Peavey Musician Mark III

[Jazz P Bass]

I have a Peavey Mark III on the bench that is driving me batty.

The low side predriver, Q7 emitter resistor (R32) gets real hot & then burns Only when a load is applied to the output.

All resistor paths have been verified.
All the transistors on the output are new.
No transistor (removed from board) checks as bad.
The idle voltages all agree with the schematic.
Main rails & the 15 volt rails are steady & ripple free.
The output voltage is in the millivolts.

I have subbed in a new TL074 & even removed U3.

The output looks perfect without a load.
When a load is applied , the bottom half of the sine wave collapses (@4 vAC output).
Then R32 bakes.

Any insights on what I am missing would be greatly appreciated.

I tried calling Peavey but I think they are on vacation.

http://music-electronics-forum.com/a...ian_mk_iii.pdf

[g1]

Ok, this is the schematic on page 5 of pdf, not page 3.
Can you monitor R54&31 to see whether the current is going that way or through Q12 base path?

[J M Fahey]

Sorry but in the linked schematic Q7 is not a driver but short protection and I canīt find R32 , definitely not at the emitter of any driver.

EDIT: that means a lot of voltage is applied to R32 which is a BE resistor so it can never have even 1V across it, so logic is either connection to power pack Q12 to Q15 is open or Q12 is open base.

[Jazz P Bass]

Ok, this is the schematic on page 5 of pdf, not page 3.
Can you monitor R54&31 to see whether the current is going that way or through Q12 base path?

That I will check.
Note: the actual board is the 'page 3' schematic, but it has the 'bass' stuff.
I found that it got in the way.
So, yes, page 5 it is.

[Enzo]

So you have a 400GH, not a 400B/G?

Please specify which.

So the amp works without load, and cannot provide current to the load on the negative side. I'll believe that the outputs are OK if you already changed them. But i have to think the neg side outputs are not conducting, that loadless output was coming from the predrivers Q6 Q7. My immediate first suspect is the three wire molex connectors between the main PA board and the output transistor board. I'd be thinking the connection to V- was open.

[Jazz P Bass]

Hold the horses.
Major blow out happened & I have to source the cement resistors.
Enzo, thanks for the tip.
I will certainly be making sure there is a solid connection on both 3 pin headers.

Edit: Yes the back of the amp is marked 'Series 400G'.
The main & the output boards are marked '400B/G'.

[Jazz P Bass]

Rap this one up!

Enzo nailed it.

The interboard connector at the B- had a 'spread' pin.

Thankyou all.

Note: in retrospect I should have realized exactly why the 47 ohm (1/4 watt at that) resistor was burning.
Without a proper high voltage rail on the output transistors, that poor 1/4 watter had to supply the load.