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Thanks Enzo. Im doing as you suggested.
To recap...
It was passing a small amount of DC to the speaker. Enough to pull the cone to full extension. Didn't measure.
I pulled the amp section and found Q7 and Q12 bad as well as R26 bad.
I replaced them and no more DC on the speaker. but unit goes into compression almost instantly and sound is distorted and very weak.
Tested all other transistors out of circute.
I put a 200hz signal into the Aux input.
At the 5 pin connector I have a clean sine wave ac signal all the way into R36.
At U14 (pin14) the signal is very distorted. I ordered a new U1. Used a TL084C in place of the TL074C. No change.
So I did some research and found that there may be issues with U2 (peavey 87478) which is now obsolete.
Contacted peavey and they said it was replaced with a 39095750 OTA adaptor assembly. I orderd and installed. the problem is worse. it goes into compression instantly with any signal at input.
I did pull the connector to the commpressor stage. sound does not change.
I know the problem is in the amp section.
I did notice a sawtooth wave form present on one of the 52V busses about 2Vpp.
I will double check both sets of plus and minus 15 / 16 volt supplys. I did this and recall they were fine.
I have tryed to run the unit without U2 but with little change.
Following from ENZO
A tiny DC is not a problem, and I mean a tenth of a volt or something. If it measures in volts, then yes, fix it. Until we know for sure the amp is not making DC, get rid of the speaker and any speaker loads. Apparently that has passed.
I have to say, I have few problems with that compressor circuit. Once in a while it fails, but not nearly as often as other things.
If it makes things more convenient, you don;t need the preamp panel. I usually lay the amp panel down flat and clip a test signal to the Molex pins where the preamp cable plugs on, eliminating the preamp cable.
U2 is the compressor chip, that 87478 IC. Pull it, the amp works without it, of course without compression now. We will put it back when the amp is fixed. Your amp cannot put out a clean signal, and U1d distorts the signal the opposite way trying in vain to compensate. That is your distortion at U1d. Pin 14. The compressor is coming on because of all this, just doing its job. Not that it matters with U2 pulled, but in servicing, either pull the four pin molex to the compressor on/off switch or at least turn it to OFF.
You found Q12 and R26 burned. And Q7. In that case, there is a REAL good possibility Q2 is bad, and probably someone over in Q13-16 land. Check Q2 aqnd R54,55,56. Those resistors like to open when transistors fail. Q7 fails when currents try to flow through it after other parts open. Check CR14 for shorts or low junction drop.
Q1, Q7 are limiters and not neded for basic amplification. If they might be involved, unsolder one end of CR14 and CR10 and lift them from the circuit. That disables those two circuits. We'll restore that later, and recheck all those parts too.
There are two long 0.1 ohm 10w resistors between the two three-pin Molexs to the transistor board. Make sure they are not open. (probably OK) Unplug the two thre wire cables there, and on the transistor board, check EVERY resistor for open. Then ther are two groups of four transistors. Note the end one of each group is wired as a driver to the other three. Check them all.
Just to humor me, make sure the two resistors top center on the page R19,R20 are OK.
Once any open resistors and bad semiconductors are found and fixed, fire up the amp. No load yet. The schematic has DC voltage references all over it. Check them all and see that they are all more or less close to the drawing. Note: most are with respect to ground, but a couple are across a resistor, NOT to ground.
Once that is all OK< apply a test signal. 200Hz is fine, I use 100Hz myself. Hell of a lot less ear fatigue than 1kHz. Scope the output bus and run it up. DO you get a full and clean waveform? If you do, then apply a load or speaker and try again. Does it now work? Or does it fail under load? A clean waveform that collapses under load is a different problem from a waveform that is never clean.
I should have entered this near the top. Ther are two sets of +/-15v. The regulated set is for the preamp. The zener pair is for the power amp itself. Make sure both sets are OK, don;t measure the +/-15 at the preamp and assume they are all OK. A missing or low 15v on the power amp ICs will cause trouble
To recap...
It was passing a small amount of DC to the speaker. Enough to pull the cone to full extension. Didn't measure.
I pulled the amp section and found Q7 and Q12 bad as well as R26 bad.
I replaced them and no more DC on the speaker. but unit goes into compression almost instantly and sound is distorted and very weak.
Tested all other transistors out of circute.
I put a 200hz signal into the Aux input.
At the 5 pin connector I have a clean sine wave ac signal all the way into R36.
At U14 (pin14) the signal is very distorted. I ordered a new U1. Used a TL084C in place of the TL074C. No change.
So I did some research and found that there may be issues with U2 (peavey 87478) which is now obsolete.
Contacted peavey and they said it was replaced with a 39095750 OTA adaptor assembly. I orderd and installed. the problem is worse. it goes into compression instantly with any signal at input.
I did pull the connector to the commpressor stage. sound does not change.
I know the problem is in the amp section.
I did notice a sawtooth wave form present on one of the 52V busses about 2Vpp.
I will double check both sets of plus and minus 15 / 16 volt supplys. I did this and recall they were fine.
I have tryed to run the unit without U2 but with little change.
Following from ENZO
A tiny DC is not a problem, and I mean a tenth of a volt or something. If it measures in volts, then yes, fix it. Until we know for sure the amp is not making DC, get rid of the speaker and any speaker loads. Apparently that has passed.
I have to say, I have few problems with that compressor circuit. Once in a while it fails, but not nearly as often as other things.
If it makes things more convenient, you don;t need the preamp panel. I usually lay the amp panel down flat and clip a test signal to the Molex pins where the preamp cable plugs on, eliminating the preamp cable.
U2 is the compressor chip, that 87478 IC. Pull it, the amp works without it, of course without compression now. We will put it back when the amp is fixed. Your amp cannot put out a clean signal, and U1d distorts the signal the opposite way trying in vain to compensate. That is your distortion at U1d. Pin 14. The compressor is coming on because of all this, just doing its job. Not that it matters with U2 pulled, but in servicing, either pull the four pin molex to the compressor on/off switch or at least turn it to OFF.
You found Q12 and R26 burned. And Q7. In that case, there is a REAL good possibility Q2 is bad, and probably someone over in Q13-16 land. Check Q2 aqnd R54,55,56. Those resistors like to open when transistors fail. Q7 fails when currents try to flow through it after other parts open. Check CR14 for shorts or low junction drop.
Q1, Q7 are limiters and not neded for basic amplification. If they might be involved, unsolder one end of CR14 and CR10 and lift them from the circuit. That disables those two circuits. We'll restore that later, and recheck all those parts too.
There are two long 0.1 ohm 10w resistors between the two three-pin Molexs to the transistor board. Make sure they are not open. (probably OK) Unplug the two thre wire cables there, and on the transistor board, check EVERY resistor for open. Then ther are two groups of four transistors. Note the end one of each group is wired as a driver to the other three. Check them all.
Just to humor me, make sure the two resistors top center on the page R19,R20 are OK.
Once any open resistors and bad semiconductors are found and fixed, fire up the amp. No load yet. The schematic has DC voltage references all over it. Check them all and see that they are all more or less close to the drawing. Note: most are with respect to ground, but a couple are across a resistor, NOT to ground.
Once that is all OK< apply a test signal. 200Hz is fine, I use 100Hz myself. Hell of a lot less ear fatigue than 1kHz. Scope the output bus and run it up. DO you get a full and clean waveform? If you do, then apply a load or speaker and try again. Does it now work? Or does it fail under load? A clean waveform that collapses under load is a different problem from a waveform that is never clean.
I should have entered this near the top. Ther are two sets of +/-15v. The regulated set is for the preamp. The zener pair is for the power amp itself. Make sure both sets are OK, don;t measure the +/-15 at the preamp and assume they are all OK. A missing or low 15v on the power amp ICs will cause trouble
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