my marshall keeps blowin a fuse

[deano1979]

hi, first off please let me know if i have posted this in the wrong subject.
I have a stereo marshall power amp that keeps blowing the fuses. When i remove one of the power transistors it doesnt blow. I have the schematics here at
http://www.drtube.com/schematics/marshall/8008.pdf
the power transistor that i replaced is tr18 (this is also the transistor that when removed the fuse doesnt blow)
if i keep tr18 in the circuit and remove tr17 and tr11, the fuses blow. I am really pulling my hair out after all it is a small circuit. ANY IDEAS DUDES
cheers

[tekman]

I'd like to help you but sorry:

http://www.drtube.com/schematics/marshall/8008.pdf

is not accesible via web for me.

Any read protection for the world set up accidently?

hth,
Andreas

[MarkMcK]

I believe you have posted this in the wrong forum. The solid state forum might be better. Perhaps more users with Marshall experience.

Next, you are not going about troubleshooting in the usual way for an amplifier.

Do you have access to an analog volt/ohmeter (or a digital with a transistor junction test capability)? If you do, just check for low imped collector to emitter on transistors.

Otherwise, you might consider wiring a 40 to 60 watt lamp in series with one leg of the mains power cord. This will limit current input and allow you check without blowing fuses. If the light stays bright, you are still drawing too much current.

Still, if TR18 is okay, what about TR4? These are the only two higher current transistors in the circuit. If the amplifier is disconnected from a load, then removing TR4 should stop if from blowing fuses. Still, it is best to test both your TR18 and TR4 transistors (both the ones in circuit and your replacements).

Best,

Mark

[Thijs666]

Originally posted by tekman
I'd like to help you but sorry:

http://www.drtube.com/schematics/marshall/8008.pdf

is not accesible via web for me.

Any read protection for the world set up accidently?

hth,
Andreas

No problem here, I can open it just fine. Are you using the latest acrobat reader? Mine is 7.0.2.