Slightly Strange Marshall Behaviour

[Buddha's Guitar Tech]

Marshall JCM 2000 DSL 50, big volume drop.

Fine at first, but cranking it unleashed a seemingly permanent volume drop.

Tried changing all preamp valves, one at a time, no joy.
Subbed a pair of EL34s.

All the voltages seem ok, no obvious sign of burning - looks and smells clean.
Here's where it gets tricky....
You know how when you're checking pin voltages, you get clicks, pops, rizzes, etc from the DMM probe?

i'm getting none of the usual noises, not on the PI, not even on the EL34 plates, grids, nothing...

Yet the guitar is getting all the way through, albeit quietly.

Could this be an output transformer thing? Perhaps NFB gone mad?

It died mid-play, BTW.

[Gibsonman63]

Impedence selector switch? I know the old ones are notorious for failure.

[Buddha's Guitar Tech]

Ah, shall maybe try a hard-wire, see what joys that brings.

[Jana]

Check the bias supply voltage--I seem to recall that Marshall put some PTC resistors in by accident. If I remember right, as the amp got warm the bias changed radically. Do some searching on it. I can't say for sure it was in the bias--I do know that they put PTC resistors in and it made for some strange behavior after the amp warmed up.

[Buddha's Guitar Tech]

When i checked, the bias was around -37.

[Richie]

put a cable in the effects loop and jumper it. See if it works with the cable jumpered in the effects loop.

[Structo]

Yes, could be the switch contacts on the effects loop jacks.

Other than that, check all your voltages, that can tell the story.

Plate voltages, power node voltages, etc.

[billyz]

Those Marshalls are notorious for the speaker jacks not making ground contact. I solder a ground strap across all of them.

They also can have bias supply issues, but mostly the speaker jacks. The ground is daisy chained from one jack to the other and the cliff jack contacts develop oxidation on the ground contact. I have even seen them burnt from arcing.

[Buddha's Guitar Tech]

Tried the FX Loop thing - nope.

Went directly from the OT SEC terminals (making sure to earth the common, of course) and same deal. With no NFB you'd expect a little something extra, but nope.

i could swear, that as i turned the volume up, i coud hear the EL34s make 'tink' sounds - as if they were being stressed, but with no great rise in volume.

Gonna do some checking, but my Fluke meter measured .5, .7, and .9 ohms for the secondary taps respective to common. Gonna do some digging, see if that's right.

[Buddha's Guitar Tech]

Tried the FX Loop thing - nope.

Went directly from the OT SEC terminals (making sure to earth the common, of course) and same deal. With no NFB you'd expect a little something extra, but nope.

i could swear, that as i turned the volume up, i coud hear the EL34s make 'tink' sounds - as if they were being stressed, but with no great rise in volume.

Gonna do some checking, but my Fluke meter measured .5, .7, and .9 ohms for the secondary taps respective to common. Gonna do some digging, see if that's right.

[Buddha's Guitar Tech]

Don't you just feel like a dufus when you miss something before asking for help?

Turns out it was the OT - primary.
Isolated, i checked the DCR across the legs and got 60-odd ohms. Then, magically, 80-odd. Aha!

Wired it back together, strummed a little guitar, poked the OT lugs (with a metal rod, because i'm Chuck Norris) and lo and behold, i controlled the fault.

Thanks for the input guys, it always helps to get the noggin greased.

[Reeltarded]

lol

If I wake up in the morning, it's because Chuck Norris let me live.

Good deal.