Peavey scem

[Valvehead]

Yes -i have searched...cant find it...

need a scem AND a layout (pcb isnt labeled) for a :

Peavey Mark III bass head 400B , this one is dated 1978 .

pcb says 400 B/G module

Id rather get it here than wait on Peavey to send one

[dmeek]

Here it is:
Peavey Mk III Bass .pdf

[Valvehead]

Hmmm...somethin aint jivin ' ....

the layout shows 4 parts in a row c26 / r6 / L1 / r44

on my pcb--see pic -- looks like 4 resistors to me ? Right next to the Q1

also--the layout says Q1 but the scem says Q12 for the transistor with the corroded leg in picture

the scem also says R6 is the Pre gain pot .

where the layout shows a resistor .

Im attaching a pic of my pcb

my pcb says 400B/G module on it.

[Jazz P Bass]

Peavey 400 B/G Schematic

Here is the 400B/G schematic.

[Enzo]

In your photo, I see the four parts. The left one is a cap. See a number of the "resistors" on the board are pale green? Those are little ceramic caps.

L1 and R44 are in parallel, you can see the traces through the board in your pix even. The original inductor looked like a little resistor also, but in most of the amps, the inductor was replaced with another 4.7 resistor during production. Compare this amp to the VERY similar 400BH board, which shows the two parallel 4.7 ohm resistors, no little inductor.


I see Q12 in the photo. Q1 would be out of the picture to the upper left, and is a small TO92 type. The two TO126 guys on the small heat sinks are Q2 and Q12, just as the layout says. They are your predrivers. Q12 is next to your mystery parts. I mean, someone even wrote Q12 right on the thing.


R6 on the POWER AMP is a 100k resistor. Upper left corner of drawing, pin 5 of U1.

R6 on the PREAMP is the pre gain control.


They are two separate boards, two separate circuits.

[Valvehead]

ok--gotcha . Yes- i wrote Q12 on it

do you know what that Q12 , 5332 would cross to ? Looking at that badly corroded leg i want to replace it.

Also wondering what caused R6 to burn up ...

[Enzo]

I can't think of anything in the world would blow R6 up. But the burnt resistor in your group of four parts is R61, not R6. And that almost certainly means Q12 is shorted. And if Q1 is bad, I'd get REAL suspicious of the larger transistors on the output board.

I would probably replace Q2 along with Q12 because the failure current path probably included it. And alsway check the resistors associated with any bad transistor, so R26 as wellas that pair of parallel 4.7.

5332 is an MJE253 and 5331 is an MJE243, but really, I;d just CALL the parts folks at Peavey and get the real parts.

[Valvehead]

hmmm... when i zoom in on the pdf --the layout . it looks like R6 to me --i dont see a 1 ? are you sure on that enzo ?

[Enzo]

I'll bet my lunch money on it.

If in doubt, follow the traces from th part and see what it connects to on the board.

Click on the schematic Jazz posted in post #4, that layout is a lot less dark and cloudy than the one in post#2, the 61 is then obvious.

[Valvehead]

ok thanks --looks like a 47 ohm

[Valvehead]

OK--- Ive replaced the previous mentioned parts. Amp powers up and has a loud hum. With volumes on 0 .

There are 2 pcb with 4 output transistors on each. Unplug one--still hums

Plug it back in , and unplug the other pcb---hum is gone.

Ive checked all the big 5W? resistors--ok

with both boards plugged in, all the DC voltages match perfectly on all pins of the 8 output transistors with amp on .

Where should I start looking ??

[Enzo]

First off, I bet that loud hum is DC on your speaker leads. Does the speaker cone move one direction and stay there? That is a good way to melt speakers. DO not connect a speaker until yo0u are sure the amp is not making DC.


It is not a good idea to operate the amp with those power transistors disconnected. But since you did it, the section you disconnected to make the hum stop- probably has a shorted transistor or more.