Quad Amplifier Repairs
Main caps bulging and fuses blown in right channel. Output transistor was found to be short circuit. Replaced the output transistors (with 2N3773) the drivers and on board electrolytic caps. The 15V Zener for the IC -15V was short circuit, IC faulty and the 100µF tantalum was a resistor.
This one was not working with hum through the speakers and one channel down. Found the smoothing capacitors leaking, this was an early model either 6886 or 9889 serial no! The o\p transistors and regulator transistors were s\c, replaced these and noticed the unusual bias setting the 22k resistor to the bootstrap circuit. I decided to change to the standard transistor as used in the later model just as well as the 1S920 diodes were short circuit in one channel. I replaced the main capacitors with much smaller ones on a PCB board.
Blown transistor was the reported fault. On switch on there was no regulated 67 V, needed the 3 transistors replacing in the regulator circuit and a diode, then found LHS o\p transistors were s\c and the drivers, the temp compensation transistor was replaced and the pre-set for the quiescent set. Okay after all that.
This was pulsing the speaker on one channel. I found one of the outputs was s\c and replaced with 2SD676's. I had to replace drivers with TIP42C, lots of dry joints on both channels, okay after.