The initial problems were sound cut-off at the amplified speaker terminals when loading the amplifier and scratchy potentiometers/s必利勁
After first inspection I found several suspects which could be the source of the problems. All had to be addressed for ruling out:
– between 25 and 38mV DC between speaker terminals
– intermittent contact in the pre-power amp switch on the back
– intermittent contact in the muting switch on the front
– corrosion on the stabilizer/protection board and both driver boards
– overheating of components on the stabilizer/protection board and both driver boards
The first step was to clean the contacts, switches and potentiometers on the front panel and the switch on the back. It’s always good to have them cleansed so they will function without pops and noises, but, as I already thought, it didn’t solve the problems of the audio cut-off. Next was a closer inspection of the corrosion. It was a result of aging glue, applied on the bigger capacitors to fasten them properly on the circuit boards. To get the stabilizer/protection board and both the driver boards out, I had to disassemble half of the amplifier and even de-solder some wires from the stabilizer/protection board and mark them so I would know where to solder them back. It turned out that the glue on the stabilizer/protection board had been eating into some resistors and diodes, also causing damage to several solder joints of those damaged components. I decided to remove all the glued components, clean the boards from the old corrosive glue and replace the parts. Since a few more capacitors where suspect of leaking and heating I decided to replace all the electrolytic capacitors on the stabilizer/protection board and both the driver boards with low leakage caps, whereof several with a bigger capacitance and voltage rating, to reduce the stress on those components. (In my opinion several capacitors where chosen with slightly limited ratings.) The last step was to re-flow a lot of solder joints while I had the boards out. More than a few had signs of intermittent contact or the possibility to future damages.
After the repairs I calibrated the bias-current on both driver boards to factory specs and adjusted the voltage between the speaker terminals to 0 Volts.
This amplifier was here before, where I had cut out the thermal fuse inside one of the transformers. It was open and I was not able to repair it, therefore removed the fuse from the circuit.