Bassman won't stay fixed!

[trem]

Hey Chuck -
I think I follow.....
Pull the ground side of the cats off of the board, and use a jumper to make the ground connection.?

[Enzo]

How common a disease might be is irrelevant. It only matters whether YOUR amp has it or not.

Ground your meter as usual, and find an eyelet with B+ on it, like the plate resistor for one of the 12AX7s. Now press your red probe into the fiber board right NEXT to the eyelet. You SHOULD get zero volts. If your meter measures any voltage "on the fiber board" then it has become conductive. Try this in several spots to be sure.

I have had success with a heat gun. I'd blow the hot air on the fiber board, and I can watch the surface change, just like defogging a car window.

[trem]

10-4 Enzo....
And you are right of course. If it is l happening to me, it does not matter if nobody else is afflicted with it.
But I am still curious how common of a problem it is. I guess we will never know for sure unless Fender, or somebody that repaired A LOT of Fenders, kept some kind of records to substantiate the problem
Thank You

[Chuck H]

Well, Bruce has repaired a bunch of old Fenders and reports that it's virtually non existent. Someone mentioned that they see it more on the waxed boards in SF Fenders. I'll second that. Two of the three I've seen were SF amps. The other was a Traynor.

Enzo. Wow, that's a lot of moisture if you can watch the surface change as you dry it. I did mention the waxed SF boards, which would certainly show something when heat is applied, but you would have recognized that for something different. But as much moisture as you describe would certainly be conductive. What I've seen has been something else though. It happens to the black boards when excessive stripping and resoldering are done. I did check for moisture too. I have a real nice moisture meter with needle sharp tempered probes. Even tested dry I could read voltage on the board. There's a lot of talk about how this phenomenon is usually an incorrect diagnosis and how the real culprit is ultimately found to be something else. But when I can test the board as dry with a high end tool and then read voltage directly off the board material I'm a believer.

trem, you would need to lift the tube end of the cathode circuit. The grounded end will still be at 0V even if the board is bad provided your amp is grounded correctly. You need to lift the end that floats above ground. That way your 1.5V (approx) of bias won't be thrown off by an extra .6V from the board. Same principal with the coupling caps. Float everything forward of the end that goes to a grid. .6V on the grid is just as significant as an extra .6V on the cathode. (.6V being an example figure)

[Enzo]

How common then? Common enough that I see it now and then. In terms of all they amps made with eyelet boards, a very small percentage. The first time I encountered it, it was baffling at first. I was trying to figure out why my voltages were weird, when I rested the point of my probe on the board between eyelets, and to my surprise, got a voltage reading. Now I know to check for that, and find it every now and then. I am sure Fender doesn;t keep records of it, and I sure don't. Such statistics are really of no help in the repair game. Once you are aware of it, it can no longer sneak up on you and it takes mere seconds to look for once inside.

CHuck, when I said like a defogging window, I did not mean to imply I was seeing actual moisture condensation. I only meant the appearance of the board surface changed, and I could watch it as I moved the hot air down the board. Just as when you flip on the refog, you can watch the change take place moving across the window.

All I see is the surface appearance make a change. The surface build-up of wax/tar/nicotine/whatever that apparently also absorbs moisture from the air, once heated evaporates or melts or does something, so the dull semi-glossy surface turns more into a matte appearance. Whatever the goop is coating the board, it has gotten conductive. And whatever it is, the heat gun seemks to break it down, or evaporaste it or do something to render it harmless. So I suspect I am seeing the same thing you encounter. I am not drying a wet board, I am using a hot air blast to do whatever it is doing to drive away the conductive surface. I do believe the problem is absorbed moisture, but I don't know that it is particularly measurable as moisture per se. It could just be that water molecules are joining with wax molecules to cause enough conductivity to be a problem. COnsider static electricity. During the winter heating season, humidity is so low that static builds up on everything. At other times, things may check as perfectly "dry", yet there is enough moisture adsorbed onto surfaces that static discharge is no longer a problem.