1978 Marshall Mk2 repair

[Buschman]

It crackles for several minutes when I turn it on. It does'nt crackle when the preamp tubes are out. I will make the noise when I put a v3 tube in with the power tubes. I thought it was related to the mid pot but that didn't fix it. This is becoming a bit vintage so I hate to just shotgun fix it and would like to leave everything as original as possible. Any ideas?

[selloutrr]

assuming the preamp tubes are functioning properly and not microphonic.
a broken lead that is held by presser, or a cold solder point,
is the amp dirty? try cleaning the tube sockets and pins, and look for any burn marks that could be arching B+ voltages. check your coupling caps and it maybe a dying carbon comp resister. If it's a resister this sucks only because they often test good out of the circuit and then pop once the voltage is applied. try the easy before you stress out about the resister.

[Structo]

Most likely has carbon film resistors right?

As selloutrr said, clean and tension the sockets, check solder joints and grounding.

Are any of the pots scratchy?

[billyz]

Some tubes do this on warm up. But it sounds like a Plate Load resistor on V3.

[Firestorm]

Is this a JMP? Which model? V3 is the PI, right? It makes noises when just V3 and the output tubes are installed?

[billyz]

V3 should be the PI on a 1978 Mk II. 82K and 100K , just like a fender.

[Buschman]

It makes noise with only the PI and power tubes installed. Sockets are clean and it seems cap or resistor related because it goes away after a few minutes. It is a JMP. Thanks guys. Nice to see a fellow Texan here Billy

[selloutrr]

You have swapped your PI tube with a known good tube?

if it goes away after it warms up it sounds more like a cold solder point it makes intermittent contact until it heats up enough to melt the solder. turn the lights off and get a chop stick put your hand in your pocket and try moving components and look for arching on the traces. or cold solder joints with the lights on. it could also be a dying transformer but lets not talk about that.

are you sure there is no burning on the sockets that make a trace. ( test continuity on all pins ) not going where it shouldn't go (tubes removed)

[Firestorm]

Touch up the solder joints on V3. Then try replacing the 82K and 100K plate resistors. Then. maybe, the coupling caps. Is this a new (NOS) tube? Outgassing makes noises like this.

[Buschman]

I have tried several tubes in v3. I will change the resistors and reflow some solder joints and give an update.

[Buschman]

I replaced the 82k & 100k on v3. Problem solved. Thanks for the help.

[selloutrr]

Awesome!