Peavey Centurion Mark III head with high DC voltage output

Thread Starter

Ale97

Joined Apr 16, 2021
6
Hello, I have a 1980 Peavey Centurion Mark III head that was buried in my garage unused for 15 years.
I knew it had problems (it's not mine, i got it from a friend who was throwing it) so before doing something i checked all the components and everything looked great.
Once done I plugged it to an old speaker I don't care about and the output was -35V DC... Just imagine the noise.

All the components look perfect and surprisingly there was near to no dust on them. The TO-3 transistors (Q6-Q7-Q10-Q11) look good but I don't know if they are dead.
No fry smell, no sparks...

I also tested pre amp output lines and it's all fine, <3mV DC, also measured in AC with bass plugged and the output rises as i play something.

I have all the schematics of the preamp and power amp circuits.
(If it's ok with copyrights and all, I can upload also the board design that i got from radiomuseum)

I'm totally lost. Any suggestions would be appreciated!
 

Attachments

abrsvc

Joined Jun 16, 2018
71
One or more of the output transistors are shorted. You can not tell this by looking. You need a meter to check them. This is a common failure usually a result of a bad speaker or cable. Remove the TO3 outputs and check them for shorts.
 

Thread Starter

Ale97

Joined Apr 16, 2021
6
One or more of the output transistors are shorted. You can not tell this by looking. You need a meter to check them. This is a common failure usually a result of a bad speaker or cable. Remove the TO3 outputs and check them for shorts.
Thank you for the advice, I tested all of them and they seem to be good. Vbe and Vbc are 0.47V on all of them. In reverse they are open loop and also C-E, E-C gives open loop.
 

abrsvc

Joined Jun 16, 2018
71
With the transistors removed, measure the voltage at each lead (E,C and B) and post the voltages here. The -35 has to come from somewhere. I'll take a look at the schematic tomorrow and try to get you a few test points, but for now, check the above.
 

MrChips

Joined Oct 2, 2009
23,232
Don't plug in any instruments. Don't connect any speakers.
First check for proper power supply voltages, +42V, -42V, +15V, -15V.
 

Thread Starter

Ale97

Joined Apr 16, 2021
6
Don't plug in any instruments. Don't connect any speakers.
First check for proper power supply voltages, +42V, -42V, +15V, -15V.
Should I solder those TO-3 transistors back before measuring?

Update I checked all the bjts and I found that 2 of them (can't find any datasheet) have only 1-2 open loops, if I pick 2 pins they give 2 voltages in forward bias and reverse bias. The others give 4 OL and 0.6V Vbe, Vbc.
 

Thread Starter

Ale97

Joined Apr 16, 2021
6
With the transistors removed, measure the voltage at each lead (E,C and B) and post the voltages here. The -35 has to come from somewhere. I'll take a look at the schematic tomorrow and try to get you a few test points, but for now, check the above.
The measures I posted are with transistors removed and meter set to diode testing mode.
Results straight from meter are:
~470 Base to Emitter
~500 Base to Collector
Everything else gives OL.
Values are pretty the same on all of them
 

MrChips

Joined Oct 2, 2009
23,232
Should I solder those TO-3 transistors back before measuring?

Update I checked all the bjts and I found that 2 of them (can't find any datasheet) have only 1-2 open loops, if I pick 2 pins they give 2 voltages in forward bias and reverse bias. The others give 4 OL and 0.6V Vbe, Vbc.
Leave the output transistors Q6, Q7, Q10, Q11 off the board for now.
 

abrsvc

Joined Jun 16, 2018
71
Measure the connections on the board where the transistors normally reside. Turn on the amp and measure each position. Note: do this with the output transistors REMOVED. These will be voltage measurements.

Test each transistor while removed using the meter in diode mode (I assume that is what you already posted).
The measures I posted are with transistors removed and meter set to diode testing mode.
Results straight from meter are:
~470 Base to Emitter
~500 Base to Collector
Everything else gives OL.
Values are pretty the same on all of them
You keep missing the instructions here. Put the meter in DC Voltage mode. With the transistors removed. Check the voltages on each pin (E,C,B) with the voltmeter and the power ON. Post those values here. DO NOT use Diode mode with the unit on.
 

Thread Starter

Ale97

Joined Apr 16, 2021
6
I waited a bit so I got the chance to test the board with a variac.
First I tested red supply alone(+-15V and +-42V)
+-42 is there.
I found that U1 and U2 had -15V ok and on the +15V pin there were only +4V.
Followed the trace and the problem was the zener diode (CR26, 1N4744) limiting at +4V.
Interesting that touching it randomly fixed everything, obv I plan to replace it.

I also changed the capacitor next to it (C25) because it was cooked. Changed also C26.
Now my question is... Can a broken capacitor in parallel with the zener drop 11V?

Tomorrow I'm going to test again with these new caps and see what I get on those to-3 pads and orange ps line.
The diagrams I posted in first place are for older versions, the one I have is 81 version.
260C_81.PNG


You keep missing the instructions here. Put the meter in DC Voltage mode. With the transistors removed. Check the voltages on each pin (E,C,B) with the voltmeter and the power ON. Post those values here. DO NOT use Diode mode with the unit on.
Sorry hahaha I'm not native english so I didn't quite understand what you meant.
 

Thread Starter

Ale97

Joined Apr 16, 2021
6
Solved! Works so damn well...
The problem was that broken zener, changed it with a 5W one instead of 1W.
Thank you for the suggestions!
 
Top