Woe is me, conductive fiber board

[Randall]

I have a 70's Master Volume Super Reverb in with all sorts of weird things going on. High frequency squeal and reverb reducing volume output being two. I noticed bias was drifting a lot, and then I found the reverb control was adding -20v to the bias voltage when turned up full, thereby reducing volume. I must admit I was like,"WHAT?" Then it occurred to me to check the board, and sure enough 5v everywhere. This one of the wax coated boards, so I am not very confident I can correct this. I hit it with a heat gun, and it looked wet, but I think that was just the wax melting. It's a cloudless and slightly breezy 78 degrees today, so I put it in the sun on my driveway.

We shall see.

[mikeydee77]

I (uk based) have only come across this kind of issue roughly on some home build amp that was built using some low voltage tag board intended for model railways I suspect. From what I read on this forum I get the impression this is more of an issue in humid climate like Florida etc. Is that about right? The board absorbing moisture?

[Randall]

That is my understanding, yes. I have seen it several times. Guys playing outdoor gigs in summer or by the water. Wax covered boards were the worst idea ever if you ask me.

[Leo_Gnardo]

I From what I read on this forum I get the impression this is more of an issue in humid climate like Florida etc.

I see Fenders in upstate NY doing this year-round, though there's an annual rash of them July, August, September when humidity/temp assume jungle proportions.

Of course the full-board replacement, with proper board material and turret or eyelet is ideal, but beyond acceptable price for most customers.

What I do is place tie strips atop the board, and recreate the audio circuit on them in leak infested areas. Standard ol' tie strips of phenolic material.

Another potential (pardon the pun) solution might be to employ tie strips, and place the high voltage nodes on them. Haven't tried it yet.

Can't say I've seen interaction between reverb & bias as you report Randall, but anything's possible.

In really bad cases, cloth insulated wires passing under the circuit board can also pick up leakage currents resulting in aardvark noises. It seems "impossible" but I cast aside denial & replace the wires with PVC insulated, run atop the board because I'm too lazy to fish them through under the board. Might look the dog's breakfast but when I'm done, no more gas music from Jupiter noises, hooray!

5 volts is a whale of a lot of leakage. I usually see a tenth that or less. Sometimes a film or disc cap will get leaky and I'll see a couple volts on the side that should show none. Of course replacing those leakers is the solution.

[Helmholtz]

5 volts is a whale of a lot of leakage. I usually see a tenth that or less.

Voltage reading depends on meter input resistance. A 10M meter (like a Fluke 87) may read 10 times higher than 1M meter.

[Leo_Gnardo]

Voltage reading depends on meter input resistance. A 10M meter (like a Fluke 87) may read 10 times higher than 1M meter.

Got a Wavetek DVM, and with it occasionally notice freedom from loading caused by lower impedance meters like my ancient (nearly 50 years old!) Fluke. Please forgive my forgetfulness, Wavetek is either 10M or 100M.

[John G]

Hi, you could try and remove the screws holding down the eyelet board, separate the two layers with 6mm or greater insulators and see if the condition improves.
I no then the leakage is lateral either across the top of the board or on the bottom side, or both sides. Also, you could isolate the offending areas and rebuild the cct up of the board, not pretty but can work.
John

[Chuck H]

Another potential (pardon the pun) solution might be to employ tie strips, and place the high voltage nodes on them.

This is a great idea. Though with all the plate resistor and coupling cap nodes (which I think constitute a high enough voltage to be potentially problematic?) the effort might be a wash. I did start a project to determine what would need to be elevated on a typical BF Fender board to eliminate any probability of trouble hoping to post it here. I soon gave up because it was becoming redundant compared to replacing a board.

FWIW I DON'T do repairs for a living and my own record with conductive boards where the probability exists is about 50%. Which seems bad.

[The Dude]

There's not really more of a PITA than this and it seems quite unnecessary and stupid. Couldn't someone do a little more research before choosing a board material? It's like building a multimillion dollar house on a loose sand foundation. I feel for ya, Randall.

[Helmholtz]

Conductive boards are still a kind of mystery to me.

I always thought the conductivity would be a bulk effect and that the wax coating sometimes seen with old Fender boards was used to seal the board against moisture penetration.

But then I saw a YT vid where the (seemingly experienced ) guy cured the conductivity problem just by careful cleaning the board with isopropanol on a cotton swab around eyelets and components.
Before he got something like 2V and zero volt after. Amp problems gone.

That indicates that it might actually be a surface contamination (salts and moisture?) problem only.

[eschertron]

That indicates that it might actually be a surface contamination (salts and moisture?) problem only.

That suggests to me that we could be drawing ions out of soupy, humid air by the charge on an eyelet in a manner similar to the process of electroplating. And the effect probably snowballs as the area of charge increases with the collection of conductive ions.

[Chuck H]

I think it's any number of things happening. In some cases it's a combination of humidity in the air and carbon in the board. It other cases it's more likely contaminants as I've seen conductivity on one gray fiber board and read about it with a couple of gray fiber boards here. And then sometimes excessive work or modification seems to bring the issue at which point cleaning and drying doesn't seem to help any more. I've only seen this with the black boards and I interpret this as potentially being heat and high voltage creating permanent conductive pathways within the carbon infused paper. At which point you're truly sunk and replacement is the only option.

It doesn't matter if it's one or a combination of the above possibilities happening, the outcome is the same and successful repair without question marks can only be done by replacing the board. Even if the problem can be fixed by cleaning and drying it's just a matter of time before it happens again. Even more remarkable to me than Fender choosing this material seventy years ago is that some amp parts vendors are still selling it to unwitting builders and repair techs today!?!

I never counted but this is one of several conductive boards Randall has dealt with here.

[Helmholtz]

If I would encounter a conductive board in a vintage amp, I would try the surface cleaning method.
If that cures all problems it means that there was a conductive surface layer and chances are that it'll work fine for years.
After all it took 50+ years for the layer and problems to develop.

Heating the board will only drive out the water but not remove hygroscopic chemicals.

[Randall]

You are not going to clean a wax covered board without first removing the wax, which would mean removing the board. No thanks. If I am going to remove the board, I am going to replace it.

Now another question, does anyone know if this 1977 Master Volume Super Reverb board is the same or close enough to a proper pre CBS AB763? CBS did not use the AB763 identifier on the cabinet sticker or schematic AFAIK, at least on this amp. I am exploring ordering a turret board from Hoffman (EL34 world) to be able give an estimate for replacement. I can't seem to get a straight answer out of Doug Hoffman.

In other words, is a CBS Master Volume Super Reverb still AB763?

[Helmholtz]

Only the BF SR is AB763, built before '68.
There were at least 4 different SF circuits until the early 70s.

Dunno about the boards.
I would try to find and compare layouts