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Old 24th October 2013, 06:59 PM   #11
Hanatsu is offline Hanatsu  Sweden
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Join Date: Apr 2011
Dunno if I should post in a new thread, but since it almost covers the same issue I'll post here

I found an old Coustic AMP360 and one channel is out. Reading 27V+ on left channel speaker output and the red "peak" LED lights up as soon as I power it up, even without load. At idle it draws ~1A so there's no abnormal current draw.

I'm not very good at troubleshooting electronics but I assumed it was shorted output transistors and replaced them (TIP142/147) but it did not help, turns out the old transistors was ok. I measured the voltage over the output transistors with an o-scope and it shows a steady DC 27V over both Base, Collector and Emitter. Removed the transistors again and there was still 27V DC on the base, I suppose it should measure close to 0 when there's so signal? I measured the resistance of the small transistors around the output stage but nothing seems shorted.

No visible sign of damage anywhere on the board, checked all the big caps, the bias pot and the small transistor in the middle of the output transistors but they seem all to be ok.

Anyone got a schematic over the board? Any ideas?
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