Tweed Twin scratchy volume pot on guitar

[Helmholtz]

all black ink is made with "lamp black" which is carbon

There are non-conductive chemical alternatives like aniline black or Nigrosin.

I'm not convinced of the free carbon theory. It would require the carbon particles to touch for conductivity, meaning a very high particle density.
Also not all black fiberboards show conductivity.
I recently measured a new black board and found resistance values of several hundred MOhms between very close contact points.

The fact that baking/heating a conductive board reduces conductivity (at least temporarily) indicates that water absorption is involved.

It might be a combination of free carbon and moisture, though.

Anyone seen a conductive grey board?

[Leo_Gnardo]

Anyone seen a conductive grey board?

Grey or tan, seen those in Traynor amps from late 60's, early 70's. They were more or less copies of Fender amps in many cases. Haven't yet run across a conductive board in those.

As far as non carbon pigments / coloring agents, sure they exist. But, as usual, manufacturers pick the cheapest way and carbon black fits the bill.

Simply stickiing ohmmeter probes into old or new black paper boards, whether onto eyelets or straight into the board, I've found resistances that explain their misbehavior. No fancy lab meters here, mine all have a 20 meg ohm limit.

What makes the problem even worse is the random nature of the conductivity, often resulting in audible noise. I first became aware of the problem late 70's when I had replaced all the components in a Deluxe Reverb and noise persisted. Somewhere in my files I have an article from an organ repair periodical of that time mentioning the board conductivity problem. So, even way back then it was noted and advice given how to deal with it.

[Helmholtz]

Simply stickiing ohmmeter probes into old or new black paper boards, whether onto eyelets or straight into the board, I've found resistances that explain their misbehavior. No fancy lab meters here, mine all have a 20 meg ohm limit.

My method using a DCV meter having an input resistance of 10M in series with 300VDC allows measuring resistance up to GOhms.

Board resistance above 500M won't disturb a tube circuit.

[Leo_Gnardo]

My method using a DCV meter having an input resistance of 10M in series with 300VDC allows measuring resistance up to GOhms.

Leave it to you Helmholtz, there's a great idea. and a big thank you!

[Helmholtz]

Leave it to you Helmholtz, there's a great idea. and a big thank you!

BTW, it's the same method I use for measuring leakage of caps.

[Helmholtz]

Do we even know what board material this clone amp uses?

[The Dude]

The 68k input mixers look to be board mounted; so there’s opportunity for a crappy board to leak some voltage onto them.
https://el34world.com/charts/Schemat..._twin_5f8a.pdf
Hence the query of post #8.

Thanks for the head's up. I was thinking the resistors were mounted on the jacks like many of the Fender amps, It might be a good idea to modify it in that way if it's suspected that the eyelet board is at fault (as has been stated already). .

[Chuck H]

There are non-conductive chemical alternatives like aniline black or Nigrosin.

Ok. But I don't expect you'll see those alternatives used in any cheap, commercial product being manufactured in east Asia.

I'm not convinced of the free carbon theory. It would require the carbon particles to touch for conductivity, meaning a very high particle density.
Also not all black fiberboards show conductivity.

No. All don't. But some do. So where does that leave us. I don't believe it's always due to moisture either. Two conductive boards I've battled were silver face Fenders that have the heavy wax. Heating them melted the wax to flow and created a lovely, even sheen and finish on the board surface, but did nothing to mitigate the conductivity.

I think it may come down to just how much pigment is used. There must be a fine line between "enough to make it solid black" and "enough to make it conductive".

I also think there may be extenuating circumstances sometimes. A particular layout that places high differentials near enough to align a conductive pathway. Then other circuits that are in proximity can be affected as well. And this is a real phenomenon. In fact the "tombstone" bulb mounts for older fluorescent lamp fixtures have been known to develop enough conductivity to melt with age and the constant high voltage exposure. As to the black fiber boards, I think that excessive heat and resoldering may also play a roll since I've seen problems develop that didn't exist before after extensive rebuild work or modding.

Anyone seen a conductive grey board?

No. I've never seen a conductive board that wasn't black.

[nosaj]

The environment Randall and I live in can be especially moist, and damp and sometimes with a little salt spray.
I have a traynor yba1 (1979) tan board became conductive. Partly in the middle of replacing the board with a G10 board with eyelets.
nosaj

[Enzo]

The wax adsorbs moisture. If you just heat the waxy coating, it spreads and looks nice, but the trick is to hit it with a heat gun until it evaporates off the board and nice DRY looking surface is exposed. Lovely sheen means contaminants are still there.

[Chuck H]

The wax adsorbs moisture..

??????

[nosaj]

??????

Beeswax is known to absorb moisture. I think early Vox transformers were susceptible
nosaj

[Enzo]

Water molecules join the wax molecules, so the only way to get rid of it is to remove the wax, not just smooth it out.

To absorb means to mix with, to adsorb means to cling to.

[Chuck H]

My understanding of wax has always been that it was hydrophobic. And that this is why they used it on the boards. I don't know wax to absorb water. Unrefined beeswax may have a small casein type element that allows emulsification? Like butter is mostly fat but does have a small water bourn element that must be removed to make it "ghee".?. I don't expect the wax that was used would have been full of honey and larva!?! Refined beeswax is certainly hydrophobic. And I don't know that beeswax is what was used either. And even if it was it must be predominantly hydrophobic. I would think enough so that it's not even a consideration. But I'm always open to learn something.

[Enzo]

I don't know what the "wax" really is. All I know is it seems waxy, so I call it wax. In any case, I have had success when I run a heat gun on it to the point the wax evaporates away. perhaps moisture collects UNDER the wax?