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Harman Kardon PM660 problems

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dcline

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I usually repair guitar amplifiers. But I've been getting into repairing vintage receivers and other devises. I'm self taught and have been doing pretty good so far. I just repaired a Harman Kardon PM660. Well, repaired it for the most part. I had to replace a couple output transistors, a driver transistor and a couple 1/4 watt resistors. It's out in my shop playing right now and sounds great. But sometimes you turn it on and it doesn't work. I get a periodic pop like a breaker is kicking on and off. I play with it a bit and turned it off, let it sit a minute and it started working again. And as I said it's sounds fine. Been playing for about 20 minutes now.
Anyone have any ideas what might be causing this problem? I don't want to give it back to the owner and have this happen to him. The speaker impedance is correct. Thanks in advance.
 
I'm starting to notice that the amp is kicking out some major heat. And not it kicks out once in a while then kicks back in. Any ideas?
 
It sounds like you haven't repaired it - there's probably more than the parts you have changed that are faulty. It's unusual that just changing the output transistors repairs an amp, there are usually other faults earlier on in the DC chain which caused them to blow in the first place.
 
Check for DC on the outputs. The click your hearing is probably the DC protection relay kicking out. awlso check the voltage across the fusible emitter resistors. That voltage is probably creeping up as the amplifier is warming up. Some of the small resistors usually change value in the output stage.

Coupling caps anywhere can cause it too in certain amplifiers.
 
We'll do.
It's definitely heating up. Works fine for a while when you first turn it on then the problems starts as it gets warm. The cooling fins in the output section are really kicking out some heat. I shut it down this time before it had a chance to start kicking off the breaker.
I've checked every component in the driver board. I guess I should assume it's in the power board eh? Thanks again
 
What voltage to you get for DC at the output - No signal?

What voltage do you get across the Sandstone emitter resistors (Around 0.22 to 0.5 ohms) or so when you initially turn the unit on and say 5 minutes later when they are too hot? What value are the resistors? Compare to the other channel?

Did you adjust the bias after making the repairs?

Is this a design where matched pairs matter?
 
Pm660

hellow guys, this looks like one nasty Amp. I hope you could get arronud this one. I my self would think a little more on doing work on it. It looks like you have a popcorn problem. this is due to differential amp stage. Don't forget like our friend said "Bias also".
 

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That's not for the faint of heart. I used to troubleshoot the amps that a commercial serving center couldn't handle. I did get a 400 W/ch disco Amp where the components puddled on the PC board. 5 shops would not touch it before I got it. Someone put a direct short on the outputs.

Ti give you an idea of 'problems", I built my own varient of the "Leach Amp" (Google)i n the early 80's.I made a mirror image of the board and didnn't realize it. I made a few trace changes and swapped the NPNand PNP's and it finally worked. This AMP boasts a 0-800 kHz frequency response and 100 v/us rise time unrolled off.

I also had to build a slow turn on ckt to handle 50,000 uf of capacitance.
What's the voltage across between the C and E of Q449
 
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