Conductive circuit board?

[stokes]

So I got a BF Bandmaster AB763 in that looks like it was in a swamp. Lots of rust. I cleaned up the tube sockets and pots, changed the caps replaced the 2 prong power supply cord and connected thru the current limiter, no shorts so I plug it in and check voltages all dc values about 4-5v hi, cathode v's almost dead on. So far it looks good.I put a signal thru it and its very unstable on the scope. As the volume turns up the signal jumps up and down i.e. from 1 to 2 on the knob its normal 3, 4, 5, it jumps until it gets distorted then comes back down and repeats until the volume is at max.When I can find a spot that shows me a nice clean sine wave, the wave is "rolling" looks like a roller coaster as it goes across the screen. I found a couple leaky coupling caps and things got a little more stable but no where near good. I have heard about these old circuit boards being affected by extreme humid or wet conditions but in all my years I've never actually seen one in this condition. Anybody here ever encountered a board in this condition? Is there any kind of test to see if the board is the suspect? I have a feeling the pots may be a big part of the amps problem and thats where I'm going next, and I would suspect there are other issues, but just wanted to hear any experiences with humidity affected boards.

[The Dude]

That sounds more like a bad volume pot. Hook your ohm meter to the pot and rotate the pot. See if you detect drastic resistance jumps as you go through pot rotation. Also check to see if there is DC on the pot. If it is a conductive board, you could try drying it out with a hair dryer. It's more often moisture in the board than the board itself.

[garytoosweet]

I have seen many conductive circuit boards in old Fenders.

Sometimes you can verify by pressing a meter probe into the black board. A sharp tip helps.

[stokes]

When I did the last v check after I changed the leaking couplers, I checked the pots, no dc. Like I said, the pots are a suspect and that was my next adventure. I gave them a quick spray already but I'm gonna try a good soaking. I really have to wonder if this amp wasnt underwater at some point,lol. I just told the owner I'm having a face plate made with a new name "Swamp Master" instead of Band Master. It is absolutely the crustiest old Fender I ever saw. He says his uncle had it in a garage for "many years". Since it has so much rust and oxidation I have to consider the board as well.

[stokes]

I have seen many conductive circuit boards in old Fenders.

Sometimes you can verify by pressing a meter probe into the black board. A sharp tip helps.

I did try that, got nothing, good sign I suppose.

[Chuck H]

Well it's clearly been subjected to extreme environmental conditions (at least) if not some acute event (flood?). It's fair to suspect ALL solder joints in this circumstance as well. And certainly the possibility of a conductive circuit board. The above mentioned test by placing probes right on the board has worked for me. It's been especially helpful for me to test near high voltage nodes and grid nodes where board voltage can make the most trouble.

[Mick Bailey]

A good warm up with a hairdryer will often reveal whether a board is conductive. Heat it until the wax re-flows and see if the problem disappears (if it does it will only be a short-term fix). Conductive boards will usually show a few volts at random points with respect to ground. Often you'll get DC on your input lead and the guitar's pots will be scratchy as a result.

[Chuck H]

A good warm up with a hairdryer will often reveal whether a board is conductive. Heat it until the wax re-flows and see if the problem disappears (if it does it will only be a short-term fix). Conductive boards will usually show a few volts at random points with respect to ground. Often you'll get DC on your input lead and the guitar's pots will be scratchy as a result.

Yep. This is my experience as well. I don't know what sort of affect/s happen once a board goes conductive but they do seem to stay that way to some degree forever after. Sometimes it's possible to just float components over a troubled area. Like a spot on the board that allows voltage to affect a grids bias. More often though I just replace a board. Which is of course a real PITA.

[stokes]

Making good progress with this wreck.Changed out some severely out of spec resistors and some leaky caps.Used a ton of spray in the pots and stabilized that issue. I'm still a bit concerned about the condition of the board. When I used the ohmmeter and probed the board I get no readings but with the amp on and using the dcv meter I am seeing from .09 to .49 vdc in various areas. Like I said, I have never dealt with a board that is conductive and never had to probe one. Is the voltage I am seeing problematic or is it ok?

[Chuck H]

Well you have to take the readings in context. You're experienced enough to recognize this...

If you have .49VDC on an eyelet that is associated with a preamp tube grid then it could certainly throw off the bias for that tube. Since preamp tubes bias at only a volt or two. But .49VDC near an HV power supply node is entirely ignorable. If you don't have weird voltage readings on the preamp tubes representative of incorrect bias and you aren't finding high bias readings on those grids you might be on the lucky side of this.

[The Dude]

Are there still symptoms or problems with the amp? I'm reminded of a quote from our once fearless leader (paraphrasing), "If it works, stop fixing it".

[TomCarlos]

I don't know if this will help or shed new light on the topic, but back in 2015 we had a thread titled Conductive Eyelet Board. See if that thread offers you ideas.

[stokes]

Well you have to take the readings in context. You're experienced enough to recognize this...

If you have .49VDC on an eyelet that is associated with a preamp tube grid then it could certainly throw off the bias for that tube. Since preamp tubes bias at only a volt or two. But .49VDC near an HV power supply node is entirely ignorable. If you don't have weird voltage readings on the preamp tubes representative of incorrect bias and you aren't finding high bias readings on those grids you might be on the lucky side of this.

Exactly what I was thinking, Chuck. The near half volt is at the end of the board nearer the power tubes. The preamp cathodes should be 2 v's as per the schem, I am seeing 2.2 v's on the cathodes which I was not concerned with. All the hi voltages are about 4-5 v's hi which is normal with an amp of this vintage. Gonna connect the variac and see where the volts are with 110 vac supply.
"If it works, stop fixing it". Yeah, Dude, I'm just about there. The signal is looking good on the scope. I still have concern about the integrity of the components, when I opened the amp and turned the chassis upside down to open the cap pan, 2 of the ceramic trem caps fell out, the solder joint didnt fail but the wire broke at the connection I assume from corrosion.Also wondering how much moisture the carbon comps have soaked up. I have the chassis on its side and have had a small ceramic heater blowing warm air into it overnight for the past 2 nights and will continue this til I sew it up. Gonna give it a run thru with a guitar into a speaker and see what its got later today.
Thanks for that link TomCarlos, I'll give it a look.

[Helmholtz]

When I used the ohmmeter and probed the board I get no readings but with the amp on and using the dcv meter I am seeing from .09 to .49 vdc in various areas.

Testing with an Ohmmeter won't work because board resistance is typically way above the meter's limit.

An example: A board resistance of 200M between a 200V plate voltage and a grid with a 1M grid leak will raise grid voltage by 1V.

The voltage method is much more sensitive, but the results vary with meters having different input resistance.
A meter with a 10M input resistance may read 10 times higher than a meter having only 1M.

[Chuck H]

Testing with an Ohmmeter won't work because board resistance is typically way above the meter's limit.

An example: A board resistance of 200M between a 200V plate voltage and a grid with a 1M grid leak will raise grid voltage by 1V.

The voltage method is much more sensitive, but the results vary with meters having different input resistance.
A meter with a 10M input resistance may read 10 times higher than a meter having only 1M.

Ok. But you can still measure DCV on grid pins, right? I mean, it couldn't be that what your meter is reading is not accurate because of board resistance, is it?

I guess another issue could be AC voltage. If there are large voltage swings at tube plates and the board is conductive then these swings might still be transferred intermittently (cycle peaks) to other circuits. This wouldn't be seen in any static testing.