"Dr RaTsTaR" wrote in message
...
On Mon, 15 Oct 2007 23:47:21 GMT, "Arfa Daily"
wrote:


"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



The generator is producing a sine wave at 800Hz, 6.1 mV AC. I am
injecting it directly into the power amp input, which is where the
preamp was plugged into.

The Standard 130 watt power amp module is used in different Peavey
products as a back end. In this configuration it uses a guitar amp
front end. The two metal chassis plates are the front (preamp) and
back (power amp) of a wooden box that contains them.

This amp module is built with a PA front end with 4 inputs, or it can
be built into it's own self-contained unit with a simple front end as
a power booster, which is really just a power amp.

Thanks for your interest,,
Doc RatstaR

I've had a look at your results, and the first thing that I feel that I must
say, is that the output waveform from your test circuit is simply not good
enough to do any serious evaluations of what is going on. It really does
need to be a sine wave. How can I put it ? What you are doing, is a bit like
trying to find a steering fault on your car, with one of the tyres flat. You
either need to have a proper signal generator, where you can properly
control the output level and frequency, or use a second sound card with a
signal generator program running. You can easily build yourself a generator
using what's generically called a 'function generator' IC or a less
sophisticated one using a phase shift oscillator.

Looking at your results, it's very hard to say if what you are seeing is
fundamentally 'wrong'. All amps will have a degree of residual hum, albeit
usually very small. However, when you start looking at output stages with
the vertical scale cranked up into the mV / cm range, the stuff on there can
start to look horrendous. What you see in "Main output no input" is the
residual hum, and at +/- 30mV or so, I would not have immediately declared
this excessive. Some crossover distortion is apparent, but at such a low
drive level from the hum, it may be perfectly natural for the output stage
to produce such.

It's hard to say exactly what we are seeing on "main out signal", because
the output level is still extremely low, compared to what you would have
here for even a modest 'in use' output level. On that point, I am surprised
that you think that the input level for the power amp is likely to be 10mV.
I can buy that for the maximum input level to the preamp, but the input to
the power amp is likely to be volts. Do you have some documentation that
suggests such a low input level ?

As far as your output tests go, a drift in the DC conditions of a few mV
either side of zero over a period of a half hour, is quite normal, and would
not usually indicate that there is any problem in the output stage worth
looking for. The drive waveforms for the bases of the outputs do look pretty
poor, but actually look more like an overdrive condition than anything else.

All in all, it's very hard to say what is going on with the results you have
shown. First you need to get a proper sine wave with an adjustable output.
You need to load the output stage, either with a proper impedance speaker
(not very friendly to the neighbours) or a dummy load of some description.
For the purposes of getting proper test results, power resistors stacked up
in parallel to get to 8 ohms or whatever, will do. You then need to drive
the output module to a point where it is delivering some power - that is
*volts* of output across a load, not mV. If you don't have enough output
from your generator to do this, then make use of the amp's guitar front end.
Drive it with 5mV and 'scope the output of the preamp to make sure that you
have got everything set such that it is delivering a clean, but large drive
signal to the power amp. The amp's "master", being normally on the end of
the final mixdown from the preamp, will then serve as your level control.

Once you have a clean easily identifiable waveform going in (that's a sine
wave ... ) and some watts being developed across a load, you will be in a
position to make a realistic evaluation of what - if anything - is actually
going wrong.

As far as doing a blanket replacement of the electrolytics is concerned, I
would strongly advise against that, if you are not an experienced engineer.
I would agree that electrolytics are a source of problems in most electronic
equipment, and that engineers working on this stuff will sometimes use the
shotgun approach where these caps are in a known troublesome environment,
such as a switch mode power supply, but with kit like this, you are far
better off to actually find the fault, if any, rather than risk creating
another which, with the best will in the world, is easily done, particularly
by someone inexperienced. The slightest slip-up in a DC coupled output stage
such as this, can be disatrous, resulting in the magic smoke escaping from
many of the components ... If you really want to learn proper fault finding,
it's not the way to go.

I'm sure that there are others on here who have also been mending PA amps
for many years, who would agree with most if not all of what I've said, so I
hope you will take it as valid observations and advice.

Arfa