -80vdc, GND, PTT/MOX (135), AF-DET MUTE(131).

MOSFET Data Sheet

The chirp fix  board was Velcro mounted to the back side of the SSB OSC shield and the 4 wires run - no cutting required. You can zoom in on the image below and follow the white wires from the fix board to see the connection points.

3. AGC too Fast for SSB
   
   Older SSB rigs tended to have faster AGC than I like to hear. I  changed C146 from 0.1uf to 0.2uf.
   
   
4. S-Meter Not Accurate Above S-9
   
   Over S-9, this puppy is way too generous. This is usually old caps and resistors but I think it's designed this way. No fix.
   
   
5. CW Note Still Poor After Applying Service Bulletins
   
   The transitions are just too large for the OA2. I might try supplying the VFO 6.5vdc from a more isolated supply off of the filaments with a 3 legged voltage regulator - some day.
   
   
6. Modern VFO Stabilizer - X-LOCK
   
   I've wasted hours tweaking the temp-comp trimmer for best behavior and there is a limit of reasonability. I hate the continued small drift even after a hour warm up. I adapted a modern, microprocessor controlled stabilizer X-LOCK. This puppy keeps the VFO within 10 Hz - smoking !!! It is a brilliant new design based on older "Huff-Puff" designs. See the extremely well written manual for additional details X-LOCK Manual.
   
   
   
   The External VFO is perfect for this enhancement. The VFO is powered from the 280VDV line using 2 series panel lamps, a 18v zener, another series current limiting resistor and a final 6v zener to supply the VFO transistors - weird. If the panel  lamps die, the VFO dies plus the lamps get very hot. I initially tried a 12v zener with a 5K 20 watt resistor but this is a stupid waste of power and another heat generator. I found that a simple rectifier and cap off of the 12VAC line made more sense. The 3 connections are: 12VDC (diodes and cap on J2 pin 7), VFO output to sample from (on terminal board coax connections), simple varactor diode network inside the VFO can with short leads using the handy 2 empty termial lugs.
   
   I used a 12pf coupling cap instead of the supplied 22pf to eliminate warble and still have the required varactor steering range from cold to hot. I programmed the "post tuning delay" to 10 seconds vs. the default 2 seconds. This gives one time to get precise with the clunky mechanical tuning mechanism before it locks down. The RIT logic is just a minor annoyance when setting the dial within 10Hz. The RIT is not operational when one is using the National External VFO. Just turn off freq quickly, 2-3KHz then set for precision.
   Note: you will have to recalibrate the VFO end points - arrrrgggg!
   
   24 hour longer term testing shows VFO output is held within 20Hz.. This exceeds the designer's specifiication and makes me smile. Without X-LOCK my VFO drifts 300+ Hz from cold to hot. This was very good for 1967 mind you.
   
   
            

10 Hz Resolution Modern Digital Display

The DD-103 Digital Frequency Display from Electronic Specialty Products in Florida works well. Since he has no NCX-5 pre-programmed offsets, it's a real challenge to program but it works very well. Unlike the DDF for Colllins, you have to manually change bands and modes since the only input is the VFO signal. Programming involves defining the offsets for each band and mode. I sent all the offset data to the manufacturer and perhaps he'll add to the pre-programmed selections for both the NCX-5 and NCX-1000.

• Receiver sensitivity is surprisingly good compared to FLEX 5000A, Drake TR-4CW and Collins KWM-2A. My test was simple recovered audio comparison on 20 meters with weak (S2) European stations. The noise floor was also very low. I was very impressed with this olde dog.
• AM operation is possible using the RIT. Zero beat on SSB, change to AM Mode, use RIT to achieve tolerable received audio (bandpass is too narrow for good  AM). On transmit, make sure you turn the MIC gain way down to barely bump the plate current just like the manual describes. This corresponds with a proper pattern on the scope. Unfortunately, the transmitted AM signal is a carrier with only one sideband.

Service Bulletins

NCL-2000 Restoration

The NCL-2000 I acquired is not in the near new condition the NCX-5 MKIIs are in - lot's of chassis discoloration - fugly but not material. In spite of my fine and noble plans to the contrary, I made the mistake of powering it up to see if it played before going through it.

The power supply and tubes were in reasonable shape because it was making 1,300 watts output (the factory rating) on 75 meters. This is NOT a 1,500 watt PEP amplifier. My NIST calibrated power meter shows 1,300-1,400 max with new tubes. 1,300 watts out  is pushing two 400 watt tubes quite hard but effectively.

The documented tuning procedure is the most arcane I have ever seen. There is no microprocessor control of anything. Take to time read the lengthy CW tune up procedure and do not deviate from it. I supply enough drive in CW mode for 600 ma plate current and 10 ma screen current. One can then switch to SSB mode and operate at full power without any additional tuning. This is not bad once you get the hang of it.

On Christmas eve, I was testing the hanging TR relay problem, trying a few difference bands etc. I accidently rotated the MULTIMETER switch  from SCREEN to EXCITER TUNE with exciter drive still on. A very large explosion happened with flashes of light coming out of the rocker switch cracks. This kind of explosion usually indicates an RF deck/DC voltage issue.  Both 12 amp AC mains fuses were vaporized.

The manual has a notice buried in the ALL CAPS paragraph warning about trying to tune into bad loads. The "MULTIMETER" switch has a non-multimeter posistion that can be a killer. It should have been a stand-alone warning in large, bold type. Something like the following would have been handy:

In general, make it habit never to change any BAND switch position or transition in/out of "MULTIMETER" - EXCITER TUNE position with the PLATE voltage ON. If you do this and you have any exciter drive present, major league internal damage will occur.

Hoping the now rare RCA 8122 tubes were not toast and the bang bang was only associated with the primary side,  I put the amp on the bench. All of diodes and capacitors were ok in all power supply sections (this is goodness so far). The only explosion evidence was the carbon-burned PLATE RELAY contacts. The contact pads were burned bad enough that I needed to replace K3 for sure. The prior cap replacement job was so poorly done, I decided to rebuild everything on general principles to remove the fugly factor.

Repair Summary - First Pass

1. The explosion damage was fortunately limited to the PLATE RELAY (K3) and the HV secondary lead that must have been hanging by a thread and got blown off. I replaced K3 with IDEC RR3B-U-AC12 , leaving the plastic cover on, all caps, resisistors, and diodes as well as the broken secondary connection on the HV board. It looks nice and works now - no smoke escaping!

2. The TR relay hangs at random. I tried a heavy duty degauser for grins (heard rumors about this possible fix), no luck. I am pretty sure the contacts were burned down and sticking vs magnetism related. I burnished/sanded the contacts and replaced the weakened relay spring with the stronger spring from the fried PLATE RELAY with limited success for only a few days. I had to replace with an IDEC RR3B-U-AC12 not-quite-the-same replacement - it barely fits but is high quality.
3. I replaced the 8122 anode parsitic chokes with a Rich Measures ("voodo") choke set. Two 3W 100 ohm film resitors with 6 turn nichrome wire, 0.25in coils. This did not work well as Carl warned. In the end,  I changed to the National recommended design.
4. Grounded cathodes - I might move to 12-25 ohms next time I open it up to reduce distortion, should not be at ground potential, found an unsoldered pin 4 on one tube - a factory inspection flaw.
5. WARNING - the time delay tube does not time properly when hot.  The 8122's require 60 seconds of warm up. I've observed 40 seconds when I power off and back on quickly.
6. Lid interlock switch intermittent - the 2 prong lid plug inserts into very sloppy receptical on the chassis. Mybybe heat has caused this over time. One would not think this simple arrangement would fail but mine did. The symptom is the PLATE light would not stay on after depressing the PLATE voltage rocker switch. I bent the prongs out a bit and sprayed with DeOxit to solve the problem.
   
7. The aluminum knob inlays were tarnished. I tried some aluminum cleaner with less than stellar results. Any ideas would be greatly appreciated from the National restoration community.
8. I replaced the 8122 anode parsitic chokes with a Rich Measures ("voodo") choke set. Two 3W 100 ohm film resitors with 6 turn nichrome wire, 0.25in coils. This did not work well as Carl warned. In the end,  I changed to the National recommended design.
9. I replaced the 8122's with good looking eBay pulls for $55.00.
   I thought that RCA and Burle were the only 8122 manufacturers. I just purchased 4 NOS Eimac 8122s on the 40 meter swap net - real beauties. I also bought one  NOS Burle - dripping with tubes now! The two I installed are almost perfectly matched and I have seen short peaks on 1,500 on 75 meters but there is no reason to run them that hard. The difference between 1,300 and 1,500 watts on the receiving end is negligible.
   
   
   

                                             RCA 8122 Data Sheet

Repair Summary - Second Pass

1. I replaced the anode parasitic compressors with the ones designed by the National Service Manager (QTY 3 120ohm metal film 2 watt with 1in loop). I am now going to redo using old school, carbon 2 watts since these modern metal film resistors don't like 15 meters - no doubt minor inductance added.
   
   
   
   
   
   
2. I installed two nearly new, matched 8122's. The Eimacs seem to require  less drive.
   
3. I cleaned all band switch contacts and discovered wipers some are burned. I heard that OAK made these switches and have some on order from Surplus Sales so I can replace the burned wipers. Hobby Bench has some nice little brass nuts and bolts to replace the rivets. Thee tank coil and swiitch can be removed as one piece to make the tear down a little easier. On closer inspection, It looks like some wiper repairs have been made in the past.
   
   
   
   
   
   
   
   
   
   I ordered some old 2" ceramic OAK switches from Nebraska Sales for parts. It looks like someone has already done one wafer repair and did not use proper silver wipers or brass bolts - sloppy and not optimal for sure!
   
    

 

The End

Final, final version with olde school 2 watt carbon resistors in the anode chokes, now DONE !

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NCX-1000 Restoration
The NXC-1000 was the last transceiver National produced prior to going out of business. So far, its perfromance is pretty impressive. It it large - roughly the size of the NCL-2000 linear amplifier and almost as heavy. It is solid state except for the driver tube and the ceramic 8122 (same tube as the NCL-2000). In reality, it was a prototype that slipped into production status before it was ready. Unlike Collins designs, field maintenance was never seriously considered. The output is 500-600 watts PEP (depending on mains voltage) and has zero final tube protection outside of the human operator.

I am close to finishing the restoration and I will post lots of pictures when I complete a new transmit path bandpass filter. The original 3 slug coils fused together and the coils broke so there is no way to repair properly. The replacement filter will exceed the capabilities of the original.

More as it happens...