Here we have the repair / modification of an electrifying Cheetah MS6 vintage analog synthesizer that likes to ZAP! Customer already reported that he gets electrical shock when he is plugging in an (metallic jack of an) audio cable into the Cheetah, or touching the end of an already pluged in audio cable. It also destroyed a mixer input channel. Last but not least it blows the house circuit breaker.
If you want to take a look at this picture:
You can see the battery and the fuse and a white substance between these two. Whatever it was, maybe tip-ex, it was conductive. I could measure ohms between on side of the fuse and/or one side of the battery. Depending on the orientation of the mains plug you have mains voltage present all over the circuit. This was supported by the missing earth/ground connection. There was no wire between the „earth“ marked spot on the pcb to chassis or to ground. But schematics also calls for. No wonder why customer got shocked or destroyed some of his gear.
What I did?
First I cleaned of the substance to have a closer look. I spotted a burned hole:
I decided to cut the mains trace that was connected to the fuse, solder a new wire to the pcb, in series with a „cable“ fuse. So no more mains voltage can flow to the battery terminal. The „old“ fuse is „out of order“.
Then I added the missing connection between pcb „earth“ and mains earth/chassis which makes it safe to use now.
It passes the VDE701/702 tests, no ore mains voltage, it’s no longer dangerous.