Crackling/buzzing with higher volumes in a Carvin X100B combo.

[52 Bill]

The black discoloration would not come off.

What did you use to try and remove it?

Would this produce the result of having very low output with the inner pair of tubes removed, but normal output with the outer pair removed?

The two pairs of output tubes are basically wired in parallel, so removing two tubes should cause the same amount of volume drop. If the two inner tubes are not playing as loud as the two outer tubes, then there is a problem with the way that they are connected, or the burned board is involved, or the off board rewiring is not correct, or ...

[bclemons]

Isopropyl alcohol. The outer tubes produced the very low output. The burned area is over one of the outer tubes. The noise that got me looking at the amp was present with just the outer tubes in place, I just had to turn up the clean volume to almost 10 to make enough output to cause it.

[tedmich]

Burned boards can be difficult to rehab, as the carbon conducts HV too well. That could be a throw away sadly.

[52 Bill]

Isopropyl alcohol. The outer tubes produced the very low output. The burned area is over one of the outer tubes. The noise that got me looking at the amp was present with just the outer tubes in place, I just had to turn up the clean volume to almost 10 to make enough output to cause it.

The noise could be the sound of the board arcing along a carbon trace. You really need to inspect the board very carefully for signs of arcing.

The low volume from the outer pair of tubes is probably a mistake in the wiring when the amp was last repaired. Pull all of the output tubes and take resistance readings from pin to pin of the two right hand sockets and on the two left hand sockets. Pin 8 to Pin 8, Pin 7 to Pin 7, etc.

Between pins 5 there should be about 4.4K ohms and between pins 4 there should be about 940 ohms. All of the others except maybe pins 1 should read at zero ohms.

[bclemons]

The left hand pair (the ones with the terminal board) measured as you expected. The right hand pair measured correctly on 4 and 5. The resistance between 3-3 and 8-8 on this pair measured infinite resistance.

[drewl]

A " Throwaway"???

Don't be ridiculous. You could always rebuild it into some thing else, hardwire it.

[The Dude]

......Isopropyl alcohol......

Try acetone (carefully with cue tips).

[bclemons]

A " Throwaway"???

Don't be ridiculous. You could always rebuild it into some thing else, hardwire it.

Yeah, if the boards trashed, I'm thinking plexi with those big ol' transformers.

[J M Fahey]

Pity you don't have access to a scope because this is the kind of problems where they excel.

Just feed a constant tone at the frequency and level that caused the strange noise and check different plates,pots, jacks, connectors, until you find one place where signal arrives clean and leaves wonky.
You still have to find the exact problem, but it narrows the search area considerably, often to just 3 or 4 suspects which, worst case, can be shotgunned.

Of course, you can always buy a Ceriatone or similar board and junk the old one.

[Alex R]

Could be oscillation, internal electromagnetic feedback. Need a scope to be sure though. Clipping in a filter cap across each one in turn has about a 60% chance of stopping it if it's that, might be worth a try.

[tedmich]

A " Throwaway"???

Don't be ridiculous. You could always rebuild it into some thing else, hardwire it.

so the board would get....thrown away?

[drewl]

No, lots of good parts on it!

Without being that drastic it shouldn't be hard to find where the problem is.

Start by monitoring the power supplies and voltages at each stage while hitting the amp to find where it is crapping out.

[tedmich]

I am the king of reuse Drewl,

I never meant to imply he should toss anything but the burned board, which is likely irreparable, like the 5150 PA board above

[52 Bill]

Unfortunately, I think that this is one large board for the entire circuit. I don't quite think that it needs to be scrapped just yet, but that's just me.

[g1]

Yeah, I was under the impression it was one large board too. Worst case would be to chop out a section of it, and build that section on a sub board.