Marshall Valvestate 8100 / 8080 Shorting Output

[Sloan]

PROBLEM
I've got a Marshall VS8100 here that had shorted outputs and blown fuse. I replaced the outputs (TR8/TR10) with TIP147/TIP142. Powered up with no load it blows the fuse and TR8 (tip147/bdv65).


TEST DATA
TR 4/5/6/7 - in circuit OK
R 84/85/73/74/75/77/98/99 - in circuit OK
D1/D2 - out circuit OK

Amp will power up fine with no outputs in the circuit.


SCHEMATICS
The date inside the chassis is '92, so i'm guessing this is an 'issue 1' model
Power amp: http://www.drtube.com/schematics/marshall/8080-pwr.gif
Preamp: http://www.drtube.com/schematics/marshall/8080-pre.gif






There is a thread about the similar 8080 (same schematic) here that suggests swapping R74 for a lower value: http://music-electronics-forum.com/t5913/
Wondering if that would also apply to this amp as well.

Is there are any common suspect components in these amps I should focus on first before I go through checking everything?

[Jazz P Bass]

I see that you used Tip141 & TIP147 transistors.
You really should use the same rating.
TIP147 is the same rating as TIP142.

[Sloan]

I see that you used Tip141 & TIP147 transistors.
You really should use the same rating.
TIP147 is the same rating as TIP142.

oops that was a typo, tip142/tip147 is correct. edited original post

also, i've got through almost every resistor shown on the power amp schematic and they all seem ok. update later...

[Enzo]

Check power resistors R118,R96 0.33 ohm 5w for open.

And critical stability resistor 10 ohm 5 watt R119, check for open.

Unsolder one end of D4, D5 and lift from circuit during tests.

You checked resistors around it, but bias transistor TR9 is critical, how is its health?

The op amp and the D1,D2 circuit run on the +/-15v rails. Are those supplies OK?

Powering up with no outputs tells us little more than that th power supply doesn;t blow fuses.

[Sloan]

Check power resistors R118,R96 0.33 ohm 5w for open.

And critical stability resistor 10 ohm 5 watt R119, check for open.

Unsolder one end of D4, D5 and lift from circuit during tests.

You checked resistors around it, but bias transistor TR9 is critical, how is its health?

The op amp and the D1,D2 circuit run on the +/-15v rails. Are those supplies OK?

Powering up with no outputs tells us little more than that th power supply doesn;t blow fuses.

Thanks for the reply;
I went ahead and removed the failed TR8, as well as TR9 & TR10 in order to power up and check voltage. R101 and R102 get extremely hot.

D4/D5 check out of circuit OK
TR9 tests good with diode test (no shorts)
+/-15v is present at IC8, R78, and R83

other data from later yesterday:
TR 11/12 in circuit OK
D4/D5 in circuit OK
R 27/58/59/61/62/78/80/81/83/96/97/100/101/102/115/116/118/119 OK

[Sloan]

Hmmm, after replacing TR8/9/10 with all new parts, the amp will now power up without blowing the fuse, but R75 begins to literally burn up. I replaced R75 and also TR4 as it seemed to be faulty after further inspection, but R75 is still smoking upon power up.

[Jazz P Bass]

I got it now.
You are simply swapping parts out hoping to fix the amp.
Good luck with that.
All of the transistors should be sitting on the edge, waiting for a signal to come out of IC88, pin 7.
What is the voltage at pin 7?
What is the voltage at the emitters of TR6 & TR5?
TR6 & TR5 , when pin 7 tells them to, will move the bases of TR7 & TR4 according to the polarity of the signal. These are the drivers of the output transistors.
At idle, the bases of the output transistors must be "slightly" on. Like 0.6 volts.
TR12 & TR11 are protection transistors.
They will remove output trans base current when the output goes too high.
That is how it supposed to work.
So if R75 is getting hot at idle, then way too much current is going through it.

[Sloan]

Previously, I went through and checked almost every component of the power section for faults and didn't really come up with anything. I just got the R75 problem today when I went back to work on this thing and I'm not sure why that much current would be hitting it.

The owner actually picked up the unfixed amp today so unfortunately I won't be able to finish this, even though I made some money it's frustrating to leave a problem unsolved.

[Jazz P Bass]

My feeling is that TR5 was shorted.

[rikosound]

Hello everyone!

I'm having the same problem. Power stage blowing, the BDVs with -25 V aprox. In IC8 i read almost 13V between pin 6 and 7 and between the ground...
I've tested all transistor outside, but have no more ideas...