Peavey Amp - Distorting and -ve DC Offset

[kolbep]

Another one landed on my bench today,
a Peavey Mixing Amplifier.

It is a mono amplifier, so I cannot compare channels.
It is fine at very low levels, but as soon as you turn it up a bit, the music makes a Crunching Sound (distorting).

The amp module is nice oldschool with transistors galore.

When it was playing music, i waved my hot air station over the board, and when a 22uF elect cap (in the 1st stage of the amp) heated up, the volume got louder (but still distorting).

I then scoped the speaker output. With No input signal, the output is a smooth - 2.3vdc!
When I up the signal, then the waveform looks clean (at low level), but it is still centered around - 2.3v.

Any idea where to look (I seem to think that the -ve offset voltage is somehow key to figuring this out.)

P

I believe that this schematic is for the board (or a hightly similar one)

[HalfSpace]

Hi Kolbep,

Sounds like an output transistor/transistors could be damaged. Disconnect them from the circuit and measure them on a multimeter diode test, they should all read the same if they’re ok. Also check the emitter resistors (.33 5W) and the two output driver transistors. The driver transistors can lose gain if damaged and will not be able the drive the output transistors to full power.

HalfSpace 

[SeanB]

Change Q1 to Q5, it is most likely one is slightly leaky. This will affect the bias of the output stage. Use a 2200uF 63V capacitor to DC isolate your test speaker, so you do not cook it either. Change all small electrolytic caps in the input as well, leaky caps there will also give a DC offset.

Output stage damage generally results in open circuit emitter resistors and blown transistors, leaky ones will just run the opposite side very hot till they fail short circuit.

[kolbep]

I ran the amp for a quick bit this morning.
The Offset voltage was gone, and it was not distorting too much.

Voltage out of the transformer was + and - 29VAC

I disconnected the speaker, and scoped the output as I turned it up (while playing music). When the output got to a certain point, the +ve part of the waveform started clipping (at about 4v), and would not go past that, whilst the -ve part of the waveform still managed to go to about -8v, before it started smoking, and then started to give out a mains hum.
The smoking was either from the cement resistors in the power section, or from one of the main smoothing caps.
Then it started giving a 50hz hum on the output..   OOPS!

I pulled out all the driver transistors, and tested them. One of the transistors did not correpond to the rest.
I cannot find the datasheet for the transistors (they are MJ1502AG BM0743 MEX). But here is my testing :

Code: [Select]

T1 T2 T3 T4
+C -1 0.039v inf inf inf
+C -2 0v inf inf inf
-C +1 0.039v 0.455 0.457 0.447
-C +2 0v inf inf inf
+1 -2 0.039v 0.476 0.476 0.476
-1 +2 0.039v inf inf inf

Transistor Testing using Diode Range on Multimeter
+ and - define which probe of the Multimeter was on which pin
c is for the can of the transistor

What would an equivalent transistor be?

I see somebody previously replaced the Power and Driver transistors, so I am suspect now...

[kolbep]

And, This thing is a thing of beauty.
Lets play Spot the Cap. Try and spot the cap that I replaced...

[SeanB]

http://www.alldatasheet.com/datasheet-pdf/pdf/172350/ONSEMI/MJ15021G.html

Possibly the 1 is distorted and looks like an A. Looks like the emitter resistors are open circuit ( or are now or have gone high) and the current limiting worked till the transistors went short circuit. Time to change the lot and look at the resistors that will probably have been blown open. Check main bridge rectifier as well as it probably did not like the surge and went short circuit on one or more diodes.

going to guess the cap is the one with the green wire leg...........

[kolbep]

Well spotted...