National NC-125 Restoration

Discussion in '"Boat Anchor" & Classic Equipment' started by KW4H, Sep 26, 2021.

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  1. KW4H

    KW4H Platinum Subscriber Platinum Subscriber QRZ Page

    OK -- an update. Injecting audio all the way backwards from the speaker connection to the the output of the 2nd detector (pin 4) is clean and works fine. That eliminates the audio chain entirely. Injecting a 455 KHz signal ANYWHERE along the IF chain results in nothing, and the RF gain control, when turned up, is silent. Not a hiss or peep. Voltages at V-1 test good, and exchanging the RF amplifier tube (V-1) for a known good one results in no change.

    Correct me if I'm wrong, but I believe these tests indicate that there's a problem in the neighborhood of V-1. It seems almost impossible to me that the converter and both IF amplifier circuits would fail in unison, and V-1 and its associated gain control are in common with all those stages. Also, the area around V-1 was the most "diddled with" in the radio. Time for another break until later today, and then V-1 and its associated circuits are going to have to be dissected and compared to the schematic. :eek:
  2. N2EY

    N2EY XML Subscriber QRZ Page

    That's good!

    Well, that's not good, but it's something.

    You're wrong.

    If V1 were all messed up, the IF stages would still work and you'd hear the 455 kc. signal when injected into the IF chain. But you don't, so the problem must be somewhere just ahead of the second detector.

    The first test I would make would be to remove the last IF tube and inject a modulated 455 kc signal into the plate pin by means of a blocking capacitor. This would verify that the 6H6 second detector is actually detecting. If that test were passed, I'd check that the BFO is working, then move forward in the receiver stage by stage.

    Is it possible the RF gain control is open? Or not wired per plan? That control is in the cathode circuits of the RF amplifier and both IF amplifiers, and if it opened up, those stages would have zero gain!

    73 de Jim, N2EY
  3. KW4H

    KW4H Platinum Subscriber Platinum Subscriber QRZ Page

    Thanks very much for taking the time to help! I pulled the last IF tube (V-4) and with a 455 KHz modulated signal injected into the plate, nothing is heard -- not even a scratch. The 6H6 is known good -- I swapped it out for good measure. Checked the RF gain control and there's continuity per the schematic to the 1st and 2nd IF amplifiers. Going to have a closer look around the base of the 6H6 -- I know for a fact that if I inject audio at pin 4, all the stages past the 2nd detector work fine.
  4. N2EY

    N2EY XML Subscriber QRZ Page

    Perhaps the IF transformer between the last IF stage and the 6H6 is bad.

    What happens if you inject a 455 kc. modulated signal into the 6H6 pin 4?

    73 de Jim N2EY
  5. KW4H

    KW4H Platinum Subscriber Platinum Subscriber QRZ Page

    Yay! Found it! There's a 4.3 ohm resistor that feeds the filament for the 6H6. It was connected to the wrong pin at the source, and the 6H6 had no filament voltage. Now -- how the heck did I miss that??!!? Answer: I got bogged down in the exotic stuff and neglected the obvious. Now to complete the recap and do a re-alignment.

    Jim, thank you VERY much for the help that guided me to this. If you hadn't nudged me towards that detector this would have probably still been a puzzle.

    Last edited: Oct 16, 2021
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  6. KW4H

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    I need to repair a defective coil (L-1) -- and can't figure out how to get it off the chassis. I've snipped the wires -- but how do you remove it from the clip holding it to the chassis?

    Thanks - Steve, KW4H
  7. KP4SX

    KP4SX Premium Subscriber QRZ Page

    Look on the other side of the chassis. You'll see that the metal mount has little tabs holding it in place.
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  8. KW4H

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    Got it -- it's out. Thanks.

    OK, time for a head check. So -- something is wrong with this coil (L-1) -- it won't peak. Signals ARE passing through, though. When injecting a 12 MHz signal for alignment, L-8 adjusts fine to put the signal on the correct mark on the dial, and L-20 responds and is easily peaked by adjusting the wire loop inside the coil form. L-1, however, does nothing -- no reaction. Which, I believe, may indicate a short.

    I'm showing continuity across the primary and secondary of this little transformer -- however, due to the small size of the windings the reading is pretty much a dead short -- the typical method of looking for continuity AND some sort of resistance isn't really working. The only thing I can tell for certain is that the coils aren't shorted to each other. And I believe another safe assumption is that -- since signals are getting through, that means the primary winding is intact and inducting into the secondary, but a short somewhere in the secondary winding has rendered L-1 un-peakable.

    Where do I go from here? Do I just rebuild the winding and hope for the best, or are there some additional tests?

    73 - Steve, KW4H

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    Last edited: Oct 19, 2021
  9. K1APJ

    K1APJ Premium Subscriber QRZ Page

    I'd put a known capacitor across it and look for the dip with a grid dip meter. Looks like 270 pF or so across the secondary should put it near the center of band A.

    I'd also be absolutely certain the bandswitch is correctly wired and is properly working. Unless it got hit by lightning, I would not expect too much trouble with coils like that.
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  10. KW4H

    KW4H Platinum Subscriber Platinum Subscriber QRZ Page

    Thanks for replying. Well -- I did a closer inspection of the coils. The protective coating around the larger one was flaking off in some spots, and there were some small areas of bare metal. I'm about 90 percent certain it was shorted. Measured the wire in a micrometer and it's 22 gauge magnet wire (.025 in). It's nothing but four turns -- decided to rebuild it, but I don't have the 22 gauge magnet wire in the shack so will have to pick some up. The smaller coil on that form is about 5 turns of .014", which I do have in stock. Figured if I rebuild the transformer it might as well be the whole thing. At this point, rebuilding it is easier than thinking any more about it. :D
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