On Thu, 18 Oct 2007 15:40:46 -0600, "bg" wrote:


Dr RaTsTaR wrote in message ...
On Mon, 15 Oct 2007 12:39:10 -0600, "bg" wrote:


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


How would you go about checking the 10 watt .33 ohm resistors? I am
not sure my DMM will measure anything that low.

I can check the diode PN junctions with the diode checker on the DMM.

The distortion is not real big, but it is present. As per my latest
post here, I believe the culprit is leaky old electrolytic capacitors,
but it may not be the only trouble.

Do you commonly unsolder different components to measure them? I am
new to this game.

Thanks for your help.

Doc RatstaR

The emitter resistors tend to open up completely or have high resistance. As
long as you get continuity below 1 ohm you should be OK. I would desolder
the power transistors which lifts one leg of the emitter resistors, seeing
as you want to test the power transistors out of circuit anyway.
Your DMM diode checker can be used to test the power transistors for opens
and shorts which is a very common failure.
bg


Thank you for this information, BG. They test around .9 ohms, which is
out of the 10% tolerance, but this reading may be due to my meter and
lead connections. I will call it continuity below 1 ohm.

The main power transistors read OK as measured across the diode
setting on my Beckman DMM. I read 500 - 600 mV drop across the
appropriate pn junctions. These are out-of-circuit readings. I plan on
pulling and checking the smaller transistors today.

The power amp can play music, but it sounds like an am radio station
that is not quite tuned in to the center of the station. I did the
chopstick test and it suddenly "tuned in" when I struck a certain
transistor, but it relapsed a few minutes later and I haven't been
able to do that again. It may have been coincidence.

Someone recommended resoldering all connections of old circuit boards
to relieve the accumulated effects of thermal stress. What do you
think of this?

Rats DocStar