Marshall Valvestate VS100R Transistors Blowing

[neilm]

The amp packed in a few months ago. The fault was traced to 2 darlington transistors (BDV64 & BDV65) that had shorted, taking out with them the T1A fuse. I replaced them (using heat sink paste) but it wasnt long before they blew. I replaced them again and left the amp on (guitar & effects unplugged) for elimination. After 15 mins the temperature of three of the transistors was 40-50deg C. After an hour I went out of the room for 5 minutes and returned to find it had blown again. The transistors were hot - one was 100 deg C! The amp is a few years old but hasnt had a great deal of use. I posted this on a newsgroup and it was suggested the large .33 ohm resistors (on stilts) or T5 MJF122 may be the cause, but these seem ok. Can anybody help please? I'm not an electronics wizard but I do have a digital multimeter, thermometer & oscilloscope. Thanks in anticipation.

[Gunny]

when repairing an 8008 2 channel Marshall power amp I found one channel had the power transistors (same ones as you) shorted. The small transistors in that channel were also short circuit. Had to replace them all. It ran for a short time, meaning a day or so before they all blew again....grrr! Spent the time and money to replace them all once more and it was fine ever since.
I'd recommend checking the small transistors in the output section.

[neilm]

Thanks for that, it did cross my mind! There's 10 D shaped transistors on the PCB that has the BDV's, so I ought to remove them to test. Did you have to replace any transistors on the PCB that connects to the control knobs? (volume / gain / treble etc)

[Gtr_tech]

Check the drivers.....sounds like one's leaky.

[neilm]

Thenks for your message. Pardon my ignorance, what are the drivers please?

[Gunny]

Neilm: the 8008 amp I referred to used OP amps from the input jacks, no transistors there....and no tone controls on that model.

It's quite common for the small transistors to be fried when the output transistors fail.

[Gtr_tech]

Thenks for your message. Pardon my ignorance, what are the drivers please?

http://www.schematicheaven.com/marsh...s100r_100w.pdf

Look at page 3. Check T7 and T8. If you don't have a transistor checker that can test for leakage, just replace them. Also check the associated diodes (D6, D7) to make sure they're not shorted.

[Enzo]

T7, T8 are not drivers, they are limiters. The amp would run without them, though if they short, they cause trouble. The output devices are darlingtons and sort of include their own drivers.

You can replace those BDV things with plain old TIP142 and TIP147. I would replace all four outputs, rather than just the two bad ones.

T6 is the bias transistor and that could be open. And verify R12, R13.

[Gtr_tech]

Aahh...I stand corrupted I just glanced at the print and spouted incorrect info. Forgot about those being darlington based....and the output devices aren't drawn on the print as darlingtons. Ugh....it pays to let the coffee kick in before I need to function.....

[zbyszekskibinski]

Output transistor failure

if there's power transistor failure do not replace them right away, check all the elements around especially those responsible for bias current setting. In this amp it looks like there are no bias setting potentiometer so I think diodes D6 + D7 regulate the current flow protecting darlington set from overheating. (check these). Check all the transistors including differential stage transistors as well, they may be leaky causing power transistors to fail. Hope it helps- Paul

[Enzo]

T7,T8 are limiters, they have a sample of output current from across the output transistor ballast resistors. If the voltage drop across .33 ohm R21 or R25 gets too high - from high current through it - then that voltage becomes enough to turn on the base of T8. When T8 conducts, it shunts the signal at the base of the outputs through D6. Until the voltage across R21/25 gets to the level needed to fire T8, all those parts are invisible. They have nothing to do with biasing the circuit.

T6 is the bias transistor, and indeed it is not adjustable, so if it doesn;t conduct enough, the power transistors are allowed to turn on hard and fuses will blow. For testing you may short T6 emitter to collector and the amp should function - assuming there are not further problems. That would be one way to determine if the bias circuit is the trouble. There would be an increase in crossover distortion in the output, but that wouldn;t harm anything.