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This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Hi, I'm relatively new to the forum as I have a problem with my (recently acquired) Beomaster.
I have just finished re-capping the board and as far as I knew, all was running smoothly. Until I felt that the heat sink was running much hotter in one area than the rest of it. Along with this, the left channel has significantly less treble than the right.
Upon further investigation, the overheating heat sink is that of IC200 (in the left channel) which runs 15-20 C hotter than the rest (IC: 201, 300, 301).
I'm prepared to replace the idle current trimmers, but I would have thought that if one of them was bad, this would cause both IC's in the left channel (200, 201) to overheat?
Any thoughts?
Thanks, Gaspar
Double check your no-load current adjustment. It sounds like you have it set too high. You should be at 12mV (0.012V) across the emitter resistors. It should run cool to warm. Not hot.
I've ordered some trimmers, replacing asap...
I forgot to add...Recheck the no-load current again after the Beomaster has been on for ten minutes (with volume at zero and no speaker connection). It should still be at 12mV across R256.
Ok sure, will do. Thanks
Ok, so the trimmers have arrived and I have replaced the old ones. But before I removed the old ones I measured the mV across the emitter resistors, as this is how the machine was set when working (at least, the right channel). It gave a reading of 1.2mV!? Not 12...
So I replace them with the new trimmers, and get a range of 2.1 to 0.9mV (max to min). I set them to 1.2, exactly as I found the old ones.
It sounds unusual to get 1.2 and not 12. I am however using a Fluke multimeter, and I definitely know what mV are.
The situation remains the same, channel R is fine but one transistor from L is overheating. Except now I know that the problem isn't the trimmers.
What else could it be?
Are you performing the no-load (idle) current measurement/adjustment per the service manual? ...No speakers connected, receiver cold (off for a while), volume control at zero? Then measure millivolts across 2R256? Then another measurement across 2R356? If the test conditions are per the manual and you are not able to get 12mV then you have other problems with your amplifier.
I've followed exactly those steps. I've since swapped transistors over to see if maybe it was the transistor that was malfunctioning. The problem persisted in the same heat sink as before.
I guess it's just a case of trial and error, testing step by step. Fortunately I have the patience for this, and (although not as good) a spare Yamaha amp to fill in for the BM.
Many thanks for all your help so far!