Marshall Valvestate 8100

[craig4739]

I picked up a Marshall Valvestate 8100. It was blowing fuses immediately upon powering up. I removed the two power transistors, powered up the amp and the fuse held. Testing the transistors indicated that they had blown. Ordered new transistors and installed them. Powered up the amp, got a very brief buzz out of the speaker and then the amp went quiet. Fuse didn’t blow but I cut the power immediately. Tried to power up again 30 seconds later and the same thing happened. What's my next move?

[52 Bill]

I picked up a Marshall Valvestate 8100. It was blowing fuses immediately upon powering up. I removed the two power transistors, powered up the amp and the fuse held. Testing the transistors indicated that they had blown. Ordered new transistors and installed them. Powered up the amp, got a very brief buzz out of the speaker and then the amp went quiet. Fuse didn’t blow but I cut the power immediately. Tried to power up again 30 seconds later and the same thing happened. What's my next move?

Recheck the new transistors to see if they have been damaged. Did you test the other transistors in the power amp as well? How about the emitter resistors?

With no speaker connected to the amp, is there any voltage on the output? Are both plus and minus power supplies working and up to spec?

Do you have a light bulb limiter available?

[J M Fahey]

Agree.
Clearly there were other parts damaged, sorry.
Recheck the new transistors, they might have died too.

[craig4739]

Checked out emitter resistors before I replace transistors and they checked out good. No apparent grounding from emitter, base or collector. Will pull transistors tonight and check power supply. Any other thoughts?

[52 Bill]

Just check through the entire power amp for all of the usual things, shorted transistors, open resistors, etc.

[craig4739]

Checked out a few more things.....Power supply checks out good. Components in power amp check out good. In running a signal through the amp, it is good from the beginning until I get to the effects loop and then gets very distorted. on the other side of the output of the first IC. Putting a signal directly into that IC and reading the output ...it comes out clear. the only thing in the middle are two jacks and some resistors. Am I missing something?

[52 Bill]

Checked out a few more things.....Power supply checks out good. Components in power amp check out good. In running a signal through the amp, it is good from the beginning until I get to the effects loop and then gets very distorted. on the other side of the output of the first IC. Putting a signal directly into that IC and reading the output ...it comes out clear. the only thing in the middle are two jacks and some resistors. Am I missing something?

No you're not missing anything, try cleaning the jack contacts. Or just plug a spare cord between the in and out and see what happens.

[craig4739]

Just tried plugging in the spare cord......no change