Quad Amplifier Repairs
Quad 99 Power Amplifier
This was blowing one of the secondary fuses. I expected this to be difficult to work on but was surprised how easy it was to disassemble (apart from the big transformer). The two transistors on the heat sink were toast, hoping no others had expired I replaced them and et voila it was sorted. The transistors were MJL21193\94 and are getting expensive nowadays.
Quad 405
This was sounding funny! Looking inside there were the two 10ohm resistors burnt out in the Zobel network (resistor and capacitor in series across the output). Unfortunately the 0V return for them was on the input 0V and not on the output 0V. Moving the 0V resistor lead to the junction of the two capacitors across the +/-DC supply remedied this. I also found around 50mV on the left output, dry joints on the board removed this; the tantalum capacitor and the bootstrap capacitor were also replaced.
Quad 303
This one was dead, the transistors on the heatsink had expired as had the drivers and a few others. This had been DADA'ed, the PCB's were not original and the transistors on the PSU board were toast too. I always say if your going to blow something up make sure you do a good job!.
Quad 405-2
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.
Quad 303
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.
Quad 303
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.
Quad 405-2
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.