A friend has a Rotel RA-820BX4 amp that had stopped working, one channel misbehaving, etc.
I took it to have a look. After turning on it became clear that one side was cooking ... the collector resistors were cooking (and had been for a while, the paint on them was flaking).
I checked biasing, and the L channel was way off. The R channel seemed dead. Did a careful check, and resoldered some dodgy solders, and it came back. Checked bias again, and fired it up.
Seems to work fine. Yay, me.
But the left channel is still running very hot on a few parts - the Right is cool as anything. Anyone have any thoughts?
Partial schematic below. Resistors that get hot are R642 and R644 and transistors (which get "ouch" hot), Q624 and 626 (they're on their own heatsink which gets distinctly warm):
I took it to have a look. After turning on it became clear that one side was cooking ... the collector resistors were cooking (and had been for a while, the paint on them was flaking).
I checked biasing, and the L channel was way off. The R channel seemed dead. Did a careful check, and resoldered some dodgy solders, and it came back. Checked bias again, and fired it up.
Seems to work fine. Yay, me.
But the left channel is still running very hot on a few parts - the Right is cool as anything. Anyone have any thoughts?
Partial schematic below. Resistors that get hot are R642 and R644 and transistors (which get "ouch" hot), Q624 and 626 (they're on their own heatsink which gets distinctly warm):
