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Author Topic: National NC-140 Restoration  (Read 4346 times)
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KJ6RSG
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« on: December 23, 2020, 08:41:41 PM »

Ive got a beautiful National NC-140 receiver. It powers up and picks up some signals, but I don't have a decent antenna at the moment and I think the radio itself is out of calibration. I'm going to go through the calibration procedure, but I want to make sure all the parts are still operating in spec first.

I took a look at the specs and its got 3x 40mfd/250V electrolytic capacitors in the power supply circuit. I believe they are paper capacitors and I would like to replace them to be on the safe side. The problem is that all three are in a single package and I'm not sure how to find a replacement for that. Any suggestions?

All the other capacitors are silver mica.  Am I safe to assume they should be good to go? Or should I check all of them? Should I test all the resistors as well?

The main tuning dial has trouble turning at certain positions. Is there a recommended way to clean and lubricate the tuning capacitors?

Is there anything else I should look into when servicing this guy?

Once serviced, I plan on setting up a long wire antenna for SWL. Are there any other more sophisticated antenna designs you would recommend trying out for general SWL? The receiver has screw points for the ground and antenna. Could I use it with a coax cable? If so will I need to make some sort of balun to go between the radio and coax?
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WD4DMZ
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« Reply #1 on: December 23, 2020, 11:14:45 PM »

When replacing the electrolytics on these old receivers, if they are in a can on top of the chassis, I leave the original can in place for a correct appearance and remove the wires to them. Then put individual caps underneath the chassis. Since the new caps are so much smaller there is usually plenty of room for them.

Often it is just the dial cord slipping. I carefully remove the spring end of the cord from the variable cap and shorten the cord just a bit. Also, will put a small drop of light oil on the bearings of the cap and on any of the small pulley wheels the cord rides on. A hemostat comes in handy and I can usually restring a cord without and major disassembly.

Over the years I have done this on dozens of radios with good results.

For an antenna attachment I use a short piece of rg58 terminated with spade lugs on on end and a pl259 on the other. I literally have 10 receivers tied in this way at the moment. Some older radios do not like the 50 ohm coax but most do.

Rich
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KJ6RSG
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« Reply #2 on: December 24, 2020, 06:24:09 PM »

Thanks Rich, thats a great idea for replacing the caps. The receiver has 40mfd 250v caps. Do I really need that high of a voltage rating? I'm having trouble finding them online.

The manual says it is designed to match a 50ohm unbalanced transmission line. I guess that means I am good to go with coax?
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W4AMV
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« Reply #3 on: December 24, 2020, 07:24:34 PM »

Go to Just Radios at:

https://www.justradios.com/cart.html

or try antique supply as well others. Go with the required WVDC, 250V. He has radial and axial. Choose which fits best for your chassis layout.

At Just radio the caps are under 2 bucks each. However, there is a min ship charge of 25$. However, I would look over your schematic, sight all the paper caps like 0.1 and .01 uF at 400 or 600 V and get some of those and combine with your order.

I am restoring an old National receiver too and finding at least a few aged paper caps after 70+ years that are very leaky.  

Have a happy holiday, Alan
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Jim, W5JO
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« Reply #4 on: December 24, 2020, 08:19:45 PM »

If you choose to replace what is there then Hayseed Hamfest will either have it or make it for you.  Saves a bit of space.

https://hayseedhamfest.com/
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KJ6RSG
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« Reply #5 on: December 28, 2020, 08:34:42 PM »

I'm going through and checking the resistors right now. The originals are all rated at +/- 10% and they are pretty much all at 10% off spec with some more then 10% off.

Would it be worth it to install new set of resistors?
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W4AMV
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« Reply #6 on: December 28, 2020, 09:56:45 PM »

Simple answer, NO.

If you do not enjoy learning how your radio works and troubleshooting the receiver, then go ahead and rip out all parts that seem suspicious, like R's that are more than 10 % out of spec. Same goes for all those C's. That must be bad, after all there all old.

I do not subscribe to that approach, unless, it is a known failure mechanism that can take out other unobtanium components.  A number of receivers fall into that class, so called killer parts.

If you do not have a reasonable restoration kit of parts, R's and C's as I mentioned earlier, than I think it would be a wise investment and purchase a kit.

I am working on a 70+ old receiver now and it is nearly 90 % up and running. I had too replace one resistor and two paper caps. Those replaced parts were the cause of two receiver failures, no audio and no BFO.

Just my opinion.
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KJ6RSG
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« Reply #7 on: December 29, 2020, 01:45:07 AM »

Thanks for the advice. I went ahead and ordered all the resistors for this radio because I needed to hit the minimum order size, but i'll probably just replace the out of spec resistors for now.
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W3SLK
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« Reply #8 on: December 29, 2020, 09:32:35 AM »

W4AMV said:
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Re: National NC-140 Restoration
Reply #6 on: Yesterday at 09:56:45 PM
   Reply with quoteQuote
Simple answer, NO.

If you do not enjoy learning how your radio works and troubleshooting the receiver, then go ahead and rip out all parts that seem suspicious, like R's that are more than 10 % out of spec. Same goes for all those C's. That must be bad, after all there all old.

I do not subscribe to that approach, unless, it is a known failure mechanism that can take out other unobtanium components.  A number of receivers fall into that class, so called killer parts.

If you do not have a reasonable restoration kit of parts, R's and C's as I mentioned earlier, than I think it would be a wise investment and purchase a kit.

I am working on a 70+ old receiver now and it is nearly 90 % up and running. I had too replace one resistor and two paper caps. Those replaced parts were the cause of two receiver failures, no audio and no BFO.
Sage advice. It also stands to reason that you may either induce another problem by making a mistake, (I've been there and done that!), or you could be following a mistake made by the previous owner(s). I bought a HRO-50T that had been in the middle of somebody trying to convert it to a -60! I poured over the schematics and repaired what had to be repaired and got it to singing in a stock mode. By doing so, I learned quite a bit about the radio and how it functions.
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Mike(y)/W3SLK
Invisible airwaves crackle with life, bright antenna bristle with the energy. Emotional feedback, on timeless wavelength, bearing a gift beyond lights, almost free.... Spirit of Radio/Rush
WD4DMZ
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« Reply #9 on: January 02, 2021, 02:37:38 PM »

Just this morning I was using a Heathkit HR10 with the Globe Scout and the dial was slipping so badly it ended up pegged in the corner. The cover is easy to remove but restringing is a task. So, I put a very small amount of sewing machine on the bearings of the tuning cap and also applied a very light film on the guide the dial indicator slides on.

Voila! Done in 5 minutes.

If you have not yet dealt with the National dial slippage just try this easy fix first.

By the way, that cosmic blue National is one of the last radios on my wish list. Glad to see you are using one.

Rich
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Jim, W5JO
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« Reply #10 on: January 02, 2021, 03:39:55 PM »

Dial cord slippage can be helped by applying a but of liquid rosin on the cord.
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