Leaking coupling cap question on a BF bassman.

[dragonbat13]

I'm working on an ab165 bassman.

I'm chasing down some pops and sometimes scratchy pots, sometimes heavy distortion.

I'll have to test further to get a better description of the noises going on.

Anyhow I tested some coupling caps real quick and found the deep switch cap leaking around 300 mv. The ceramic treble cap on the bass channel has 7mv. The bass channel has 12mv.

I'm curious if this is too much leakage.

I'm not positive these caps are bad, but I figured the ceramics don't last forever and that deep switch cap is a little high for a blue molded. All the other blues in the stack read 0.

Am I trying to fix something that ain't broke, or should all three be replaced due to age, or in the case of the blue the 300mv on the deep switch?

[dragonbat13]

I guess I'll just disconnect the bass cap and replace them.

[pdf64]

Are you sure it's the caps leaking DC, not the crappy fibreboard?

[dragonbat13]

I have a feeling it's the board. But I want to know what the threshold of a leaking cap in circuit is.

[Stevem]

For me if I have a cap with 200 vdc on one end and 200mv on the other due to bleed it’s gone south.
All Fenders ( and most any amp ) will sound better when the board is bypassed and the components mounted right to the pots, especially when you can get rid of even 6 inches or more of grid wire runs to get to and from the controls.

[dragonbat13]

Thank you.

I'm going to replace it and check some others.

I'll be replacing the board at some point with a glass type eyelet. I plan on moving a few resistors, such as grid resistors around a bit. But I like to keep the amp somewhat to the original design. It's not original of course, but this isn't a museum grade amp anyway. Although it is nice enough to keep it somewhat the way fender built it.

[Stevem]

Another very important thing about installing coupling caps other than disc type is that they are rolled up .

So the lead on one end of the cap goes right into the center of the cap with the foil and whatever is used for the insulation is rolled up around that center and then connected to the other lead.
The proper insulation of these caps is if the signal flow out of it is feeding a down stream grid of a gain stage then that center connected lead should be used as the output.

On line you can read up on how to test a cap to find which lead is more shielded if the cap is not marked for such.

Installing caps the right way cap help to stop signal bleed and oscillations from taking place .