"bg" wrote in message ...

Dr RaTsTaR wrote in message ...
Hello,

I am attempting to repair a Peavey Standard 130 watt power module and
would appreciate some advice. I am not experienced in amplifier or
electronics repair but I have taken all the electronics courses that
the local Jr College (Mesa Jr. College, San Diego) had to offer. I
have a lot of theory but not any practical experience.

The amplifier will pass a music signal to the speaker when I patch an
audio signal directly into it, but the music is cracked and fuzzy,
very distorted. Here we have bypassed the pre-amp front end
completely. This unit is in a Standard Power Pak, which is a guitar
setup.

I have the schematics and have been poking around with a computer
sound card oscilloscope (Daqarta). I have also constructed a simple
tone generator so that I can use a stable signal to trace.

What problems might allow a simple amplifier like this to still work
even though it is distorting the signal? I have checked the main power
transistors in circuit using a diode checker on my Beckman DMM.

I recently successfully repaired a Peavey XR600E using this same diode
checker to identify the 3 blown transistors in the left side. I also
had the right channel to compare readings to, so that was a great
help.

Any help you can provide would be greatly appreciated.

Thank you.

d0ct0r RaTsTaR
The most common failure in a power amp is the output stages or the power
supply. There will be a transistor or several transistors in parallel that
reproduce the positive half cycle, and one or more to reproduce the
negative
half cycle. If one side goes out , you still have sound but only one phase
of the sine wave, thus, big distortion. Check all of the output
transistors
with an ohmeter. Check the driver transistors too. Check the low valued
emitter resistors (usually less than an ohm).
bg


Also, be sure that your 'tone generator' is not over-driving the input of
whatever stage you are connecting to, and is completely AC coupled, and is
not loading or otherwise shifting any bias supply that may be present at
whatever point you are connecting in at, which is a little unclear from your
post. I am assuming that you are going in at the input to the power amp when
you say that the preamp is "bypassed"? Does this amp have an effects send /
return jack pair ? This is usually a good place to 'break in' post preamp /
pre power amp. Also, as James suggested, looking at the shape of the
distorted output aginst what you are putting in, will give a very good clue
as to what is going wrong. Just as a matter of interest, what shape wave is
your home-built tone generator producing ? It must be a sine wave for
general testing. Most audio stages do not like having a square wave thrust
up them ...

Arfa