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 Post subject: National NC60 Shortwave Volume Control Question
PostPosted: Apr Tue 17, 2018 2:25 am 
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Joined: Mar Sat 24, 2018 3:41 am
Posts: 7
Hello Everyone, I have a National NC60 Shortwave receiver I’m restoring, and have a little issue with the volume control. As the volume knob is slowly advanced from low, the sound increases much too quickly; not gradual as is desirable.

I’ve checked the 1 Meg potentiometer with an ohm meter, and it appears to be OK; no erratic changes in resistance as it is moved and a total resistance of 980K. The adjacent resistors (47k & 10meg) test OK.

The set has been recapped and has new filter caps. Also, checked the AVC voltage and it goes more negative with strong stations as it should. The radio plays and receives well.

I did try adding a 50K resistor to the center terminal of the potentiometer. That didn’t help and created a new issue; the radio played at low volume when the knob was at it’s lowest setting! So you couldn’t “turn it down” all the way…

I don’t want to try and replace the volume pot. and switch, but is there a way to vary some of the components in this circuit to cause the volume to increase at a slower and more gradual rate?
Thanks in advance for any help or advice!

Steve


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NC60gray.jpg
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 Post subject: Re: National NC60 Shortwave Volume Control Question
PostPosted: Apr Tue 17, 2018 3:26 am 
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Joined: Dec Wed 25, 2013 7:57 am
Posts: 2195
Location: USA
It sounds like a previous owner may have substituted a "linear" potentiometer where it should be "logrithmic" or "taper." You can check with a meter as you turn the knob.

Connect a 250k ohm resistor (you can experiment with values) betweem center tap and far end of the potentiometer.

Check here:
http://www.learnabout-electronics.org/R ... rs_09a.php


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linear to taper.jpg
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Last edited by AJJ on Apr Tue 17, 2018 2:01 pm, edited 2 times in total.
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 Post subject: Re: National NC60 Shortwave Volume Control Question
PostPosted: Apr Tue 17, 2018 9:24 am 
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Joined: May Sat 12, 2012 1:33 pm
Posts: 1469
Location: Rochester, NY.
I agree. The volume control may be the first to wear out and be replaced by a past owner with a conventional pot. An 'audio taper' volume control is what you want.
You may be able to install a series resistance with the pot or in the audio amplifier circuit to throttle back the amplifier volume.


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 Post subject: Re: National NC60 Shortwave Volume Control Question
PostPosted: Apr Thu 19, 2018 8:05 pm 
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Joined: Mar Sat 24, 2018 3:41 am
Posts: 7
Hi Everyone, thanks to AJJ and Wally58 for your replies. I checked the radio’s potentiometer and it looks like it’s non-linear. Here are resistance values as it is rotated in approx. 45 degree intervals:

942K
776K
425K
185K
102K
51K
2.3K

I tried adding a 250K resistor as was suggested, but to no avail.

What about reducing the gain of the 50C4 Audio Amp. tube to tame the volume issue? Maybe adding resistance in series with pin 7 to reduce the plate voltage….or maybe a resistor in series with the control grid (pin 2,5) Just not sure how best to accomplish that without introducing distortion or some other problem. Thanks for any advice!


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 Post subject: Re: National NC60 Shortwave Volume Control Question
PostPosted: Apr Thu 19, 2018 9:23 pm 
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Joined: Dec Wed 25, 2013 7:57 am
Posts: 2195
Location: USA
Based on the description of the issue, it's the rate volume changes as you turn the control, not that the radio is playing too loud. So altering gain or AVC action is not going to solve the problem. Is the problem "severe" or just a minor annoyance?

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 Post subject: Re: National NC60 Shortwave Volume Control Question
PostPosted: Apr Thu 19, 2018 11:34 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 7891
Location: Cleona, PA
Instead of a 250K resistor from center of volume control to low side, try going down to some lesser values, like 200K, 150K, and so on. There should be one that gives an acceptable result.

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Reece


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 Post subject: Re: National NC60 Shortwave Volume Control Question
PostPosted: Apr Fri 20, 2018 12:19 am 
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Joined: Dec Sun 14, 2008 3:33 pm
Posts: 666
Location: Tokyo
The easiest way to reduce audio level is to disconnect the 50C5's 5uF bypass cap. That's not a radical mod: many AA5s did not use a bypass cap there. You might even notice a small improvement in sound quality.

You could try inserting a resistor between the pot and the 100pF/47K resistor node. Perhaps a 470K to start with. This will of course limit the maximum volume, but it might work as a quick and dirty solution.

Rob


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 Post subject: Re: National NC60 Shortwave Volume Control Question
PostPosted: Apr Fri 20, 2018 2:23 am 
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Joined: Dec Wed 25, 2013 7:57 am
Posts: 2195
Location: USA
I agree with Rick's and Reese's ideas. One would alter the tapering the other would lower the overall volume. I went to look at EC-1 and S-38 schematics; volume control circuit on both are very similar. I do remember my EC-1 would have decent volume on even the lowest setting with strong signals; on my S-20R, very strong stations don't go completely silent. I think this is just a feature of this design.

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