It never fails to amaze me what people or "techs" do to equipment!!!

premiumplus

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Well, I got my dc protect board from Don this week and took my new-to-me PL700B to the bench for installation.
Uneventful until I hooked it up, and heard the most horrible distortion...
Back downstairs we go, and I find a bulging 22uF 100v cap on the driver board.
The "tech" had installed it backwards, and it's been working like that since I had it...at least 2 weeks!
So I replaced it and the distortion got better but it's still there. Some other damage exists.
Now, this is the amp I just replaced all the output transistors with new ON semi's, and is waiting for the complete White Oak treatment.
I think it's going to wait until I get the new board installed before I work on it anymore. It just doesn't make sense to me to diagnose and repair the existing driver board when it's going to be replaced in a couple of weeks.

Thought I'd share that with you all. It blows my mind what people will do to equipment...and more than that, it blows my mind that the equipment will survive the "repairs"!!!
Dave
 
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premiumplus said:
Well, I got my dc protect board from Don this week and took my new-to-me PL700B to the bench for installation.
Uneventful until I hooked it up, and heard the most horrible distortion...
Back downstairs we go, and I find a bulging 22uF 100v cap on the driver board.
The "tech" had installed it backwards, and it's been working like that since I had it...at least 2 weeks!
So I replaced it and the distortion got better but it's still there. Some other damage exists.
Now, this is the amp I just replaced all the output transistors with new ON semi's, and is waiting for the complete White Oak treatment.
I think it's going to wait until I get the new board installed before I work on it anymore. It just doesn't make sense to me to diagnose and repair the existing driver board when it's going to be replaced in a couple of weeks.

Thought I'd share that with you all. It blows my mind what people will do to equipment...and more than that, it blows my mind that the equipment will survive the "repairs"!!!
Dave
Click to expand...
Been down that road before, once to many times. I understand your frustration.. I do the repairs myself. I have gotten pretty good at it.
 
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I've been amazed at just how badly I can F&^% one up with just a couple probe slips!!
 
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premiumplus said:
Well, I got my dc protect board from Don this week and took my new-to-me PL700B to the bench for installation.
Uneventful until I hooked it up, and heard the most horrible distortion...
Back downstairs we go, and I find a bulging 22uF 100v cap on the driver board.
The "tech" had installed it backwards, and it's been working like that since I had it...at least 2 weeks!
So I replaced it and the distortion got better but it's still there. Some other damage exists.
Now, this is the amp I just replaced all the output transistors with new ON semi's, and is waiting for the complete White Oak treatment.
I think it's going to wait until I get the new board installed before I work on it anymore. It just doesn't make sense to me to diagnose and repair the existing driver board when it's going to be replaced in a couple of weeks.

Thought I'd share that with you all. It blows my mind what people will do to equipment...and more than that, it blows my mind that the equipment will survive the "repairs"!!!
Dave
Click to expand...
Which cap was it Dave? I always recommend fixing what you have first before proceeding into upgrade. Sometimes you think that the upgrade will fix it but more often than not, the damage done lies in some other place and you end up with the same or worse problem.
 
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Gepetto said:
Which cap was it Dave? I always recommend fixing what you have first before proceeding into upgrade. Sometimes you think that the upgrade will fix it but more often than not, the damage done lies in some other place and you end up with the same or worse problem.
Click to expand...
It was C3 (47uF, 100 volt on a PL-14 driver board) that someone had installed backwards, and it was all bulged out. Funny thing is that it didn’t destroy the cap, it still measures around 20 uF and will even hold a charge.
I replaced it and the distortion got better, but not much.
If I feed it a sine wave from my HP function generator, the amp reproduces it well, from near dc out to 40 kHz or so. But music distorts like crazy. Any peak signal gets mangled. It’s strange. I did a quick check of all the diodes and transistors on the board, and they all have standard diode voltage drops of between 0.45 and 0.7 volts. Except the base/emitter junction of Q8/Q9, which is expected with about 300 ohms resistance between the junctions. I also looked at the electrolytics with a ESR meter, and they all look okay…but they are old. It’s just not feasible to replace them when I’m going to White Oak the amp in a couple of weeks.
The amp worked fine before I put the dc protection board in, and the board is behaving like it should; the relays cut off around 7 or 8 Hz, and then kicks back in as I raise the frequency. There isn’t anything on the board that would cause distortion, is there? The left channel is worse than the right one, but distortion is present in both channels.
I checked with my scope for ultrasonic oscillation, and there is none.
So do you have any ideas that I might look at? Oh, additionally, a week or so ago I replaced all the output transistors with ON MJ21196G’s, and checked the drivers, RCA 410’s. It’s been sounding fine, at least for running the subwoofer in a bi-amped Dahlquist system. It hasn't been driven hard, and it's biased at 360mV. Bias is steady as a rock, and the power supply is a clean +/- 108 vdc.
 
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The symptoms sound like one of the 5A AGX rail fuses blown (guessing the positive one given the events that you found).

The amp will still play but will not produce any appreciable drive current on the upper half.

Could be wrong but would check that before going any further.
 
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Yes, it's always a good idea to check the power supply first. Fuses are all good. In fact, when I got the amp I changed the fuses and cleaned and deoxited the fuse holders. At any rate, the fuses are good...I checked them yesterday.
But you're right, it does seem like there may be a sponginess somewhere in the power supply line. I inspected for frayed wires or cold solder joints on the backplane, and on the driver board. The board has had a lot of work done to it over the years and I suspect there is a cold joint or a bad trace on it. It needs to be completely resoldered, but the PL boards, at least in my experience, tend to lose trace adhesion and the foil falls off the board after being reworked.
 
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The 14's aren't as bad as the Pl400C, or the big board in the 3300, but you're right, they suck.
 
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premiumplus said:
Yes, it's always a good idea to check the power supply first. Fuses are all good. In fact, when I got the amp I changed the fuses and cleaned and deoxited the fuse holders. At any rate, the fuses are good...I checked them yesterday.
But you're right, it does seem like there may be a sponginess somewhere in the power supply line. I inspected for frayed wires or cold solder joints on the backplane, and on the driver board. The board has had a lot of work done to it over the years and I suspect there is a cold joint or a bad trace on it. It needs to be completely resoldered, but the PL boards, at least in my experience, tend to lose trace adhesion and the foil falls off the board after being reworked.
Click to expand...
Then next check D3
 
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premiumplus said:
All the diodes check good with forward drops of around 0.6 volts.
The transistors look good too.
I haven't pulled any of the RCA 410's yet.
Click to expand...
Question Dave
Had you tried out this 700 before you installed the DC protect? If so was it distorted?
 
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Gepetto said:
Question Dave
Had you tried out this 700 before you installed the DC protect? If so was it distorted?
Click to expand...
I've been using it to drive one subwoofer for a week or so. It has been perfect in that application.
I didn't want to hook it up to my main speakers without dc protection.
But it sounded fine.
This was after I put in all new outputs, so I think they should be good. Unless they were damaged after I installed the dc protect.
But this is the third dc protect board I've installed in the last month, and it's working the way it's designed to, as far as I can see so far.
 
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premiumplus said:
I've been using it to drive one subwoofer for a week or so. It has been perfect in that application.
I didn't want to hook it up to my main speakers without dc protection.
But it sounded fine.
This was after I put in all new outputs, so I think they should be good. Unless they were damaged after I installed the dc protect.
But this is the third dc protect board I've installed in the last month, and it's working the way it's designed to, as far as I can see so far.
Click to expand...
Not sure you would notice in a sub application, usually that is 80Hz and below.
 
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I'm done for this evening, gentlemen...I appreciate all the input, and I'll check back in the morning.
I'm going to have a good night's sleep, have breakfast and coffee, lots of coffee...and I ll report back after another round in the morning.
Please do let me know what you think, and I'll give your ideas a shot in the AM and get back to ya tomorrow when I know more.
Thanks,
Dave
 
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premiumplus said:
I'm done for this evening, gentlemen...I appreciate all the input, and I'll check back in the morning.
I'm going to have a good night's sleep, have breakfast and coffee, lots of coffee...and I ll report back after another round in the morning.
Please do let me know what you think, and I'll give your ideas a shot in the AM and get back to ya tomorrow when I know more.
Thanks,
Dave
Click to expand...
We will be here tomorrow Dave. Good night.
 
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