i think it might be the dc jack.

going by the image above, the neg is the pin on the right of the jack with the number 1 on it,

the neg side of the dc input is obviously going to connect with the flat spring connection on the inside of the female jack, and the pos to the pin in the female jack.

using a simple continuity test, there is no connection between the flat inside the female, and the No1 connection shown above. the pos tests out fine.

however... there is a neg continuity to a pin directly underneath the jack socket. due to the pcb being quite thick and multi layered i cannot see where this leads too, even holding it to a 300w light!

I have to admit, this pcb is some piece of work, when i was building boards (QC / test, hand finishing chips etc) back in 90/92, it was all big resistors, caps, etc. i knew all the resistor values by the colours, what the numbers meant on other components, etc, i don't even know what half of what i'm loking at in the empeg is!

surface mount stuff was barely seen, and when i say surface mount, i mean like on the empeg board, most components a fraction of the size i dealt with, and soldered on top of the board, not behind it. I've fixed many a device by tracing earthing / power faults on the pcb themselves, broken / burnt track, etc, and repairing them, but i wouldn't want to attempt this baby! heck, even my fine tips on my irons are twice the size of the smallest solder joints on the empeg!

still, i was 15/17yr old when i used to do all that, latterly more used to fixing circuits etc that you could poke your finger into if you'd not got a probe! not even done them for 4 years!