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
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