Help...my amp turned into a fuzzbox...!

[crbowman]

Any smoke?....funny smell?.........
If not, I wouldn't think it's a transformer.
Looks like you might have a switching jack in your loop. If so, make sure that the switch is making good contact.

[Blindog]

What is voltage is the B+ for the PI?

{Edit} - I figured it out - 320V ?



Did you do anything with the NFB when you fitted the PPIMV?



Are all the resistor values in the PI as per the schematic?



Andy

{Edit} Your PI seems to check out apart from the voltages on the grids (which I'm not sure you can measure accurately).

What voltage to ground do you get between the junction of the 470R / 10k / 1M / 1M resistors in the PI tail? (Guess: 33.5V)



What voltage do you get between the point where the 4 resistors join & pin 2 & 6 respectively? (Note, not the voltage to ground)? (*should* be 0.00)

What voltage at the top of the presence pot (junction of 10k & 47k NFB resistor? (Guess 11.5V)



/edit

You were pretty much spot on with the suspected voltages you asked about...B+ to PI=317, PI tail=33.7, presence pot=10.3, 0 volts from PI tail to pins 2 & 6

I'm going to check my wiring from the board to the send/return jacks for faults...

Thanks for the input

Mark

[Blindog]

Any smoke?....funny smell?.........
If not, I wouldn't think it's a transformer.
Looks like you might have a switching jack in your loop. If so, make sure that the switch is making good contact.

I've swapped out the OT, and checked and re-checked the wiring to the send/return jacks, even bypassed it completly..scratchin' my head at this point..

[ampgeek]

Frustrating to say the least!

If you are unable to narrow down the possibilities with some kind of scope/other technique, you are left with replacing everything item by item which is a pretty daunting task.

Old circuit boards and tube sockets have haunted me in the past even with a scope. I have heard that certain types of those materials can become conductive over time.

I had an old Silvertone 13**something or other that suffered from a socket failure. The scope brought me right to that section of the amp but after replacing the 7 or 8 components around it without luck, replacing it was all that was left. And...it solved the problem.

Looks like your board has a few waves in it. Is it possible that the back side is "lightly" contacting the chassis somewhere?

Good luck!
Dave O.

[Tillydog]

You were pretty much spot on with the suspected voltages you asked about...B+ to PI=317, PI tail=33.7, presence pot=10.3, 0 volts from PI tail to pins 2 & 6

I would say the PI is OK.

Time to "phone a friend" ....

(A friend with an oscilloscope, that is )

Sorry, out of ideas

Are there any clues from when it happened - it's not clear to me whether you were playing one day and it started, or if this is immediately post some modification.

Good luck, whatever!

Andy

[Blindog]

Well guys, I guess now's as good a time as any to learn to troubleshoot with a scope...I've had one for a while now, but never have taken the time to play around with it...

Andy, this problem cam about while i was playing the amp one day...kind of just crackled and lost volume, came back for a while, and did it again a while later, but this time for good, so it seems

Thanks for all the input guys.

Mark