After a quick trip to my favorite distributor to grab some cable shoes (I returned with the cable shoes, some heat shrink tube, an SSD, a rust drive and a graphics card and was told to come back for the big Creality 3D printer, they had to think about the price they could give me) I had a hunch and started working on that pin.
Rewire the power box. Not pretty, but does the trick.
Measured Voltage at metal parts against earth. A couple hunnerd mV, but that's it.
No dangerous voltage after correctly attaching the ground wires within the cabinet.
And behold:
However a closer look at the CPU board reveals this utterly shameful situation:
Why can't people take out the batteries before dying ?
Hrmpf.
No, this is not meant seriously.
Close up:
The switch matrix is toast. Quite obviously. I need to remove the battery holder, the ULN2803, LM339, the 74LS line driver and all affected resistors, diodes, capacitors. The scrub in acid, remove all gunk, throw in ultrasonic, repair vias and put in new components.
Once that is done the bulk of the repair should be finished.
Estimated time needed to bring the pin back into operation: 10 hours.
Which includes lamp / rubber swap, repair of broken coil stops and mounts and the overhaul of the CPU board.
An additional day of work to make it reasonably tidy.
To make it nice: additional 100 hours. But I don't think I will go to that length.