Fender prosonic

[nosaj]

Got a fender prosonic custom shop combo, When the drive is engaged it gets a nasty distortion.
With no signal input there is no noise, and no dc on caps where I don't think there should(that could be the problem me thinking
When i put a 1k tone into the amp on the drive channel I get DC on c5 (-1vdc), c7(-6vdc), it seems that when signal is applied the caps breakdown, am I on the right track?


Also another question involves c27 and DC on both sides, in the aspect the cap is used to separate 2 different DC right(it sounds dumb when I type it out cause it must be correct I just think I'm second guessing too much.

I'm using the the second schematic on the page

Thanks,
nosaj
http://support.fender.com/schematics..._schematic.pdf

[The Dude]

Are those negative signs or dashes you posted for the voltages on C5 and C7? Cap leakage on those would result in a positive voltage since the other side of the caps is tied to the plates of the preamp tubes.

C27 should measure approximately the same as C26 as far as voltages on either side. Both are coupling caps from the PI to the output tubes. If one of those was leaky, one of your output tubes would likely be red plating.

[nosaj]

Are those negative signs or dashes you posted for the voltages on C5 and C7? Cap leakage on those would result in a positive voltage since the other side of the caps is tied to the plates of the preamp tubes.

C27 should measure approximately the same as C26 as far as voltages on either side. Both are coupling caps from the PI to the output tubes. If one of those was leaky, one of your output tubes would likely be red plating.

Yes when Drive is engaged on c7 now getting -11.9vdc other side of cap is 143vdc
C8 -3vdc other side is 244vdc
c5 -3 other side is 244vdc

Switch to normal an the negative voltage disappears.
nosaj

[The Dude]

And you said there might be an issue with C27? What are the voltages on both sides of that cap?

[nosaj]

And you said there might be an issue with C27? What are the voltages on both sides of that cap?

302vdc one side other side is -38vdc I just figured it was separating the Bias voltage from the other circuit.
nosaj

[The Dude]

Have you tried swapping out V1 and/or V2? V1a and V2b are both in the gain channel path.

[nosaj]

Have you tried swapping out V1 and/or V2? V1a and V2b are both in the gain channel path.

I have not I did change V2 with another tube. I will swap them and see what it does.
But what exactly am I see with voltages?

Thanks,
nosaj

[The Dude]

I don't see anything that would indicate cap leakage. The voltages are negative. If the caps were leaky, the voltages would be positive.

[nosaj]

I don't see anything that would indicate cap leakage. The voltages are negative. If the caps were leaky, the voltages would be positive.

Ok i flipped the tubes and issue was gone on both channels. So i cleaned and retensioned the sockets and cleaned the jacks. So I'm guessing dirty contacts. On another note driving the input with a guitar signal no dc at points I was questioning earlier(but I also scrapped some excess flux off a track can that stuff be conductive at all?)

Thanks,
nosaj

[nosaj]

Ok i flipped the tubes and issue was gone on both channels. So i cleaned and retensioned the sockets and cleaned the jacks. So I'm guessing dirty contacts. On another note driving the input with a guitar signal no dc at points I was questioning earlier(but I also scrapped some excess flux off a track can that stuff be conductive at all?)

Thanks,
nosaj

So would the negative voltage I was experiencing be called grid leak?

Thanks,
nosaj