Restoration Resources


Manual and Schematics

The "Boat Anchor Manual Archive" (BAMA) is a great resource for hobbyists looking for manuals and schematics for old Ham gear. I downloaded the info I needed for my NC-125 from them.

You can jump straight to the NC 125 info and download it for free at NC-125 or you can see the whole list of what's available there at their Home Page


Additional Pictures

Here's a few closeups of above and below the chassis before restoration. Click on any image for a larger view.

National NC 125 Receiver National NC 125 Receiver

NC-125 Ads

An old ad for my radio when it was new. Click on an image for a larger view.

NC 125 Ad

Old Radio Restorations

Don't miss my other radio restorations - I have several listed on my "Projects" page. More…


National Radio Model NC-125

National NC-125 Receiver National NC-125 Receiver National NC-125 Receiver

It was bound to happen eventually...

Just ask Mike, WB9DLC. He is frequently my "hamfest buddy" at area swap meets. He'll tell you - N9DD can not pass by an old National receiver without stopping and admiring it for a while. There is something about the "machine age" styling of the old SW-54, NC-98, and NC-125 receivers that I can't resist.

When I got an EBay gift card for Christmas, I started scouring listings for a nice radio to restore. My success with the Zenith Trans Oceanic that I had recently finished had rekindled my interest in restoring radios.

I found a nice looking NC-125, but rather than an auction, the listing was a "Buy it Now" or "Best Offer" kind of deal. I made an offer, received a counter offer and accepted it. Since the seller was only an hour and a half drive away, I saved some money on shipping by going to pick it up myself. When you buy something as big and heavy as this old communications receiver, having the option to pick it up is something to look for. Shipping for this radio would have been $50. Who knows what condition it would have arrived in after the package delivery company had their way with it!

Preliminaries

When I got my new purchase home, I put it on the bench, opened the top lid and did some nosing around. Although the face of this radio is fairly clean, the rest of the radio was rather dirty, the copper chassis had a lot of corrosion, and there were quite a few small rust spots and imperfections on the rest of the cabinet. Oh well. It was sold as-is and was for a good price. I'll get some experience with metal cabinet repair and painting with this one. I found a speaker and a hunk of wire for a makeshift antenna and set to work.

I hooked the unit up to my variac and powered it up slowly, listening and looking for trouble. Nothing bad happened, so I twiddled the knobs with my ear to the speaker to see how she was working. Hmm.... Nothing but hiss and crackle from the speaker. I couldn't tune in any stations. It was going to be more of a project than I had hoped for, but I was looking forward to a good learning experience.

Flipping the radio over and removing the bottom cover, I discovered a whole slew of "bumble bee" capacitors that would need replacing. Boy, there's a bunch of them in this radio! They're called that because the dark colored capacitor body has stripes that make it look a bit like a bumble bee. I got out my trusty Fluke meter and checked a few resistors. Every one I checked had drifted high by at least 50%. Good thing I have a good supply of them.

Time to dig in!

I started by spraying some DeoxIT contact cleaner into the controls and onto all the switch contacts. Then I set to work replacing the old caps and out-of-tolerance resistors. One thing I had problems with while replacing components -- The solder lugs on the bottom of the IF cans will not take much physical stress. Be very careful when unsoldering leads from them and don't yank on them. I broke off a couple and had to resort to using solder lugs that I removed from terminal strips. This worked OK, but if I had been more gentle, the need never would have arisen. Be careful! As I replaced components, I would occasionally turn the radio back on to see if my work had helped restore this old baby to life. Nope. So I kept on going. After I had replaced about 75% of the components needing replacement I decided that there had to be a bad tube or open connection somewhere. Nearly all the receivers I've restored have been able to tune in something right from the start. Recapping has always just made things better as I went along. This radio was beginning to get on my nerves.

So I hauled out my old Heath TC-3 tube checker and started testing the tubes, starting with V-1. They were checking good until I got to the 6SG7 from the 2nd IF amplifier. It was deader than a doornail. "Alright! I've found the problem," I thought. Nope. After replacing the tube with a good one I had on hand, the receiver sounded the same. Grrrrr....

Next up was the 6H6 twin diode that serves as a 2nd detector, ANL, and AVC tube. Bingo! Another bad tube. I replaced the tube with a good one, fired up the receiver again and... it was happy dance time! "Yes!" I shouted with a fist pump as I heard the sound of our local AM station, WSBT, sounding sweeter than ever. Well, sort of. It was obvious that the radio still had some issues, but I was sure that I'd get them all resolved in time. There were more caps and resistors to replace and an alignment to do.

A bit of trouble shooting

After recapping the set, I was left with several problems. First, when I tapped on the cabinet, or actually, anywhere on the radio, I heard a loud, scratchy static. I kept tapping and listening and managed to pinpoint the problem to the area of the tuning coils in the front end. The problem was staring me right in the face - an unsoldered connection on L-7. It looked as if it had been that way ever since the radio was built. I resoldered the connection and problem #1 vanished. Woo Hoo!

Next up, there was a "lightning crash" sound in the receiver that varied in intensity with different combinations of RF and audio levels. If I rotated the "Select-O-Ject" "Freq" control to the left, the sound got worse. I surmised that the problem had to be coming from the area of V-6 and V-7 that provide the "Select-O-Ject" notch and boost functions.

Indeed, the "Select-O-Ject" section did not seem to be working right at all. The voltages on the tube plates were way low. I had tried to make the adjustment to this section while doing an alignment without success. You are supposed to send a 1000 kHz signal to the set with a 400 Hz audio modulation. Using the rear panel "Reject" control along with the front panel "Freq" control, you are supposed to null out the 400 Hz audio. I couldn't do it. Rotating the "Reject" control one way eliminated all audio. I had to rotate it fully the other way to make the radio sound right.

In the area of V-6 and V-7 are two 1000pF mica capacitors. Micas rarely go bad, so I didn't replace them while doing my recap. But while probing around with my oscilloscope, I noticed that the lightning-like static was present on one end of C-50, but not the other. "Could it be bad?" I thought. Sure enough, replacing it not only eliminated the static, but brought the voltages for V-6 back to reasonable values. Yes!

The voltages around V-7 were now better, but still not high enough. The "Select-O-Ject" still didn't seem to be operating right. The other 1000pF mica cap, C-52 comes off of pin 5 of V-6. "Well, the other one was bad" I thought, "why not replace this one too?" I can be brilliant at times. :-) I replaced the second 1000pF mica and success! The voltages all came up to reasonable values and adjusting the "Freq" control seemed to be doing something good. I got out the alignment notes and retried the "Select-O-Ject" adjustment. It worked just as advertised.

The only issue left was what sounded like a bit of an oscillating IF. I heard it only between stations, so it was not a real biggie. When I attached the ground lead from my oscilloscope to the cabinet of the radio, the oscillation was greatly reduced. That seemed odd. "Perhaps a 3-wire power cord would help," I thought. Replacing the old 2-wire ac cord with a 3-wire cord, connecting the green wire to chassis ground, completely eliminated the oscillation! I'm not sure I understand why, but I'm happy as can be with my new receiver! No reason to argue with success.

I found a bottle of "Flitz" polish and used some to clean and shine up the cabinet. I soaked the knobs in warm water with a drop of "Dawn" dish soap, scrubbed them up a bit and applied some polish to them too. Wow, this old baby is looking nice! With top cover back in place, I let the radio sit on my workbench for a while. Everytime I walk by I have to stop and admire it a while. Hmm... Some things never change :-)

My NC-125 Tale of Woe

Shortly after moving my restored NC-125 to my ham radio operating bench, I noticed a DX spot for a station in a country that I needed. I reached over to the NC-125 and flipped the standby/receive switch to "standby" to silence it so I could listen on my ham transceiver and make the contact. After a very short while, the NC-125 suddenly came back to life as if I had flipped the switch back to receive. I flipped the switch a couple of times and it didn't seem to be working right.

I unhooked the speaker and power and took the radio back to my repair bench. After hooking it back up to a speaker and power, things quickly got worse and the radio went completely dead. With my ohmmeter I determined that the secondary of the power transformer was probably shorted and the primary was open. I turned to the Internet Antique Radio Forum for some advice. You can see the thread on that here.

With replacement power tranformers costing at least $80, I set my receiver aside, hoping to find a suitable used one or a donor "basket case" unit to get one from. Instead, I found another NC-125 in nice shape and for a good price, so my first NC-125 is still on the shelf, awaiting a fix. If you happen to have a spare transformer or an NC-125 in poor cosmetic shape, not worth restoring, I'd love to hear from you. My newer NC-125 is very nice, but my first one will always be special. :-) I'd love to see it back in operation!


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