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Old 30th May 2021, 4:10 pm   #1
DonaldStott
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Default Yet another RF bypass cap

I've now restored a number of sets, including Bush DAC90As and DAC10s, so it came as no surprise to find in the Bush DAC10 I am currently attempting to restore that the Mains RF bypass cap had blown it's top! This is C27 (0.1μF) in the Bush Radio Service Instructions.

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What is different this time around is that the volume control (VR1) appears to have taken the full force of this eruptive event while C20, the V3 0.01μF Grid coupling capacitor doesn't look too clever either! C20 is a waxy paper cap that would have been replaced anyway.

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What a mess!

VR1 should be 500kΩ and in another DAC10 I have this is measuring 0Ω-470kΩ but this one measures 54kΩ-712kΩ and this was after a good squirt of Servisol Super 10.

If I decide to replace C27 it will of course be with a proper X-rated cap suitable for this location on the switch side of the AC mains input. Or I may just leave it out completely?

As for VR1 ... ?
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Old 1st Jun 2021, 3:01 pm   #2
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Default Re: Yet another RF bypass cap

As per usual I've now disassembled the pot and switch.

The double pole double throw switch is now working as it should.

The pot has been thoroughly degreased and cleaned - now reading 598kΩ.

Is this still too high when the Service Sheet states that VR1 should be 500kΩ??
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Old 1st Jun 2021, 3:45 pm   #3
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Default Re: Yet another RF bypass cap

With it reading that high, it would not normally concern me, what reading are you getting from the wiper to ground?
It should be no higher that 10 ohms.
A suggestion for cleaning the wiper itself is to use printer paper on the track, run the wiper over this a few times and job's a good'un.
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Old 1st Jun 2021, 4:03 pm   #4
Ian - G4JQT
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Default Re: Yet another RF bypass cap

I don't think the volume pot is made to particularly high spec as long as it's within about 30%. It doesn't matter. But it needs to be quite low on minimum or the audio won't reduce to nothing at minimum.

A good clean of the track and wiper as suggested by GBits above is a good idea.
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Old 1st Jun 2021, 10:04 pm   #5
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Default Re: Yet another RF bypass cap

I have come across pots giving out strange readings in times past and present, it's usually caused by either the track having a groove cut into it by the wiper or carbon deposits getting stuck under the wiper itself.
When the wiper starts trashing the track, it can be temporarily repaired with a HB pencil, rubbing it all over the track several times to build up a layer.
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Old 2nd Jun 2021, 8:28 am   #6
Craig Sawyers
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Default Re: Yet another RF bypass cap

https://www.blore-ed.com/product-pag...er-metal-shaft

470k log carbon track with double pole mains switch, £7.50

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Old 2nd Jun 2021, 10:09 am   #7
DonaldStott
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Default Re: Yet another RF bypass cap

Thanks for all the helpful suggestions.

I've now had the opportunity to look closely at the wiper and track using my loupe and it is clear that we don't just have a groove but a deep channel carved in the conducting surface. No amount of temporary repair is going to fix this!

Moving on to my collection of 'faulty' switch/pots I've found one where the volume control is measuring 408kΩ which I can combine with the DPDT switch section from the other one? Now to what extent is too low a value better or worse that too high a value in this location??

Just in case I have a 470kΩ log DPDT switch/pot from Bowood Electronics on stand-by.
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Old 2nd Jun 2021, 11:51 am   #8
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Default Re: Yet another RF bypass cap

The only thing to happen if it's too low would be the volume output itself increasing too fast.
The 470k would be a good candidate.
Anything between 400 - 600k Ohms should work, old parts weren't the most accurate things even when new.
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