Marshall short

[Valvehead]

Working on a Marshall Valvestate model 8100 .

I replaced a burnt output chip BDV64 , and also the other 2 chips on the heat sink. (TR8 , TR9 , TR10 ) .

Plus 2 resistors that looked cooked. Powered on with my current limiter and all looked good.

Plugged it into the wall and turned it on, looking good, then played a little...for about 20 seconds , as soon as i turned it up loud , the fuse blew and there we have smoke.

Several resistors got hot and some smoked... R113 R115 R118 . now powering up on the limiter i have a massive short. assuming i took out one or more of my new output chips ?

After replacing the above parts, what would cause this to happen ? Whats weird is the amp acted normal for almost 30 seconds !

scem :

http://www.drtube.com/schematics/marshall/pc0689p.pdf

[The Dude]

It could be that there was a stressed component you missed or possibly you got some "fakes" (see THIS thread). Unfortunately, your back to square one at this point. Check and double check everything. It only takes one defective component in a direct coupled amp to upset the whole apple cart.

[J M Fahey]

Yes, or they were *WAY* overbiased, which would explain the short lifespan.

[Amp Kat]

This is a really unstable amp and Imo it's due to a poorly designed power supply that dissipates to much heat. If you replace the parts again try firing it up without the output trannies in it and verify that all the voltages are equal on both sides of the supply rails of the outputs and the op-amp +/- 15 volt rails. Another thing that happens is the tube clip for the 12AX7 digs into the board and shorts one of the rails to ground so make sure that isn't happening. You obviously still had a short somewhere which is not an uncommon thing with this amp.

[Jazz P Bass]

Something that I noticed in the post was the lack of any voltage measurement info.

[g1]

R113 is for the power supply to the heater of the preamp tube. It is not related to the power amp failure but I'm wondering if it could be caused by the issue with the tube clip that AmpKat mentioned.

[Valvehead]

Something that I noticed in the post was the lack of any voltage measurement info.

Cant do that with a big short .

i will power it up with the output transistors pulled out and start measuring , also will check that clip -- thanks guys

[Enzo]

It your outputs repeatedly blow, I replace them with TIP142 and TIP147. Even Marshall sometimes does that. If you stock those for Fender and Crate solid state repair, its the same parts. Check the emitter ballasts, the 0.33 ohm ones. When the outputs burn up, i always replace the drivers. I don't care if they "test" OK, yours are MPSA06 and MPSA56, and they cost maybe 20-30 cents apiece. Just change them. All four of them. The drivers each have 220 ohm emitter resistors, check them. And there are 10 ohm grid resistors into the outputs, check those.

I don;t like just replacing everything in an amp - the shotgun approach - as a repair technique, but when the outputs fail, they stress the driver circuits too, and as long as the board is out, it is false economy to not change out a dollar's worth of parts to be sure.

The TIP29 TR9 is your bias circuit, check it and the two resistors with it. TIP29 is maybe 60-70 cents, why not just replace it too.

If your output jack is not shorted or other issues like that, and your new output transistors pop, then you haven;t really completely repaired it. That is why I suggest new parts in the drive circuits leading up to the final stage.

Note R83, R78, over near the op amp. See they connect to the 15v rails. TR5 TR6 and associated circuits depend on those voltages being there. So make sure they are. Those are zener derived off the main rails, and if one of those zeners is collapsing it will throw your amp off to some DC, which when loaded will stress the outputs. SO like all power supply voltages, they must be OK.

[Valvehead]

Did some testing today -

I removed the 3 main output chips , TR8 , TR9 , TR10 .
measuring at the holes, tr8 has 40v . 41v , 2v .
tr10 has -40v , -40v , -2v

tr9 has -40v , -40v , -40v

I noticed with amp running (no out chips) i have 3 resistors getting very hot .
R112
R101
R102

measuring across ZD1 i have -15v / 0v
across ZD2 is +15v , 0v

legs of TR5 have -40v, -1v , -1.5v

TR6 has 40v , 2v , 1v

so r101/102 getting hot would indicate something in the power supply ?

the tube clip looked ok--not touching anything

[Valvehead]

dont know if this helps pinpoint anything, but i clipped the ends of r101/102 that go to the 2 zeners, the resistors stay cold.

also, i pulled the tube out and r112/113 stay cold .


preamp scem :

http://www.amparchives.com/Amp%20Arc..._8080_8100.pdf

[Jazz P Bass]

dont know if this helps pinpoint anything, but i clipped the ends of r101/102 that go to the 2 zeners, the resistors stay cold.

also, i pulled the tube out and r112/113 stay cold .


preamp scem :

http://www.amparchives.com/Amp%20Arc..._8080_8100.pdf

I don't know where you are going with this.
If you clipped out the zener's, you no longer have voltage regulation at the +15 & -15 bias junction.
The resistor AND the zener make up the regulation circuit.
And R112/ 113 certainly will be cold if there is no tube installed.
No current is flowing through them?

[J M Fahey]

dont know if this helps pinpoint anything, but i clipped the ends of r101/102 that go to the 2 zeners, the resistors stay cold.
also, i pulled the tube out and r112/113 stay cold .

No, that's not related to your problem, they normally work hot.

1) test those transistors you pulled, now that they are outside.
Replace them in the amp.
2) build and use a lamp bulb limiter.
Re-measure all transistor voltages.
Label them accordingly: base, emitter, collector.
Post transistor and rail voltages.

The basic idea is that *any* transistor must have around +0.6V base to emitter and positive voltage collector to emitter if NPN and opposite for PNP.

So go, measure and post, for all transistors.
Also voltage to ground on all IC pins.
Check datasheets to know which pin is what.

[Valvehead]

thanks JM , i do have a bulb limiter . So i need to measure while the limiter is in ? wont that give me low readings everywhere ?

I just got a big box of parts for the amp today from Mouser including the 3 new chips that sit on the heat sink (tr8/9/10 )

[Jazz P Bass]

Yes they will be low.
Hopefully, so will the lamp.
The amp, as designed works.
Pay attention to the details.
Check everything.
It will work.

[Valvehead]

ok--bear with me guys.. i had time to do a few tests. i clipped the legs of the old transistors, so i cant stick them back in.

I installed 3 new ones = TR8 TR9 TR10

powered on with the current limiter , and each leg to ground i get :

tr8 b= 1v , c = 37v , e= 3mv / ( base to emit = 1v)

tr9 b= -0.5v , c= 1v , e= -1v / (base to emit = 0.6v)

tr10 b= -1v , c= -37v , e= -3mv / (base to emit = -1v)

so tr8 and tr 10 look normal--yes?


measuring TR7 , it doesnt match the other small transistors , it has +32v / +32v/ +1v on its legs--does that look odd ?

i can post / take measurements of the rest as soon as i get time (tr4/5/6/11/12 )

all the op amp ( 8 pin ) chips test normal with +13v and -13v .