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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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4th Nov 2018, 12:35 pm | #1 |
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1M pot measures 1.5M
I'm trying to solve a dodgy volume control on a GEC valve radio (circa 1956). The control is a 1M log pot., but when I measure it I get 1.47 M across the outer lugs.
Do pots degrade in this way? Is there a fix? Thanks and regards...Paul |
4th Nov 2018, 12:39 pm | #2 |
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Re: 1M pot measures 1.5M
Put the wiper at each end of the track in turn and measure the resistance from the wiper to the other end of the track. It's possible the wiper has limited travel and only part of the track is being used.
If the volume control circuitry is conventional, the extra resistance won't make much difference.
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4th Nov 2018, 12:47 pm | #3 |
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Re: 1M pot measures 1.5M
The resistance at the wiper measures about the same. I'll clean the pot and then see where we stand. The pot sits across the output of the IF stage so I guess it won't make much difference. It's just operating as a voltage divider, right?
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4th Nov 2018, 1:33 pm | #4 |
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Re: 1M pot measures 1.5M
Very common for a 50 year old carbon pot to read up to 50% high.
Be careful when cleaning, dont wipe the carbon track too hard or you will wipe it away. Clean the wipers and the centre metal track. I find a little silicon grease on the track helps stop crackling and reduces the ware on the track. Mike |
4th Nov 2018, 3:49 pm | #5 |
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Re: 1M pot measures 1.5M
You are correct, it will make little or no difference if that is all that's wrong with it.
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4th Nov 2018, 4:13 pm | #6 |
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Re: 1M pot measures 1.5M
I plan to clean it carefully with a cotton bud and DeOxit D5. Does that sound ok?
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4th Nov 2018, 4:37 pm | #7 |
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Re: 1M pot measures 1.5M
Just the one very gentle wipe with a cotton bud, deoxit should be OK, I have lost the carbon track on some pots.
Watch out in case there are carbon brushes on the wiper, they are easily lost. Mike |
5th Nov 2018, 2:56 pm | #8 |
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Re: 1M pot measures 1.5M
What I have done in cases like this is rub a soft pencil over the carbon track, then wipe off very gently with a dry cotton bud. This cleans the track and fills any slight imperfections. Good luck with however you decide to deal with it.
Regards. John Last edited by GRID LEAK; 5th Nov 2018 at 2:58 pm. Reason: thick finger syndrome |