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 http://www.diyaudio.com/forums/solid-state/39736-intermittent-amp-problem-pm80se.html) 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?
I replaced the notorious STK3102IV in September 2004 (see http://www.diyaudio.com/forums/solid-state/39736-intermittent-amp-problem-pm80se.html) 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?
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?
Attachments
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
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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