Marantz pm26 receiver/amplifier cut out (chip overheat issue)

[dominicM]

I have a Marantz pm26 receiver from the 80's that cuts out after a few minutes as well as makes speakers pop every few seconds during the cutout period. There's also heavy distortion for a few seconds when it starts playing again. Having a fan directed at he board with cover removed stops the cutouts.The Q702 chip is definitely overheating as it becomes too hot to touch very quickly regardless of volume level even with the fan running. Q701 (to the left) only gets a little warm. I am testing with only one speaker connected but it makes no difference whether it is connected to the right or left channel, Q702 still heats up. The question is what makes it overheat? The caps look fine though I know that doesn't mean they are. What could be the cause and what is first thing to try? Q702 has this on the side: NEC QPCI270H  Japan L639C but there's nothing that I could find on it.

[mzacharias]

I would start by checking the solder connections both at the driver IC's and elsewhere; output transistors, bias transistor, etc. Bias can be checked by measuring milli-volts across the emitter resistor. Base-emitter voltages at the associated output transistors is often revealing. Capacitors associated with the 1270H could be dried out, and of course you could always swap the two IC's to see if the problem goes over to the other channel.

[WaveyDipole]

Your chip might be a uPC1270H

http://pdf1.alldatasheet.com/datasheet-pdf/view/84773/NEC/UPC1270H.html

I couldn't find a circuit for the pm26, but the pm25 amplifier uses them.

[dominicM]

Yes, I believe it is,  it was hard to see properly.

[oldway]

My God, what a holy shit compared with 70's Marantz ! 

[SpaceCow]

My God, what a holy shit compared with 70's Marantz ! 

If that's a comparable model, then there was some serious backtracking! :/

[dominicM]

I would start by checking the solder connections both at the driver IC's and elsewhere; output transistors, bias transistor, etc. Bias can be checked by measuring milli-volts across the emitter resistor. Base-emitter voltages at the associated output transistors is often revealing. Capacitors associated with the 1270H could be dried out, and of course you could always swap the two IC's to see if the problem goes over to the other channel.

I tested a few caps around the IC's and they are within range though I don't have an ESR meter yet so can't be sure and no matching replacements now either. Could you be more specific which transistors to test and what values to look out for? For educational purposes how would a bad cap affect the IC to cause it to heat up?

[oldway]

For not to loose your time, replace the driver IC from the faulty channel first and see if it solve the problem.
Test the two output transistors also or, if you don't know how to do this, replace them both.
A bad cap could only affect the IC if it leaks.

[Seekonk]

My God, what a holy shit compared with 70's Marantz ! 

If that's a comparable model, then there was some serious backtracking! :/

I forget when he told me he sold the company to two brothers, the company had gotten too large for him to manage.  But, by the early 70's all the equipment was being made by Standard Radio.

Origin was making some temperature sensing diodes around that time that had intermittent problems. Do your tests with a 100W lamp in series with line power. That will save your outputs from being destroyed as toy tinker around.

[dominicM]

For not to loose your time, replace the driver IC from the faulty channel first and see if it solve the problem.
Test the two output transistors also or, if you don't know how to do this, replace them both.
A bad cap could only affect the IC if it leaks.

Swapped the chips and the same chip overheats so it looks like it is the IC itself that is faulty. That being said the sound is now quite distorted on both speakers so I am not sure if replacing the chip alone will even fix that. Any ideas as to why this may have happened?

[mzacharias]

When something deteriorates after I have just done something I always go back and see if I could have done something wrong. It happens. Cannot troubleshoot remotely. A service manual, a digital voltmeter and actual troubleshooting are key to solving problems like this. The chips aren't that expensive, might not hurt to replace them both (but get them from a reliable supplier). Voltage readings at the output transistors are important for troubleshooting purposes - especially the readings at the BASE of each output transistor, as this will tell the operating condition of the device (whether properly biased mainly). This is common with distortion problems.

[dominicM]

When something deteriorates after I have just done something I always go back and see if I could have done something wrong. It happens. Cannot troubleshoot remotely. A service manual, a digital voltmeter and actual troubleshooting are key to solving problems like this. The chips aren't that expensive, might not hurt to replace them both (but get them from a reliable supplier). Voltage readings at the output transistors are important for troubleshooting purposes - especially the readings at the BASE of each output transistor, as this will tell the operating condition of the device (whether properly biased mainly). This is common with distortion problems.

There is no manual that I can find anywhere. I tried again this morning and it works fine I think what happened was that it had overheated when I tested yesterday, I only kept it on a few seconds after all for that exact reason. The only place I found the chip is on ebay, fake ones from china are like 2 euros while seeming genuine are 13e, I might just get a used chip since this amp isn't worth spending much on. How does that logic sound? Maybe I should just get a few of fake ones instead? The amp won't be used much or at high power.

[Seekonk]

Why do people hate fakes so much.  This is low technology stuff and easy to fabricate. And if your hearing is that good, you wouldn't have owned this unit in the first place. Just buy a spare.

[dominicM]

I can only guess that people have to spend time/money to repair issues cause by fake parts and that bad experience leads to hate of fake parts in the future. Basic human psychology, bad experiences leave more of an impact than good ones. I would thing the quality shouldn't be affected otherwise that is a good reason to hate fake parts. Considering price fake part in this case is probably a good option as reliability is not that important.

[mzacharias]

Why do people hate fakes so much.  This is low technology stuff and easy to fabricate. And if your hearing is that good, you wouldn't have owned this unit in the first place. Just buy a spare.

If you'd ever tried multiple non-functioning STK replacement audio amp modules only to find they weren't made right in the first place (by installing a new old stock) you'd change your opinion pretty quick. Some newer "fakes" are pretty good, but there are still issues. I once had a bunch of in-line op-amps which had at least an 80% failure rate. Guess what - fakes. I've had other fakes with EMPTY dies - nothing at all inside.

[mzacharias]

There is no manual that I can find anywhere. I tried again this morning and it works fine I think what happened was that it had overheated when I tested yesterday, I only kept it on a few seconds after all for that exact reason. The only place I found the chip is on ebay, fake ones from china are like 2 euros while seeming genuine are 13e, I might just get a used chip since this amp isn't worth spending much on. How does that logic sound? Maybe I should just get a few of fake ones instead? The amp won't be used much or at high power.
[/quote]

I have a PM-25 manual that may be essentially the same (uses the same drivers anyway). Maybe just the U.S. version. I can send it by email. Not great quality but better than nothing.

mark_zacharias@sbcglobal.net

[dominicM]

Why do people hate fakes so much.  This is low technology stuff and easy to fabricate. And if your hearing is that good, you wouldn't have owned this unit in the first place. Just buy a spare.

If you'd ever tried multiple non-functioning STK replacement audio amp modules only to find they weren't made right in the first place (by installing a new old stock) you'd change your opinion pretty quick. Some newer "fakes" are pretty good, but there are still issues. I once had a bunch of in-line op-amps which had at least an 80% failure rate. Guess what - fakes. I've had other fakes with EMPTY dies - nothing at all inside.

I wasn't saying it was unjustified hatred just that it's human nature and I do understand it. Problem with genuine parts is you end up paying 10x time for postage than for the part and there aren't that many places to buy genuine parts in the first place. I did have 3 cheap power supplies fail all because of a transistor, genuine parts so far have outlasted the fake parts so yes fake parts should be avoided when possible but that's no always possible or economical.