Fiber board bites me in the ass again

[Randall]

Some of you may remember a DR build I did on black Mojotone fiber board that I had to tear out and do over on garolite turret board because the fiber was conductive right from the start. I vowed never to go near it again. Well, I tore down and rebuilt a Bassman chassis almost two years ago that has been sitting in my shop wating for the cab to begin. I had a piece of grey fiber just the right size, and someone here had mentioned it not having quite the risk of conductivity as the black, so I used it. The cab is now finished and I went thru the chassis to check a couple of things, and guess what? Scratchy pots. Yes, there is mV's everywhere. After much knashing of teeth I had an idea to remove the screws and lift the board up a bit, and yes the mV's went away, and no more scratchy pots. Turns out, I didn't have but only one piece of grey board, so I used the blank black piece from the original amp as an insulator. Now it would appear, it is causing me more trouble. Odd, since you might think it would act to shunt those mV's to ground. Until I remember that the chassis is painted.

Anyway, seems I will have to yet again tear up a perfectly nice board to replace the bottom board with something, not exactly sure what. Yay me.

Here a shot of it after grille and speaker install. It sounds really cool, BTW.

[nosaj]

Some of you may remember a DR build I did on black Mojotone fiber board that I had to tear out and do over on garolite turret board because the fiber was conductive right from the start. I vowed never to go near it again. Well, I tore down and rebuilt a Bassman chassis almost two years ago that has been sitting in my shop wating for the cab to begin. I had a piece of grey fiber just the right size, and someone here had mentioned it not having quite the risk of conductivity as the black, so I used it. The cab is now finished and I went thru the chassis to check a couple of things, and guess what? Scratchy pots. Yes, there is mV's everywhere. After much knashing of teeth I had an idea to remove the screws and lift the board up a bit, and yes the mV's went away, and no more scratchy pots. Turns out, I didn't have but only one piece of grey board, so I used the blank black piece from the original amp as an insulator. Now it would appear, it is causing me more trouble. Odd, since you might think it would act to shunt those mV's to ground. Until I remember that the chassis is painted.

Anyway, seems I will have to yet again tear up a perfectly nice board to replace the bottom board with something, not exactly sure what. Yay me.

Here a shot of it after grille and speaker install. It sounds really cool, BTW.

[ATTACH=CONFIG]53703[/ATTACH]

Don't know about you, but we've been burning up up here in NWFL, humidity not so great either, but mosquitos seem to be in a lot smaller number lately.
nosaj

[Chuck H]

That's a real bummer. I'm a premier hater of black paper boards and "I" wouldn't have expected that from a grey board. It takes a lot of carbon to make a conductive path. More likely your problem is moisture. You could probably dry the lower board out with a hair dryer and wax it and never have a problem again. But that still means removing the top board to do the work so why not just replace it with something less sponge like. Yay Florida climate

I've never used a paper board for anything, but I've had trouble with several. All black, some waxed, but that didn't seem to help. Drying never helped me. I'll never use a paper board (or even a black one) as long as I'm doing this stuff.

[Randall]

Well, nix that, I must have made a goof, because with the grey board raised, I still have mV's all over. As high as 500 mV over by the B+, and 30 - 40mV at the preamp end, and everything in between. Should this be less than this? Maybe I have more than one problem, because I had 16mV on the treble cap and 4 mV on the volume pot, but when I lifted the cap it measured next to nothing for leakage, and the vol pot went to less than 1 mV. I noticed the leg of the cap seemed a bit long, so perhaps it was poking the under board? I trimmed it and soldered it back in, now the volume pot has about 1 mV fluctuating, and the scratch is a lot less. The other channel has ~2mV on it's pot, so I will try to get that figured out.

So how much voltage on this board is too much, and is there something amiss with my methodology? And why does a blocking cap with the low side lifted fluctuate wildly in voltage reading from -50mV to 20mV?

Oh, and BTW, waving a heat gun over it makes it much worse, for whatever reason.

[Chuck H]

Are you using a mica cap for the treble? There's been a disproportionate number of problems with mica caps lately and it's not the first time this has happened. There seems to be a recurring history of it. That's enough for me to exclude them from future designs. Maybe there's a good and right way to make them and they're great. Maybe there's a problem with how and where they're being made now.?.

[Randall]

"Are you using a mica cap for the treble?"

No, it's a 250pF ceramic, and as I said, after I trimmed the leg and re-soldered, that part of it is fine now. Still really wondering about the stray mV's on the rest of the board, and if it really is a problem, or if I am doing it right.

[J M Fahey]

When I started in 1969 I used Pertinax, phenolic paper boards, and was unhappy because I could not get black fiber locally.
Hey, "Leo used it" so it *had* to be the absolute best, wouldn´t it?

Like Oscar Wilde says: "there are two worst things in your life, one is not achieving your dreams ... the other one is achieving them"

Boy what a disappointment!!!
Doubly so in humid Buenos Aires.

[tedmich]

Hey maybe you can use the fiber board as a ground plane?
I remember my first time trying to stick a high impedance sensor wire to a metal panel with old silver duct tape (I was young!) what a noisy week that was for my poor A/D converter until I figured it was conductive and swapped in some kapton.

[Randall]

^^ yeah, that doesn't really help much.

[nevetslab]

I had sampled some materials back in 2007 from Fralock, div of Lockwood Industries, where they were then handling Bergquest insulation materials, Fishpaper and ITW Formex. Fralock has moved away from that end of things, but I did find ITW Formex, which has a GK material that's 0.030", 0.045" and 0.060" thick, with insulation properities rated to 2200VAC, temp rating of 115 deg C, H20 absorption less than 0.06%.

Now, what is the specs of fishpaper? When I was with BGW Systems, we used to source the Bergquest material and fishpaper from them. There are multiple sources for Fish Paper material in different thicknesses. The data sheets I downloaded don't call out the insulation properities in terms of voltge like I found for the ITW GK Formex, but listed instead in terms of dielectric stength, such as 215 volts/mil for 0.062" thick material, which would be 13.3kV if I'm reading this correctly. I've attached a couple data sheets for Fish Paper that I found. I didn't find a data sheet in pdf form for the Formex. All are available in cut sheet sizes, resource-dependent.

fishpaperprop.pdf
vulcanex-datasheet.pdf

[Mick Bailey]

So I used the blank black piece from the original amp as an insulator. Now it would appear, it is causing me more trouble. Odd, since you might think it would act to shunt those mV's to ground. Until I remember that the chassis is painted.

I'm thinking that even if it was a bare chassis you'd have some problems. As the board material is high-resistance and rather than a direct path to ground you'd have voltage gradients all over the place between numerous contact points. I've had bitter experiences with conductive boards on new builds and now only use FR4. Even Fender has abandoned fibre on the hand built reissue amps such as the '57 Tweeds. Some of the phenolic tag strip even leaks. I have an old hand-cranked insulation tester that tests to 1000v and use that to check any new or unknown material.

[Randall]

Follow up. I sold the amp to a guy who lives on the water. He's a die hard Floridian charter captain, I don't think he even has central A/C in his little crackerbox house.

Anyway, about 6 months ago the amp started making noise. I brought it home and, sure enough, conductive grey fiberboard. F++K! I swear on my mother's grave I will NEVER use fiber board again!

So, I had to build another one, this time on Garolite, built up on 3/8" standoffs. It was tricky. I improvised on the original, making some changes I didn't document well, like putting a tweed preamp on channel two. I was able to pull some photos from the build thread, tho. Those were helpful. I worked on it for a while, then had to take a couple of months off. Maybe getting a new motorcycle project had something to do with that.

I put in a half day today, and finished it. Happy to say it works great right from the first power on. No mistakes! I think it is quieter, too. Dude has been extremely patient, and I know he will be very happy. As for me, I won't make an extra dime on this, in fact it is costing me extra materials and untold time. But, I made it, and I sold it, so I figure it's on me to make it right, especially because this customer abso loves this amp. So, just thought I would post some follow up as to how it looks now.

I don't expect to ever see the insides of this one again, I think it's that tight now.

Good effing riddence fiber board!

[Chuck H]

FWIW I never made money on a build. But that's as it should be since I'm not a "qualified professional" so attempting profit would be dishonest to the actual professionals. And it doesn't help that I mostly built one off custom circuits so the trouble shooting and fine tuning takes a lot of time. I mention this because you did make some changes to the original circuit on this one. But there is a great deal of learning in the process and satisfaction in the success.

Looks great. And properly functional in it's intended environment now. Kudos!

[Pedro Vecino]

Brought it home and, sure enough, conductive grey fiberboard.

Can you put a picture of the old board to identify it?
It could be very useful to know. Thanks.

[Leo_Gnardo]

Can you put a picture of the old board to identify it?
It could be very useful to know. Thanks.

What you see in every old Fender, typically dark grey almost black. Unfortunately the same potentially defective product is supplied as part of many amp kits "for authenticity." As long as you love the "authenticity" of having unexpected noises show up in your build or repair.