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 Post subject: National NC-200 hair puller
PostPosted: Jul Sun 16, 2017 3:17 pm 
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Location: Prescott AZ
I've been working on a National NC-200 reciever from about 1940 (silver anniv. model) that has had some previous work done to it.
So far I recapped the set, replaced way out of spec resistors, checked tubes, cleaned catacomb coil pins and contacts.

The audio section of the set works fine and I can get a nice signal through all the way back to the grid of the 6C8G second detector.

However, I cannot get an IF signal through the set even though both IF transformers have continuity of about 10 ohms.
I have tried putting an IF signal into both the grid and plate of the 6K7 1st IF tube, the 6SK7 second IF tube and the 6K8 grid (as specified in the manual). The interesting thing is that if I use all the power of my signal generator I CAN get a tone through the speaker when I put the signal into the grid cap of the 6K8 tube BUT the volume control (AF volume) has no impact on the loudness and it is not very loud.

This has been driving me crazy.

Any suggestions?
The schematic can be seen here.
http://www.nostalgiaair.org/pagesbymode ... 012418.pdf

Thanks, Mark

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 Post subject: Re: National NC-200 hair puller
PostPosted: Jul Sun 16, 2017 11:51 pm 
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Location: Seattle WA US
The very first place to look is dirty, bent, or shorted switching contacts in the headphone jack.

-Chuck


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 Post subject: Re: National NC-200 hair puller
PostPosted: Jul Mon 17, 2017 12:05 am 
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Mark,

Are the voltages normal on the IF tubes and detector/limiter? What you describe could well be a problem with the detector and noise limiter circuit because a fault in the noise limiter can make it function much like a squelch circuit.

Do you have a scope? If so take a look at what the signal is doing as it goes through the IF into the detector. Of if you don't have a scope loosely couple the output of the last IF into a test receiver tuned to 455 and see how it responds with different signal levels fed into the front end of the receiver.

With your description I am betting on a problem in the detector/limiter stage and a scope or signal tracer should make short work of finding the issue.

Rodger WQ9E


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 Post subject: Re: National NC-200 hair puller
PostPosted: Jul Mon 17, 2017 12:19 am 
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Thank you. I will check out the detector limiter circuit area tomorrow.
Mark

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 Post subject: Re: National NC-200 hair puller
PostPosted: Jul Mon 17, 2017 12:39 am 
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Hi Mark, Nice rx, but I can see myself being frustrated with this situation, also. I saw that you checked the tubes, but sometimes they are bad anyhow. You might try injecting 455kc into the 2nd IF output primary without a tube in the socket. If no luck there, try the secondary. Or it might be the tube socket - try disconnecting the primary from the socket and injecting into the transformer. In any case, it sounds like the problem is in or around that IF output transformer.

Good luck.

Regards, Larry


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 Post subject: Re: National NC-200 hair puller
PostPosted: Jul Mon 17, 2017 12:54 am 
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Thanks, I'll try that too.
It's frustrating when someone or more than one person have gone in and moved things around in a set .
I just took a quick look at the 6C8G limiter circuit and I see that the grid cap wire has been replaced and tacked in underneath the chassis but I can't find C20 or R18 which come off the grid and go to ground. There is nothing where the photo in the manual of the underside of the chassis show they should be. I quickly tacked them into place but still no improvment.
By the way, my early NC-200 does NOT have a pick up jack, just a phono jack.

Tomorrow the frustrating hunt will continue.
Mark

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 Post subject: Re: National NC-200 hair puller
PostPosted: Jul Mon 17, 2017 4:44 am 
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In addition to what others have suggested, have you tried with the AVC off and RF gain all the way up? Looks as though, as soon as you move to a stage with "AVC" its dead.

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 Post subject: Re: National NC-200 hair puller
PostPosted: Jul Mon 17, 2017 4:49 am 
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Yes, On this set you can turn the AVC on or off and I have tried with the AVC off and the RF gain all the way up. No change.

I will also try changing out tubes tomorrow even though all tubes test well into the "good".

There just has to be something I am missing but I'll go through all the suggestions I've gotten here.
Mark

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 Post subject: Re: National NC-200 hair puller
PostPosted: Jul Mon 17, 2017 6:00 am 
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If, while recapping the power supply, you connected B- (the negative side of the filter capacitors) to chassis ground, the AGC will be inoperative.

-Chuck


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 Post subject: Re: National NC-200 hair puller
PostPosted: Jul Mon 17, 2017 9:29 am 
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Location: Rochester, NY.
Agree. Watch the orientation of C35, the B- Bypass cap. It is easy to put in backwards between the B- bus and chassis. The chassis is more positive than the B- line.
Using the tube socket voltage check chart shown on p. 10 may quickly isolate the problem area.


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 Post subject: Re: National NC-200 hair puller
PostPosted: Jul Mon 17, 2017 11:35 am 
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You say the grid of the 6C8G, are you meaning the left grid tied to T3?

You say it was worked on by others, have you verified THEY put THEIR new parts in correctly? Don't get me wrong but the last guy may have replaced a part, putting it in the wrong place, and you simply followed suit without paying much attention.

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 Post subject: Re: National NC-200 hair puller
PostPosted: Jul Mon 17, 2017 3:33 pm 
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Mikeinkcmo wrote:
You say the grid of the 6C8G, are you meaning the left grid tied to T3?

You say it was worked on by others, have you verified THEY put THEIR new parts in correctly? Don't get me wrong but the last guy may have replaced a part, putting it in the wrong place, and you simply followed suit without paying much attention.

Great point Mike!

I have a schematic handy but when replacing paper caps and similar I generally follow the existing construction EXCEPT when it is a replacement part and that is generally clear by the solder joint and a component that doesn't match the others in terms of brand and style.

It is possible someone tried to modify the infinite impedance detector to another type.

Rodger WQ9E


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 Post subject: Re: National NC-200 hair puller
PostPosted: Jul Mon 17, 2017 4:27 pm 
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Don't ask me why I thought of that! :roll:

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 Post subject: Re: National NC-200 hair puller
PostPosted: Jul Mon 17, 2017 5:13 pm 
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OK,
finally starting to make SOME progress. I looked at the limiter circuit which is part of the 6C8G dual triode. The grid cap is the right side grid looking at the schematic. The left side Grid, pin 5, goes into the secondary of T3, the second IF transformer.

I found that a previous owner seems to have removed both C20 (250PF cap) and R18 (50K) both of which should go from the grid cap wire to ground. The grid cap wire had been replaced and not put back in the correct location as far as I can tell.
Now I can get an IF signal through from the grid cap of the 6K8 first detector (as stated in the manual) and I can peak both IF transformers.

However, the AVC and the CWO both do not seem to work. Possibly dirty switches or something else.

The biggest success is that I can now pick up a local station on the lower AM band (at 780KHZ) and one short wave band seems to be working but all the other bands are quiet including the upper AM band. I will reclean all the contacts and the pins to see if that helps.

The negative side of the filter caps is connected to B-, not ground so that is OK.
I need to check the orientation of C35, the B- bypass cap as it was replaced by a previous worker.

More later but Now I am somewhat encouraged as I can at least hear one AM station.
Mark

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 Post subject: Re: National NC-200 hair puller
PostPosted: Jul Mon 17, 2017 6:14 pm 
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Update:
Well, the B- to chassis electrolytic C-35 WAS installed backwards by a previous owner. I changed it out but the AVC still doesn't work.
Not sure where to look next. I'll check out each component in the circuit in case I missed something.

Still need to reclean the catacomb coil pins and sockets again as still only have two bands with any sound.

Mark

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 Post subject: Re: National NC-200 hair puller
PostPosted: Jul Mon 17, 2017 6:19 pm 
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When a sliding-coil receiver receives only on some bands, the pins and fingers are a good place to look. I've worked on several of these sets that seemed nice and clean but the fingers and pins were surprisingly foul. You can clean them all without removing the coil rack.

Try this: tune in one of the non-working bands and wiggle the bandswitch. If you get reception, it's a pretty good clue that dirty fingers and pins are your problem.


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 Post subject: Re: National NC-200 hair puller
PostPosted: Jul Mon 17, 2017 7:05 pm 
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Hi Joe,
I've tried the "wiggling" but still only two bands receive. I have cleaned all the pins and contacts once but will do it again and make sure none of them are mixed up.

Still can't get the AVC circuit to work. RF overloads unless I turn it down on the local station that I receive on the lower AM band.

Mark

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 Post subject: Re: National NC-200 hair puller
PostPosted: Jul Tue 18, 2017 2:15 am 
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Hi Mark, Yeah progress. For the AVC, I did't see in previous posts that you measured the B- to common after you straightened out the cap issue. It should be -45 or a little more. Did you recheck that? Make sure.

Regards, Larry


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 Post subject: Re: National NC-200 hair puller
PostPosted: Jul Tue 18, 2017 2:27 am 
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Good progress:
OK, long day but good progress. I now have all bands working after another cleaning. Sensitivity on the "B" band which is around 10MHZ is very low but I haven't done any alignment yet. I can get signals through at 10 and 15 MHZ with my signal generator so band is working but with low sensitivity.

AVC still is not working as far as I can tell. I can't hear any difference on strong or weak signal when I switch it on and off.

I will try measuring B- to Common as suggested but need to ask exactly what you mean by "common"? Is it chassis ground?

Mark

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 Post subject: Re: National NC-200 hair puller
PostPosted: Jul Tue 18, 2017 4:10 am 
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Mark, Yes, common would be chassis ground.

And on a strong station, the AVC tube cathode pin 5 should be -40 and the plate pin 8 should be about -20vdc. Suspects would be poor contact on tube socket, C32, C33, and C34. See if they are ok and installed with correct polarity.

Regards, Larry


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