Even in our clinic, sometimes staff get sick. This guy is our main player, so we had to take care of him on a first place.
For some time, there were some problems with it, that decreased pleasure of listening to it. The biggest and the most annoing was network hum. After measurements, it turned out, that power lines were affected. Naturally, first suspects were input filtering capacitors. I measured them, surprisingly they were more than fine. Even despite their's age, they stil kept parameters.
While searching the Internet, I have found interesting article about power suppling this kind of devices and the most typical mistkes in desing of their's construction. It turned out, that during last changes made in this model, I had broken it. Two wires providing AC supply were aligned with other wires. I have twisted them (green wires on photo) and moved aside from others. After that, problem disappeared almost completely.
I have measured also parameters of all electrolityc capacitors in power ending, in power supply and replaced them with new one
Another sympthom, that made itself known, was occasional crackling noise in one of channels. It was disappearing after moving filters, leverages switches. I discovered, that when problem occur, amplitude of a signal in this channel is also lower than in second one. This kind of behavour may be caused by dirty contacts. I decided to disassemble all of them and clean. During work, I made a decision to replace short wires with stiff, silver, bended wire creating 3D tracks.
As a second module for closer look I took input selector. There are more sophisticated switches with many wires attached. It would be hard to assemble them back again without notes and pictures.
The last module was equalizer. I wanted avoid disassembling this unit in the future, so I've taken out all potentiometers, cleaned and greased them. I've checked all capacitors. Most of them needed to be replaced, because of their's big value of ESR, which in this case is very important. I've also fixed tracks and soldering
Having a bit of free time I wanted to pimp look of this power amplifier. On the back side of metal case, many small screws can be found. They became a bit rusty, because previous owner had kept this device in some wet place. Of course, this 'red' steel didn't have an influence on sound, but it simply bothered me. I have scrathed them and painted on black. Now they look as brand new.
All PCBs were cleaned in ultrasonic cleaner. Thanks to that, boards got 'brand new' look.