Conductive circuit card

[Mark]

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I have several mV of DC on the pots of my 6G7a Bandmaster which is due to the circuit card being conductive. I have cleaned the board thoroughly with alcohol and dried the board in a dedicated oven for electronic components years ago and the board looks very clean. The caps are largely Mallory 150's.

I find if I touch the circuit card with positive probe of my Fluke 77 set to 300mV DC I get a small voltage and if I turn the amp off the voltage decreases.

The question is what to do about the conductive circuit card. In a perfect world there would be a replacement that looks like original. Though in a perfect world I'd also like to keep the original circuit card.

What are your thoughts?

[martin manning]

I've seen one tech go to extraordinary lengths to cure this, including stripping the board, cleaning, heating, and retreating. Maybe you could heat the board in place and apply liquid silicone, which is non-conductive and water repellent.

[Mark]

Thanks for your reply Martin, though it must be making contact around the eyelet, which would be hard to reach.

[martin manning]

Can you slide the backer board out? Often there will be solder balls and flux on it. That part could be easily cleaned and dried.

[Mark]

That is very true.

[Stevem]

Which pots and on which lugs?

That amp has atleast 9 subterranean wires and any of them due to there poor insulation from back them could now be bleeding voltage back onto the board.

I would lift up the board and remove and replace all of that wire under the board one wire at a time with new cloth covered wire running on the top side of the board see if the issue goes away, if not then it's time to make yourself up a new board anyway!

[Mark]

Thanks for your reply Steve, the mV of DC voltage is on the tone control pots and the volume control. You are correct there are quite a few wires under the board and I have had experience with a Kendrick Roughneck where the volume pot wiper wire in fact bled treble frequencies to ground. In that case I left the wire where it was and ran a shielded cable to the grid of the next triode. As a result the treble frequencies vastly improved.

Should the board need to be replaced, what material could be used?

I think garolite is the best material that I know of but as far as I've seen it bares little visual similarity to the original circuit card.

[pompeiisneaks]

Thanks for your reply Steve, the mV of DC voltage is on the tone control pots and the volume control. You are correct there are quite a few wires under the board and I have had experience with a Kendrick Roughneck where the volume pot wiper wire in fact bled treble frequencies to ground. In that case I left the wire where it was and ran a shielded cable to the grid of the next triode. As a result the treble frequencies vastly improved.

Should the board need to be replaced, what material could be used?

I think garolite is the best material that I know of but as far as I've seen it bares little visual similarity to the original circuit card.

You can still buy the old stuff, if you hope for it to go conductive again in the next 30 or so years for visual parity. I highly prefer garolite/FR4/G10 and small standoffs as well to keep it up off the chassis directly.

Your call. the other stuff is still for sale, I think Doug Hoffman sells it and I am sure there are tons of other places.

It's a choice of 'authenticity with it's known failings' or 'do it right for the long term'

~Phil

[Mark]

Thanks for the reply Phil, Hoffman only sells G10 boards, I suspect Marsh amps sells the fibre board, but I haven't gotten a reply from them.

[Stevem]

And your triple sure that the D.C. Voltage on the pot is not due to a leaky coupling cap?

[pompeiisneaks]

Thanks for the reply Phil, Hoffman only sells G10 boards, I suspect Marsh amps sells the fibre board, but I haven't gotten a reply from them.

Oh weird, I swore he used to. I guess maybe he's stopped. He's removed a lot of stuff from his store, maybe things that didn't sell that well?

~Phil

[Colossal]

Mark,

I'm not sure where you are in your repair...

I am going to be starting a Bassman build shortly and I plan to use 0.09" black G10. I want to get a nice look to my boards, to make them look traditional but do not want to use that conductive paperboard. I did a little experiment the other day, sanding the surface of a test piece of black G10/FR4 to a nice, smooth matte. It is passable for that old Fender board material with none of the problems. I hate that stuff.

For what it's worth...

[martin manning]

I had the same thought re roughing up the surface of black G-10 to give it the look of vintage fiberboard. Beware though, as there have been reports of that material being conductive too, perhaps due to the added black pigment. I think buying from a reputable manufacturer and source would minimize the possibility of a disappointment.

[dorrisant]

If color is not an issue...

https://www.cedist.com/products/fiberbo ... -062-thick

[M Fowler]

I have some of that CED gray paper boards and they are terrible to drill through, punching would work.

Mark