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Old 10th January 2011, 09:03 AM   #1
jitter is offline jitter  Netherlands
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Default another intermittent amp problem (PM80SE)

6+ years after fixing my PM80SE and it's giving me problems again. Yet again it's of an intermittent nature.

I replaced the notorious STK3102IV in September 2004 (see intermittent amp problem (PM80SE)) and has been working fine ever since. Or rather, had been, beacause a few weeks ago the speaker relays wouldn't switch on after power up and there was a smell of a power resistor getting hot. The resistor getting hot was in the voltage devider for the headphone outputs in the L channel. A quick check with a DMM revealed what I suspected: a massive amount of DC on the output, luckily the protection circuit did its work.

I desoldered the resistor getting hot to prevent it burning up and started to trace the problem. I quickly found that the bases of the power transistors in the offending channel were close to a power rail. Luckily the fault occurred in such a way that they didn't blow. After bleeding off residual charge in the power caps I measured the diode drops of the transistors for opens or shorts, but they were fine.

While measuring the outputs of the STK-module, at first I thought I measured something odd. The DC-offsets on the output pins should be 1.7 and -1.7 V according to the schematic, I thought I measured something like 10 V on the L channel on one of the output pins. When I measured again to make sure, there didn't seem to be anything strange anymore, and pressing the speaker button indeed made the relays click.

The fault seems to have cured itself for now, but given the intermittent failure, I suspect the STK-module. Is this a reasonable assumption, or is another fault more likely?
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Old 10th January 2011, 11:12 AM   #2
singa is offline singa  Singapore
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yes,it's a reasonable assumption if the stk is the driver for the power transistors.
The burnt headphone resistor should alert you of the potential failure hope
it does not take out your precious speakers?
But do check the protection circuit as well. regards.
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Old 10th January 2011, 01:34 PM   #3
jaycee is offline jaycee  United Kingdom
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The STK is indeed the "front end" of the amp. Unfortunately these particular modules were not very reliable and prone to failure.

I think if I was dead set on saving the amp, I'd probably make my own discrete replacement for the STK module, but that is a lot of design work.
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Old 11th January 2011, 04:49 PM   #4
jitter is offline jitter  Netherlands
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Quote:
Originally Posted by jaycee View Post
The STK is indeed the "front end" of the amp. Unfortunately these particular modules were not very reliable and prone to failure.

I think if I was dead set on saving the amp, I'd probably make my own discrete replacement for the STK module, but that is a lot of design work.
Maybe not. I had a look at the schematic of a successor, the PM8000. This one has a discrete voltage amplifier followed by a pretty similar power stage (it too can be set to class AB or A). PSU voltages for voltage amp and power amp are the same as in the PM80(SE).

I wouldn't be surprised if I could duplicate the discrete voltage amp and use it in my PM80SE as a replacement for the STK3102IV, but I'd like to have some input before I would attempt that. I attached the relevant parts of the schematics of both the PM80(SE) and PM8000.

Could this be done without major modifications to the rest of my PM80SE?
Attached Images
File Type: gif PM80SE.gif (35.7 KB, 76 views)
File Type: gif PM7000_PM8000.gif (80.2 KB, 73 views)
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Old 2nd April 2012, 10:12 AM   #5
bousie is offline bousie  Australia
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Hi, I have a PM80mkII which I've owned since new and a PM82 both similar to the PM80 but use discretes rather than the STK3102. Diagrams for these are available and could be a solution?

Unfortunately I'm having a very similar issue with the PM80mkII as the commonly reported popping problems of the PM80. Just on one chanel and progressively getting worse. The protection circuitry hasn't cut in yet though (however when it started I didn't give it too much chance too.)

From what I've read most of these problems with the PM80 have been caused bu the STK3102. I'm yet to track down the cause in the mkII.

If anyone has any suggestions, it'd be appreciated.

Cheers
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Old 30th April 2012, 04:21 PM   #6
jitter is offline jitter  Netherlands
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The achilles heel of the PM80(SE) is indeed the STK3102, that doesn't mean that discretes can't fail, though. But I'd suggest checking for cracked solder joints first.
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Old 30th April 2012, 08:42 PM   #7
bousie is offline bousie  Australia
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Thnaks, I resoldered everything I could and thought it was fixed as it seemed fine for a little while....but it wasn't.

I've just ordered some 2SC3067's as there is one for each channel. Will put these in next and see what happens.
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