Conductive Eyelet Board

[lowell]

Well, i think for the first time in 10 years of working on amps ive actually run into a Bassman AB165 head with a cconductive eyelet board. All amps that ive run into with this issue (crackling, hissy) in the past, ive managed to fix replacing components. This one is extra tricky and has had me pulling components AND my hair out. I stuck my meter probes into the board and get 8Mohms in certain spots, it varies depending on location. Sooooo, really in all truthfulness, how many of you have actually dealt with this issue. I know some techs assume this before actually figuring it out. I'd like to know how common this has been for you all, and do end up replacing the entire board or just part of it? Oh, and I wouldnt think 8Mohms would really cause an issue.

[Enzo]

8M is what your ohm meter measures by putting a volt on the board. Now put 400v on the board, now what? Instead of measuring resistance, turn the amp on and ground your meter and measure voltage on the board instead of resistance.

I have encountered this any number of times over the years. I have had success using a heat gun. Ever watch your frosty car windshield slowly defog when the heater blows on it on a cold day? You can watch the thaw line move across the fog. I blow hot air from my heat gun on the board, and watch the appearance of the circuit change from waxy looking to a more matte finish. As I play the hot air on it, I can watch that "thaw" line move along the board. I work from end to end until the whole thing is cleared. I have found this works for me a lot better than other methods with cleaners and solvents.

[lowell]

Ah, good point concerning 400v vs 1v. Lemme check it out. And boy am I looking forward to finding my heat gun after moving.

[lowell]

2v from board to ground...also, I unscrewed and raised the board and the problem goes away....so i guess there is some conductivity between boards and therefore from eyelet board to chassis ground. Seems that replacing the isolation board should do the trick, right? (Probably be a while b4 i locate my hear gun)

[Leo_Gnardo]

how many of you have actually dealt with this issue. I know some techs assume this before actually figuring it out. I'd like to know how common this has been for you all, and do end up replacing the entire board or just part of it? Oh, and I wouldnt think 8Mohms would really cause an issue.

Seen it a lot, and to make matters worse, sometimes in brand new kits built on the black cardboard. Ask Randall, he built a Deluxe Reverb with it about 2 years ago, had to rebuild the circuit board entirely. For this reason I recommend avoiding it to build new circuitry. Each year more amps show up especially around July, August, September as summer humidity makes the problem even worse.

I rebuild the afflicted audio circuits on tie strips bridged over the black board. If DC stray current is going to leak here there and everywhere, let it. It's not going to jump thru the air and get on the phenolic tie strip material. The total solution is replace the board with a proper turret or eyelet board built on a better material like epoxy-fiberglass but that gets expensive and time consuming. No amp owner I've met wants to spend that kind of money so the tie strip method works for us.

8 Megs is plenty enough to raise hob with your audio. All kinds of noises crop up as you mentioned plus pops & squeeks that sound like bad tubes. On occasion I've even found cloth-insulated wires sandwiched between the circuit and underboard pick up noises thru unexpected conductivity of their "insulator" layer, they need to be replaced too. It's a noise jungle in there.

[Chuck H]

2v from board to ground...also, I unscrewed and raised the board and the problem goes away....so i guess there is some conductivity between boards and therefore from eyelet board to chassis ground. Seems that replacing the isolation board should do the trick, right? (Probably be a while b4 i locate my hear gun)

If you have 2V measured to ground, and breaking contact with the insulation board stops it then it must be that the insulation board is conductive (which I've never seen, but...) and can only transfer voltage to the circuit board when in contact with an HV eyelet. But then why should you be able to read voltage from the circuit board at all even in contact with the insulation board if the circuit board wasn't conductive?!? Right? Ok, moving on... If the insulation board is what is causing the problem, but is contacting the chassis (as most of them do) then the differential would be 0V were the insulation board the only offender. It's pretty clear to me that the actual circuit board must be conductive as well. Why the voltage disappears when you lift it is a mystery unless you have -V on the chassis!?! Something about the testing method is skewing the results being evaluated I think.

I've seen my share of conductive circuit boards. Always the black paper turds. I've seen two that became conductive without being in high humidity. Where Enzo's heat gun method didn't work. With the amp stored and/or used in appropriate climactic conditions to be sure and still conductive and I had to replace the board. I suspect excessive soldering heat or repeated heating and HV can align the carbon used as a pigment in rare cases and conditions. One was a customer amp that I was doing extensive mods to and the other was a personal project that I had already worked on a lot. I've seen two other black turd boards that were acceptably repaired with heat, but not 100%. I don't do repairs professionally and probably haven't seen a dozen black fiber boards so four of those seems like a disproportionately high number to me. I've said it so many times that long time members her will be sick of hearing it from me. I hate those black paper boards.

[J M Fahey]

Not suggesting you do it if you are charging a customer by the hour and you want to make it work ($$$$/time) for both you and him, but in some cases where it's an amp I own or customer is fed up with "ghost" problems and gives me the green light, I have cloned the board out of 2mm Pertinax, the brown phenolic material, and transferred parts and wiring.

Yes, it takes most of a rainy Saturday or Sunday afternoon, not justified on a busy weekday, but it cures the problem forever.

I also drill old self tapping screw mounting holes out and use standard 1/8" bolts and plastic separators, so no need for the "insulating" board but the new eyelet board is 1/4" away from chassis.

It really cures the problem forever.

[Randall]

Ask Randall, indeed. The amp I built with the black fiber board changed what voltages I could measure on various parts of the board over a 3 day rainy spell we had a couple of springs ago. Scratchy pots, weird sounds, voltages that didn't make sense. And it decreased with the heat treatment, and came right back later. Proof enough for me. It was a full two months before I could bring myself to continue on with that project, I was so mad. I ended up replacing the entire board with a nice epoxy-glass turret board, and never looked back. That amp was as quiet as a mouse pissing on cotton after that.

[Chuck H]

That amp was as quiet as a mouse pissing on cotton after that.

Damn! A new bar has been set. Now I need to build an amp as quiet as a mouse that isn't peeing at all!?!

[gbono]

I have a 66' bassman that I suspect has this issue. Where do I get a good turret board - as mentioned? maybe just use a piece of "bare" FR4? Not going for concourse judging on this amp

[defaced]

When I first started doing turret boards I got my stuff from turretboards.com.
Bare board: Circuit board material
Bassman circuit boards: Circuit boards

If you want to source your own stuff, I have bought the G10 from McMaster Carr and Keystone Electronics makes the turrets which I bought from Mouser. Anvils and stakes can be fabbed from standard hardware store bolts. Add in a drill press and you're making turret boards.

[gbono]

Thanks -1/8" G10(FR4) use the Keystone turret terminals with a -4 suffix. Can you use the same staking tool for different terminals?

[Leo_Gnardo]

I have a 66' bassman that I suspect has this issue. Where do I get a good turret board - as mentioned? maybe just use a piece of "bare" FR4? Not going for concourse judging on this amp

If you're looking for a ready-made board, inquire at Doug Hoffman's website. Doug used to make turret boards of all sorts 15+ years ago, now he sells the parts to make them and has about a dozen recommended shops that can fab one up for you. With or without parts installed - you'll have to ask about that.

I'm sure Watts / turretboards as defaced suggested could do it. If you want to shop price or for a nearby source check Hoffman.

[defaced]

+1 For Hoffman. I forgot about them, but I have bought many things from them and they've always been good.

Regarding the staking tool, the Keystone staking tools are sized for various terminals - I think the sizing is listed in the Keystone catalog. I've always wanted to buy one of the real stakes, but I went the cheap route and bought a center punch that I use for staking turrets of all sorts of sizes.

[lowell]

66' bassman... Thats big amp!!!