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National NCX-3 Frequency instability on transmit

Discussion in 'Radio Circuits, Repair & Performance' started by WA4ZYN, Jan 11, 2013.

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  1. WA4ZYN

    WA4ZYN Ham Member QRZ Page

    Hello. I have recently acquired an old National NCX-3 tri-band transceiver and I am trying to get it air-worthy. Receive is good. Unfortunately, listening to it on another (reliable) receiver, I can hear it drift up and down-frequency on transmit on CW. I'm pretty sure that it would do the same on SSB. I am hoping someone out there can help me get up to speed on diagnosing and fixing this problem. I have generally had good luck utilizing my skills and Google in repairing other rigs. This rig doesn't seem to have a lot out there on the internet for me to go on. Any suggestions where to start?

    Thanks in advance for any and all suggestions.
    Francis WA4ZYN
  2. K9STH

    K9STH Ham Member QRZ Page

    Unfortunately, the NCX-3 was never the most stable unit around. First of all, are you giving the unit a warm up time of at least 30-minutes? It takes at least that long for the unit to stabilize. Another thing that can contribute to drift is that the machine screws and nuts that go through the chassis have come loose and/or corroded over the years (remember, the NCX-3 is 50-years old) and need to be tightened.

    A "fix" that works in various Heath equipment is to replace the 6AU6 VFO tube with a 6AH6. In the NCX-3 the VFO tube is a 12AU6. Unfortunately, they did not make a 12AH6 (at least that I can find in any receiving tube manual). However, you can replace the 12AU6 VFO tube in the NCX-3 with a 6AH6 by adding a 15-ohm, 5-watt, resistor in series with the heater ("filament"). This should help the stability considerably.

    Glen, K9STH
  3. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I had an NCX-3 back in about 1966, and of course it was years old then. As Glen said, it's a long warm-up cycle and not terribly stable, right out of the carton when it was new. The saying back then was, if you wanted stability, the label on the panel had to read Collins or Drake, or then a bit later, Heath SB-series (which were also very good).

    My solution back then was to never turn the rig off. I just left it on 24/7.:eek:
  4. W9GB

    W9GB Ham Member QRZ Page

    Francis -

    Glenn and Steve at touched upon the high points of the National NCX-3.
    Carl, W1QJ may jump in -- he worked in National's service department.

    The Cumbria Design's X-Lock Frequency Stabilizer could be one solution.
    I have not seen anyone perform this on the National NCX-3.

    Dale, W4OP did install X-Lock on the Hallicrafters FPM-300

    W4OP radio restorations
    Last edited: Jan 11, 2013
  5. WA4ZYN

    WA4ZYN Ham Member QRZ Page

    Thanks! I'll look into that 6AH6 tube. That would be an easy thing to try. Why is the 6 volt tube more stable than the 12 volt version?

    Thanks again.
  6. WA4ZYN

    WA4ZYN Ham Member QRZ Page

    Well, that is a do-able solution. I'll go turn it on right now. . .

  7. WA4ZYN

    WA4ZYN Ham Member QRZ Page

    I hope Carl does jump in. I'd like to meet him. I got the NCX-3 because it was identical to my very first rig back in 1973. . .felt a little nostalgic. I hope I can fix a couple of its problems. I had one QSO with it the weekend I got it (after bringing it up on variac, and checking it out on a dummy load before connecting an antenna), and got a good signal report, but was told it had a chirp. I found a service bulletin dealing with that problem, and have made the suggested mod. Seems to have cleaned the signal up. Sunday I made a QSO with a local friend, and he said it was drifting a lot. So, on to the next set of problems. I have ordered replacement electrolytic caps for power supply and radio, and have replaced the voltage regulator tube. I figure it's some voltage fluctuations on full power. It also doesn't have full power on 20 meters and 80 meters. That will probably require touching up driver and mixer coils.

    Thanks again. I'll read up on the Cumbria X-lock. I built their morse code reader and had a blast doing it.

  8. K9STH

    K9STH Ham Member QRZ Page


    It is not the 6-volt heater that makes the difference. The 6AH6 was proven to be more stable when used in VFO circuits than the 6AU6. There are slight differences in the inter-electrode capacitance and in the general "make up" of the tubes. However, when used in the same circuits, the 6AH6 produces a considerably more stable signal than the 6AU6. The 12AU6 is the same tube as the 6AU6 except for the heater voltage. If there were a 12AH6 tube available it would be a simple thing just to unplug the 12AU6 and plugin a 12AH6. Unfortunately, at least as listed in several tube manuals (from different manufacturers) it seems that there are no 12AH6 tubes. Therefore, to use the 6AH6 in the NCX-3, one has to add a resistor in series with the 6.3 volt heater to allow the 6AH6 to replace the 12AU6.

    You still have to allow the unit to "warm up". But, when "warmed up", the VFO should be more stable with the 6AH6 installed.

    Fortunately, the 6AH6 replaces the 6AU6 with no changes in the circuitry. The only thing that may be necessary is to slightly adjust the trimmer capacitors (not the inductors) to calibrate the VFO. This is necessary because of the slight difference in the inter-electrode capacitance of the two different types of tubes.

    Glen, K9STH
  9. WA4ZYN

    WA4ZYN Ham Member QRZ Page

    Thanks. That's good to know. I'll look for those trimmer capacitors so that I'll know what to adjust. I have ordered that tube and that resistor for cheap (NOS) on ebay. Hopefully those will arrive this next week. I'm looking forward to getting to work on this rig.

    Thanks again.
  10. K9STH

    K9STH Ham Member QRZ Page

    The resistor only has to dissipate about 3-watts. However, using a 5-watt resistor gives a safety factor.

    Glen, K9STH
  11. KM1H

    KM1H Ham Member QRZ Page

    Youre getting senile George[​IMG]

    As mentioned the NCX-3, as well as most others including early Drakes, all drifted in the days of a tube based free running LC VFO. OTOH I ran one mobile for a few years and after a warmup it was mighty stable as were many others I worked on at National 1963-69. VR tubes have a finite life and if its not stable you will have a warble.

    The 6AH6 mod may work, never tried it but also consider a new 12AU6A, look for Sylvania or Telefunken.

    You say the VFO is wandering around....does it ever settle to a drift in one direction only? If so a small TC cap may help, Ive done this to a lot of older tube gear including Hammarlunds and Hallidrifters.

    The carrier oscillator could also be acting up, be sure the voltage is stable. Try monitoring it on another receiver. The trimmer cap wasnt the best quality.

    Sometimes a tiny drop of DeOxit in the socket pins will help, install the tube several times to break the oxide barrier.

  12. WA4ZYN

    WA4ZYN Ham Member QRZ Page

    Got the resistor a few days ago, and the 6AH6 arrived in the mail today. Will let you know when I get it all replaced. Probably won't be this week, tho.

    Thanks again to you and everyone for all the tips.
  13. WA4ZYN

    WA4ZYN Ham Member QRZ Page

    Hello again. Well, I'm making progress with the ol' NCX-3. I got power supply electrolytic caps replaced, and got rid of the chirpy CW signal.

    I got the new 6AH6 tube in (with the appropriate resistor in series with the filament) as you recommended. So far, so good. Unfortunately, I forgot to refer back to your note before re-aligning the VFO. I started with the instruction manual instructions for aligning the VFO, and doggone it, I moved the slugs in the inductors, trying to zero-beat. Now, nothing lines up, as you probably expected. I guess I need some good old-fashioned wisdom now, as I can't line up one end of each band and stay lined up at the other. . .therefore, the closest I can get it is to 15 kc high on the high end of 80 meters (and of course 20) and 10 kc low on the low end. And with 40 meters, I can't get even close to the upper end (it's somewhere higher than that), and 20 kc lower than 7.0.

    I admit it: I did a stupid thing, and now I'm clueless where to go from here. If you have any wisdom, please, I'd really appreciate it.

    Thanks for any and every bit of advice.

  14. KB4QAA

    KB4QAA XML Subscriber QRZ Page

    I too have gotten 'lost' before during an alignment. The helpful advice I got was to use the oscope to find the right signals. Working by ear or other earlier alternate instruments just wasn't sensitive enough.

    Taking a quick look at the manual, and without knowing specifics, perhaps you need to drop back a stage and check the IF alignment, since I didn't see any inductors in the VFO step. Some general points:

    -Be sure you are following the setup to a "T", including any dropping resistors, etc.
    -Check for control settings that may be specified in the prior stage or step that need to carry over.
    -Keep the alignment signal on the low side, don't overload the circuit or scope, sensitivity will be better
    -Using a GDO is quaint and good for getting in the ball park, but the oscope is best for precision
    -Reminders to myself: Be calm, focus, methodical, patience. Don't work when tired, sleepy or distracted.

    The NCX-3 manual is detailed and frankly a bit tedious with many repeated steps. The controls interact, so it may take some time to find the right 'peaking signal' and drag it back to the path. Good luck! bill
  15. WA4ZYN

    WA4ZYN Ham Member QRZ Page

    Also, here is what I've been using since I don't have an oscilloscope, signal generator, or Grid Dip Oscillator. I hope I am not embarrassing myself, but I've been using my TS-430S to a dummy load at 1 or 2 watts out as a signal source, with a short wire to input coax SO239 on NCX-3 as an antenna. I set the 430S to 4.0 and 3.5mHz as the alignment procedure calls for. I have tried zero-beating using inductor adjustments at each end of the band; zero-beating using trimmer cap at each end of the band; adjusting inductor at one end and cap at other. . .you name it, I have tried it.

    I had started this whole problem out by trying to stabilize the VFO. I followed the suggestion of another ham, and changed the 12AU6 tube with a 6AH6, which is supposedly a more "stable" tube(?) Maybe I shouldn't have messed with it, but the VFO was moving all over the place during QSO's. I have replaced the filter caps in the power supply, have plans to replace the ones in the xcvr itself, and have cured the "chirp" on CW with the addition of RF choke and 0.01mF cap around the voltage reg tube (OA2), as per Service Bulletins from the 1960's.

    So, I hadn't planned on making any IF adjustments. I doubt if I have proper instruments for that. It seemed OK before I changed the tube, just the instability problem. Have I gone too far? Should I have left well enough alone?

    Thanks for your time, interest, and any and all suggestions.

  16. WA4ZYN

    WA4ZYN Ham Member QRZ Page

    I just wanted everyone to know that I have completed re-calibrating the VFO in my National NCX-3. Unfortunately, what I had to do was remove the new 6AU6 tube and re-install the old 12AH6. Just never could calibrate the VFO with that new tube, but it calibrated within 10 minutes with the old 12AH6 installed again. The VFO does seem to be more stable tho, what with all the capacitor replacements I've put in.

    Thanks again to everyone for your helpful suggestions. It's so good to know I have others to help get me out of binds.

    Francis WA4ZYN
  17. WA4ZYN

    WA4ZYN Ham Member QRZ Page

    I want to thank everyone who helped me with my National NCX-3 over the past several weeks. I spoke with several hams tonight on 40 meters SSB and got great signal reports. I am very pleased with the results of the repairs.

    Francis WA4ZYN
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