Polytone minibrute

[bigdrums925]

Hi all: Looking for a schematic for a Polytone Minibrute bass amp. I have replaced the faulty transistors and is out of the self destruct mode, but signal gets lost before making it through the power amp to the speaker. Hope I don't have to do this without a diagram. After entering the power amp, the signal goes through a 10uf 50v cap and to a couple of TO3 transistors which chop up the signal looking something like square waves. Then enters another transistor but nothing after that. All three of those transistors check ok and I think the voltage rails are good too.

[nosaj]

Hi all: Looking for a schematic for a Polytone Minibrute bass amp. I have replaced the faulty transistors and is out of the self destruct mode, but signal gets lost before making it through the power amp to the speaker. Hope I don't have to do this without a diagram. After entering the power amp, the signal goes through a 10uf 50v cap and to a couple of TO3 transistors which chop up the signal looking something like square waves. Then enters another transistor but nothing after that. All three of those transistors check ok and I think the voltage rails are good too.

You kind of answered your own question If signal enters a transistor the n nothing after that . That should be a really big clue.

nosaj
schems


http://music-electronics-forum.com/t34687/

[Jazz P Bass]

First off, what signal amplitude are you using going into the power amp (at the 10uf cap)?

If that signal looks clean, then you must explore why the 'choppy, squaring' is occuring.

The transistor pair after the cap is the Long Tailed Pair input. (highly doubt that they are a TO3 case)


After that the signal goes to the PNP transistor Q2.

Q2s Collector then drives Q3 & Q4 (output stage drivers)

I would gather a few static (no signal) voltages on the three leads of Q2-Q4.

That may tell you where the upset is.

PowerAmp PA378B.pdf

It would help if you stated the failure so far.

You may have damage to the ballast resistors, the bias circuit or even the LTP transistors.

[bigdrums925]

Sorry they are TO-225AA or ISO 125 style. Signal is coming in as a clean sine wave
Voltages on Q3 are: E 0 C 33.6 B -.5
Voltages on Q4 are: E -1.7 C -33.6 B 0

When I got the amp Q5 was shorted and I replaced it.
Both q3 & Q4 were shorted and replaced them.
Someone else had tried to repair this and many of the molex connecters were mixed up, so I straightened that out
I popped r19 while test power up on the variac so replaced that 3.3K

At this point there is almost no current draw running up the variac to full 120VAC, but the +/- 33.6 rails are there.
I believe I installed Q3 & Q4 correctly one Q3 has the metal back touching the diode, Q4 has the plastic front touching the diode.
Pre amp seems to pass signal fine

[Jazz P Bass]

Voltages on Q2?

[bigdrums925]

Q2 voltages:
E 33.4
C 33.3
B 32.7

[Jazz P Bass]

Something is wrong with those voltages.

Q2 is a PNP transistor.
Pulling the Base low will turn on the Collector.

With a Base voltage of 32.7Vdc, that does not qualify as 'low'.
So I think it is shorted E to C.

Plus, your last post of voltages does not agree with Q2's Collector being at 33.3 Vdc.
It ties to Q3's Base through a 100 ohm resistor (R11).
(Q2's Collector and Q3's Base will ultimately be around 0.5 Vdc)
I would check R11 as it and/or a pcb trace appears to be open.