On Mon, 15 Oct 2007 22:14:47 -0400, Meat Plow
wrote:
On Mon, 15 Oct 2007 23:47:21 +0000, 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
What model is the amp?
On Mon, 15 Oct 2007 13:11:19 -0700, Lord Valve
wrote:
Load the amp with an 8-ohm resistor. Input 1 KHz sine.
What does the trace look like? Any DC on the output?
Any crackling or spitting noises? Hum? Excessive
AC line current draw?
LV
Thank you for the tips, Lord Valve.
All measurements taken with the tone generator built from a circuit
posted on
www.geofex.com. Tone generator outputs 800 Hz at 6.1 mV AC.
(Original circuit runs around 1 kHz at 10 mV AC. I had to substitute
some resistors/caps.) Tone generator is connected directly to the
input of the Peavey 260 power amp module, which seems to be a generic
power unit for several different Peavey products.
Trace measurements were taken with the DOS version of DAQARTA, a sound
card oscilloscope program from
www.daqarta.com.
DC ON THE OUTPUT - NOISES
Generator off, amplifier energized - Initially quiet, 0.1 mV DC on
output.
20 seconds -2.1 mV DC, low buzzing/humming noise begins, remains
constant. No other noise present at any time.
1 minute -6.4 mV
2 minutes -13 mV
5 minutes -19.7 mV
10 minutes -24 mV
30 minutes -25 mV
30 minutes, generator on - -31 mV DC
The DC present on the speaker output changes from around -25 mV at
idle to +25 mV upon shutdown, at which time it rapidly declines, over
10 to 20 seconds, to 0.
EXCESSIVE AC CURRENT DRAW -
It has not blown the 5 amp fuse that is standard for this unit.
I did not check for hot caps. I will run the unit for 30 minutes,
unplug, bleed down the caps, and then feel for unusual temperatures,
just as an exercize.
DEATH CAP -
I see in the schematic and on the board that there is a .022 uF 600 V
cap connected to the primary side of the power transformer input lead
and to ground. A dpdt switch allows one to select which of the
incoming power leads will connect to this cap. This sounds like the
death cap mentioned by Mike Schway in the AGA General FAQ, item 70.
The amp does have a 3-wire power supply cord factory connected and the
green does go directly to ground. Should I remove this cap? There is
also a .022 uF 125 VAC cap across the two power lines which I know is
necessary.
TRACES -
I am including the traces in a zip file. I hope it is alright to post
here. If the system won't allow me to post a binary I will be right
back with a hyperlink to the file. Is a hyperlink the preferred way to
do this?
EXPANDED means that the time base displayed has been shortened from 25
ms to 6 ms to show more detail in the individual waveform.
TONE GENERATOR OUTPUT * what it is. The waveform is a little crumpled
at the bottom. Still, the amplifier should be able to reproduce this
without a problem. The wave is consistent, and that is what matters.
MAIN OUTPUT - NO INPUT * shows the speaker out with no input. There
is what appears to be a 60 cycle signal peaking between +20 mV and -20
mV. Sharp spikes of released energy occur whenever the trace crosses
the zero line.
MAIN OUTPUT - SIGNAL * is the speaker output waveform with the input
signal energized. The tops of the waveforms are rounded, and peak
around 50 to 100 mV. The bottoms are narrow V-shaped spikes that
extend down to -150 to -200 mV.
COLLECTOR 6530 WITH INPUT * displays an 800 Hz waveform with a
distinct 60 cycle spike. The mate to this transistor,
COLLECTOR 6533 WITH INPUT * shows the same waveform minus the 60 Hz
spike.
431C BASE WITH INPUT EXPANDED * for whatever it is worth. Is this some
kind of crossover point for the positive and negative cycles?
A RECAP - (In more ways than one....)
According to AGA, Technical FAQ, article 21, item 7, line 5, a Mr.
Zenon Holtz advises us that, (regarding noisy amps and electrolytics)
"Usually, in really old gear (over 25 years), just replace them at the
first sign of problems - they have a finite life."
There is a date stamped on the inside rear of the metal mounting plate
that reads "Aug 18, 1977". It doesn't appear that the amp has ever
been worked on.
So I guess the first order of the day is to do the recap job. Should I
replace the mylar and ceramic disc caps, too? I am a total newbie
here, so any and all advice is welcome. I have the time to do it
right. Is there any difference between cap makers? Are there any
favorite suppliers?
I have ordered a few parts from Peavey in the past, they are very fast
and do not charge for shipping. Perhaps they would be the place.
Do the 'scope traces show any signs of other trouble? Or do all the
glitches relate to crappy cappys?
All the diode junctions I could test in circuit seemed to show logical
results, considering they are often paralleled with other components.
However, when I take the board apart to do the caps I intend to check
what components that I can.
Thanks for any and all help,
d0ct0r rAtstAr
Apparently I can't post binaries to this newsgroup. Here is a link to
the 'scope traces if anyone is interested.
http://www.intergate.com/~gander/Peavey%20Standard.zip
Doc RatS