Conductive board blues - possible safety issue

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Conductive board blues - possible safety issue

Postby Tim Hicks on Wed Dec 11, 2013 1:38 pm

A friend of mine who is not a regular web user has encountered a problem with some black tag boards sourced from China. He had built a Tweed Deluxe using one of these tagboards and the fault showed up initially as if one of the pots was faulty, giving a crackle and hum when moved between settings. By process of elimination he eventually narrowed it down to the tag board material being conductive, as the pot and all the other components tested were absolutely fine. Further testing showed that the board material would begin to smoke if higher voltages were passed across it. Red boards from the same supplier were fine but some of the black ones were faulty. He is wondering if the material has been treated with a black carbon based coating.

Has anyone encountered similar problems?

I mention this because the fault could be potentially dangerous for an inexperienced amp builder.

Thanks
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Re: Conductive board blues - possible safety issue

Postby Unit_1 on Wed Dec 11, 2013 8:10 pm

if you measure resistance in two spots just touching the epoxy board it actually measures ohms other than zero?

if so you guys should contact the seller immediately to stop any more importation

did you verify that the under board wires are hooked up correctly?

another thing that happened to me once was a blob of solder had formed on the bottom of the board, so i clipped it off which made a sharp spot. when i sandwiched this down on top of the bottom layer of insulation it poked through the wire carrying the b+

this went directly into the top of the power tube cathode rc and BOY did that 5 watt resistor start smokin' !!! :Zap:

had to undo all the solders to the chassis components, lift the board and *there it was*
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Re: Conductive board blues - possible safety issue

Postby Tim Hicks on Wed Dec 11, 2013 8:34 pm

Thanks for the reply.

I will answer as best I can but I am not the amp builder, he's a friend of mine who is very competent (he used to work in the electronics industry in the days when valves and point to pont were the norm).

"if you measure resistance in two spots just touching the epoxy board it actually measures ohms other than zero?"

Yes that is what he has found.

Everything was wired correctly and apart from the noisy pot was working OK and in fact he only discovered the root cause of the problem when he started to work on a second new black board from the same supplier. He found that the second bare board with no components fitted was conductive!

I believe he has contacted the importer and he also warned another guy who he knows uses the same source. Its ironic that he tries really hard to source the best quality components from the UK, Germany and the US and this is just about the only one that comes pretty much out of the box from China.

Thanks again for your reply.
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Re: Conductive board blues - possible safety issue

Postby Unit_1 on Wed Dec 11, 2013 10:22 pm

WOW :shock:

i've checked boards before but never found any that did conduct

thanks for the heads up

i buy g10 epoxy board from Norva plastics and cut those with a diamond tile saw which keep them from stinking while i cut, then swag in my own turrets to save buck$

just checked my stock: non-conductive as one would expect insulation to be!!!!
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The ability to play/make music is a gift that not everyone gets. Those of us who have it should use it.

Asimov:Individual science fiction stories may seem as trivial as ever to the blinder critics and philosophers of today — but the core of science fiction, its essence, the concept around which it revolves, has become crucial to our salvation if we are to be saved at all.
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Re: Conductive board blues - possible safety issue

Postby Tim Hicks on Thu Dec 12, 2013 11:25 am

I asked around my other contacts and my pal who makes fine guitar pickups has found black "insulation tape" that caused similar problems for his pickups, because it had carbon black in it! Not perhaps the smartest choice of ingredient for insulation tape.....

I guess we have to assume that things may not always be what they seem with some imported materials from China.
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