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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 13th Jun 2005, 3:41 pm   #1
SA-Draadloos
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Default Worn pots

The Bush EBS 3A (export Dac90A) receiver I mentioned previously is now finished its restorative work Problem is the volume control pot. There are only 2 volumes available - medium loud and loud. I usoldred the pot and put an ohm meter across it - to discover the track is open (isntead of being 500K) somewhere I managed to track this lose of contact down to just above the bottom end.

What suggestions (if any) can be made?? I read this web site owners story of the dac90A at http://www.vintage-radio.com/recent-...h-dac90a.shtml but is conductive paint hard wearing? Is it a temp fix or will it survive the weekly use ??

Thanks all

James
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Old 13th Jun 2005, 5:08 pm   #2
Ed_Dinning
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Default Re: Worn pots

Hi James, if it's only a small way up the track you could try tightening the rivet (if it has one ) or the use of copper EMC foil with a conductive adhesive could be added on the last little bit of track.

HTH Ed
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Old 17th Jun 2005, 10:15 pm   #3
igranic
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Default Re: Worn pots

I have had some success with the following technique using epoxy resin andthe silver conductive paint formulated for repairing car rear screen heaters (about £5 for a very small bottle from cheaper motoring shops).

Carefully examine the control to ascertain the level of damage. If the crack penetrates the substrate (usually bakelite or SRBP) it will need stabilising to prevent thermal creep causing the crack in the resistive coating to re-open.

To stabilise the substrate, degrease the whole assembly then carefully clean the reverse and sides of the substrate (not the carbon track) with a glass fibre pencil (such as CPC part SABU10191). Coat the reverse and sides with a thin layer of epoxy resin and allow to harden completely.

Now the important part: If the crack is at the extreme top or bottom end of the track, one can usually paint over the crack with the silver conductive paint, but cracks in the area swept by the wiper can be more tricky since the transition from carbon to silver and back again can often lead to loud crackling if the paint is applied too thickly.

However, the wiper rarely covers more than a fraction of the width of the resistive track. Provided the crack is hairline, then simply painting a short silver stripe across the crack on either side of the wiped area (but being careful to leave the wiped area untouched) will reconnect the track and usually provide a usable and crackle free result. A minute amount of contact grease or liquid (not aerosol) electrolube, applied to the wiper, will help ensure trouble free operation.
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Old 18th Jun 2005, 2:42 am   #4
Norm_Leal
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Default Re: Worn pots

Hi James

Although conductive paint is expensive it takes very little to repair an open pot. Usually the wiper goes over a small portion of the carbon element. Conductive paint can be used beyond where the wiper makes contact so it will hold up.

Here in the US you can buy "Loctite rear window defogger repair kit". It's used to repair those conductive lines in car windows. It comes with a very small bottle of conductive paint/glue. I use it to repair opens in some pots.

Norm
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