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Old 5th January 2013, 05:30 PM   #1
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Default Volume pot repair

Hi. I am repairing a Toshiba sc-330 power amp which has an issue with the left channel volume pot.

It only works when in the middle position around no 5. If I turn it to a position around 6-8 it will work but requires gentle touching to get it to stay on. After 8 it wont wont work at all. I discovered that pulling the knob out will make it work in any position but the sound goes off again when you let go.

I have tried spraying a little switch cleaner into the pots whilst turning around etc as per suggestions in other threads, but to no avail.

Any suggestions?
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Old 5th January 2013, 05:53 PM   #2
Mooly is offline Mooly  United Kingdom
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The new thread

Check the soldering on the pot leads first. It might help to see a close up picture of the pot and PCB. Fitting a new one should be straightforward, and cleaning (if that's all it needs) is do-able on some pots. That involves removing the pot and opening it up (assuming its the metal type with flanges that can be bent open).

Should be an easy fix though.
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Old 5th January 2013, 08:14 PM   #3
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Ok. Im happy with it for now. Iv got both channels on 5 which is plenty loud anyway! Il come back to it when I get some other projects out of the way.

Thanks a lot.
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Old 6th January 2013, 06:18 AM   #4
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It seems like the circlip holding the shaft/wiper assembly in position is missing or the wiper is damaged such that it no longer contacts the track properly. That can happen in handling or storage or even removing/refitting the knob. You can easily replace a lost circlip from another pot. in the unusual event it has been lost but otherwise, you will surely need a new pot. 'Should still be able to get economical old style 24mm dia. dual gang types (I assume, from the vintage), depending on your location.

Or..if you like nursing things along, a fat, felt washer between pot and front panel was once a common DIY "fix"
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Last edited by Ian Finch; 6th January 2013 at 06:33 AM.
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Old 13th January 2013, 10:38 PM   #5
ftpols is offline ftpols  Japan
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Default Volume pot repair

Quote:
Originally Posted by peacocksuit View Post
Hi. I am repairing a Toshiba sc-330 power amp which has an issue with the left channel volume pot.

It only works when in the middle position around no 5. If I turn it to a position around 6-8 it will work but requires gentle touching to get it to stay on. After 8 it wont wont work at all. I discovered that pulling the knob out will make it work in any position but the sound goes off again when you let go.

I have tried spraying a little switch cleaner into the pots whilst turning around etc as per suggestions in other threads, but to no avail.

Any suggestions?
Any kind of switch/contact cleaner can delaminate the graphite resistive element inside the pot. It's best to spray/inject a small amount of Isopropyl alcohol inside the pot and rotate the pot about twenty turns.
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Old 13th January 2013, 10:52 PM   #6
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Try WD-40 if the cleaner does not work - - bit crude but often works well enough, if you can't replace the pot.
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Old 14th January 2013, 03:45 AM   #7
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Quote:
Originally Posted by peacocksuit View Post
..... I discovered that pulling the knob out will make it work in any position but the sound goes off again when you let go...
I think that's the clue to a mechanical fault - not so much a cleaning requirement, whether cleaning helps much or not.

The problem to clean pots is that quality types like the ALPS are sealed. How do you know that you are doing anything dribbling solvent down the shaft or hoping there is a way to reach the resistive track by squirting at the tiny gap in older style, cheap pots? Solvents like IPA or alcohol are usually safer than the butane and diesoline in WD40 etc. but the problem of actually cleaning the tracks remains. Without dis-assembly and thorough washing, we are most likely wasting our time.
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