This time a Technics surround receiver on the work bench:
It’s a SA-DX850 with a power issue, on startup it gives F 70 Error code.
I’ve found a post on a German website about the SA-DX750 with similar error. On this post they were referring to a PCB connection problem. I took the service manual, schematics and took a quick look if this could be the case at this SA-DX850 as well.
In fact, it did. As I measured the resistance of the two wire points that should be connected, the resistance wasn’t near 0 ohms.
Reassembled the receiver again and YES, it is working again!
But that is not all. The volume knob is very jumpy and not reacting very smooth. It’s not a normal stereo volume potmeter, but a digital one. Such type of dial goes by the name of “Rotary switches”. They act like two switches that opens and closes one at the time.
The direction of rotation is detected by the phase of the pulses those switches give. One switch will act a little later than the other one. Over time the contacts in those rotary switches will become oxidised and will loose its conductivity.
Cleaning the contacts with a fibreglass pencil will fix it and no need to replace the complete rotary switch:
I also remove al lubricant, clean all thoroughly with IPA (Isopropylalcohol) and reapply some new grease on the shaft. Reassemble again and we’re good to go.