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

    The NC-125 restoration is such an interesting project, I've gone on ahead and started a thread. The NC-98 can wait -- after looking deeper into this NC-125 that I picked up yesterday, I'm anxious to dig in.

    Yesterday I did some preliminary evaluation and ran across a 12SG7 instead of a 6SG7 for V-4 (2nd IF amplifier). This radio is filled with 6 volt tubes -- that 12 volt had no place. The modifications are old -- they've been in place in the radio for what appears to be many years.

    So -- here's the puzzle -- this would seem to indicate that the 6 V winding on the transformer went bad and someone decided to replace it with a separate transformer (see the filament transformer notation in the photo). But the thing I can't figure out is why 12 volts would be sent over to V4 (see the green circle on the below photo), when that replacement transformer was already supplying 6 volts? Notice the joint circled in red -- that's 6.3 VAC, which appears to be supplying filament voltage to the rest of the radio. Has anyone ever heard of a NC-125 modification involving the use of a 12SG7 instead of a 6SG7 for V-4?

    Digging more into this later today. In the meantime I'd appreciate any feedback on what I'm seeing here.

    73 - Steve

  2. KM1H

    KM1H Ham Member QRZ Page

    Hmm, an interesting kluge up in the air.

    I cant tell if that transformer has 4 or 5 wires . If 5 then the secondary "may" be 12.6V AC with a center tap offering 6.3-0-6.3 which feeds all the tubes. Just a WAG before I go back out on the lawn tractor
    KW4H likes this.
  3. KW4H

    KW4H Platinum Subscriber Platinum Subscriber QRZ Page

    Yes, it’s 12v with a 6v center tap. The use of the 12v is mysterious.
  4. KM1H

    KM1H Ham Member QRZ Page

    To the 12SG7, since the owner didnt have a 6SG7 and the transformer was on hand maybe? Trace out the wiring.
    Of course a 6SK7 would also work with a bit less performance at 10 MHz and increasing to 30 MHz.

    Back to yard work.
    KW4H likes this.
  5. KW4H

    KW4H Platinum Subscriber Platinum Subscriber QRZ Page

    Well, well ... here's what stepping away for a while and going to church will do: a fresh outlook. As it turns out, every voltage on the secondary of the main power transformer is operating. The radio is getting 6 volts from the correct source. That 12v transformer adds a little jumble of wires but it does, in fact, have the center tap unconnected. Somebody in the past went to a ridiculous amount of trouble to wire up a 12v filament voltage for that 12SG7. The 12v transformer was also wired a little bit strangely -- one lead from the secondary goes directly to pin 7 of V-4, the other should either go to pin 2 of V-4 (or to ground), but instead is connected on the opposite side of the heater voltage line, which goes to pin 7 of all the tubes -- which is why I initially thought it was supplying 6 vac as well as 12 vac.

    It's all coming out and I'll put a 6SG7 in its place. Time for a walk. :rolleyes:
    KM1H and N2EY like this.
  6. KM1H

    KM1H Ham Member QRZ Page

    Sounds like progress, I did my walk at 8AM.
    The combination of the 6SG7 RF amp and the 6SB7Y is a big improvement over the usual 6SK7/6SA7 combination, those are a PnP swap. The 6SG7 in the first IF adds a bit more overall sensitivity and I notice its cathode resistor is also factory selected.

    I have just opened a box with a nice clean Hallicrafters SX-23 inside, owner wants it to play like new since his grandfather had one. Not among their best performing radios but it sure looks nice.
  7. KW4H

    KW4H Platinum Subscriber Platinum Subscriber QRZ Page

    A little update. The 12v transformer is now removed, as well as the 12SG7 (replaced with a 6SG7). The heater wiring to V-4 is restored to the original design. The NC-125 powers up fine but the performance is horrifying -- some reception but mostly hiss with a little hum. Looked over the front end and it's been tinkered with and there are some dangling connections. If anyone has a NC-125 and would be willing to post high resolution photos of the area around the trimmer capacitor and L1 it would be greatly appreciated. Some of the wiring and connections have been moved around and reconciling it with only the schematic is pretty intense. I've found some photos from online searches but the either the resolution isn't good enough or the angle is off. I need to see exactly what's connected to the trimmer cap. Thanks!
    Last edited: Sep 27, 2021
  8. KW4H

    KW4H Platinum Subscriber Platinum Subscriber QRZ Page

    The RF gain control is going to need to be replaced. National lists it as a wire-wound 10k ohm pot, part number K349-1. Anyone know what sort of taper National used in the NC-125 for that control? Linear? Audio? Logarithmic?

    73 -Steve
  9. W9GB

    W9GB Ham Member QRZ Page

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  10. KW4H

    KW4H Platinum Subscriber Platinum Subscriber QRZ Page

    Thanks -- I did an intensive web search and found those photos and others, but they're "just this much" out of resolution in the area I'm trying to work in. In the NC-125 on the bench, someone disconnected R-55 and had connected a ceramic disc and another trimmer, neither called for on the schematic, and all three of them were flapping in the breeze. I removed the unnecessary ceramic trimmer and the capacitor, and R-22 is connected back to the post on the trim air cap where it belongs. However, they also soldered around on the wiring between the rotary switch wafer (S-1A) and the trimmer. I'm trying to get the connection between the grid of that first IF amplifier (V-1) and the band switching back to what it should be but can't quite find the right angle and resolution in the available photos to see those points. There's also a mica down in that area that has the appearance of having been miswired. I'm digging in the schematic, but a picture can be worth a thousand words in this situation.

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