Old National NC-155

[kc4umo]

Last week end I went out to the shop and saw this sitting at my door with a note on it.
 "Thought you may want this for parts ect".

I did a YouTube video on this one.

https://www.youtube.com/watch?v=ORXsGghi_ss




Later Saturday evening I had a chance to put it on the bench and found no DC high voltage. I replaced the regulator tube (5Y3GT) and got the high voltage back. The original tube looks like it once had an internal arc going on near the top of the plates.  Also the voltage on the OB2 tube was very high.  I pulled it and could see the tube has went into meltdown also.

After that the receiver fired up and came to life and received pretty darn good. What I did not like was the 160 volt line is checking about 185 to 190. Still way to high. So going to replace all the resistors in the circuit. Got to get this voltage down.

I found the resistor on the plate of V10 (OB2) has high thermal stress. I checked it and it is at 480 ohms and has been changed to a 5 watt. The schematic says it should be 2.2K at 2 watt. Not sure why this was changed to the current value.



I know these are nothing great but you do not see that many of them. My plans are to do a full restore just to add to my collection. This old girl is starting to clean up nicely and the chassis is shining.

Figured since a lot of you guys here are very good at what you know if you could give me some pointers on trying to tame the high voltage.
I am just a sucker for old stuff and when it is free.....

[K9DXL]

If you've already done this, I apologize, but...
Have you checked the line voltage at your outlet?  At my house, I get about 122 volts at the plug, so I run my old radios from a variac.  They perk along just fine at 110 to 112 VAC.  I think most of these older radios were designed to run in the 115-117 volt range.

[kc4umo]

If you've already done this, I apologize, but...
Have you checked the line voltage at your outlet?  At my house, I get about 122 volts at the plug, so I run my old radios from a variac.  They perk along just fine at 110 to 112 VAC.  I think most of these older radios were designed to run in the 115-117 volt range.

Yes and a good point.
My line voltage runs about 122 volts. So I test all of this older gear on one of three variacs.
I did bring the voltage to about 112 and it is still 10% high.

I did find this a bit ago
http://www.ohio.edu/people/postr/bapix/NC155.htm

A hot transformer
 While the manual specifies a voltage rating of 105 to 125 volts, it also specifies a power draw of 75 watts. The actual power draw at 120 volts was 90 watts. The B+ voltage line is specified at 160 volts on the schematic. Even with my variac at 115 volts input, the B+ was actually about 10% high.

I decided on a circuit modification, something I rarely do. I added a 150 ohm resistor between the 5Y3 rectifier and the first electrolytic. That reduced the B+ to the specified 160 volts with 115 volts AC input. As expected, it also reduced the power draw by a couple of watts. The transformer still gets hot but a bit less so. I recommend using a bucking transformer or variac to reduce the AC input for all boatanchor radios but especially so for this one. I would not operate it for any length of time at a voltage higher than 115 and preferably somewhat less. Since its voltage rating notes 105 volts as the lower end, running it at that level will not affect performance. For more information on the transformer problem, see the notes under the  NC-270.

Thanks for your reply

[wb1ead]

Hi Buddy UMO..nice surprise waiting at ur door!..been looking for one of those just as a curiousity thing..that and the NC-270..there was an article I downloaded relating to the Heath VF-1..it was about swapping the OA2 with the OB2..they are directly interchangeable..but this was to gain better regulation seeing it was a VFO I suppose..not sure if that wud help ur problem or make it worse..whether it wud allow less B+ voltage or not I'm not qualified to say..however I wud change out that nasty resistor with the correct value and if a 5 watter were available then for sure give it a shot..seems like this wud have some effect on B+..BTW our AC here is currently at 121 volts..our only National at the moment is an NC-125 and that seems to be OK so far..I was wondering just how sensitive ur model is on 15/10mtrs?.....keep us posted...73 de DAVE

[kc4umo]

Hi Buddy UMO..nice surprise waiting at ur door!..been looking for one of those just as a curiousity thing..that and the NC-270..there was an article I downloaded relating to the Heath VF-1..it was about swapping the OA2 with the OB2..they are directly interchangeable..but this was to gain better regulation seeing it was a VFO I suppose..not sure if that wud help ur problem or make it worse..whether it wud allow less B+ voltage or not I'm not qualified to say..however I wud change out that nasty resistor with the correct value and if a 5 watter were available then for sure give it a shot..seems like this wud have some effect on B+..BTW our AC here is currently at 121 volts..our only National at the moment is an NC-125 and that seems to be OK so far..I was wondering just how sensitive ur model is on 15/10mtrs?.....keep us posted...73 de DAVE

Hi Dave and yes, was a nice little surprise. Thank you very much

For the time being I am just running it with the variac to keep the voltage down.

As far as the 0A2 and 0B2 tubes.
Guess you can think of it as this:
0A2 is basically a 150 volt zener diode.
0B2 is basically a 108 volt zener diode.
So the A tube may raise the voltage even more.
Think I have that right.

The other night I was listening to 15 meters on it and it seemed pretty good.  I was using a 40 meter dipole. Only attached the center lead to the rig. no ground. So was just a comparison check. 40 meters was blasting with some broadcast on AM. That sounded really good. 10 meters was somewhat dead here. Not even much on the Kenwood. However I did here some local mumbling on 29.0 AM but not enough to make out anything.  Funny I could not hear it on the Kenwood or Icom with proper antennas. Go figure.

Just as soon as I get a few other rigs out the door I am going to get back on it. May be Christmas holiday before I can.  I have a project TS-50 I am chomping to finish and a miserable TS-940 with PLL issues on the bench. Not even counting the back log of various HF rigs and V/UHF rigs along wit ha few scopes in for repair. Always something to do here.

But I will most likely do a video on the final repair/alignment and restore of this old rig. So please stay tuned

Thanks for your comments.