Incorrect voltage from A1200 floppy power connector

[mikolas]

Hello,

I'm getting -5V reading from the 12V line of the floppy connector, the 5V line gives 5V as it should. I have shorted some of the connections before as well as connected the power cable wrong way around (to a now dead 3.5" hard disk :-). This has caused some traces on the motherboard to almost melt, heating so much that there are some burn traces. However the mobo works fine, even the floppy connected to the power that gives wrong voltages.

I measured the voltages, just in case, before feeding extra power via the floppy connector and I'm lucky I did as I think something would have gone out with a smoke. Now, does anyone have any clue what on the motherboard might have burned to cause this strange behaviour? -5V on the 12V rail sounds too absurd to be true... I'm a bit desperate to salvage this rev 2B motherboard as my BPPC does not seem to like rev 1D motherboard at all.

[mikolas]

Upon more thorough visual inspection I found a broken trace between a via and E5610 + C592. If I'm reading the schematic correctly, C592 is part of the floppy power supply decoupling and this caused the voltage drop.

I cut the remains of the burned trace and soldered a piece of wire between the via and E5610 and everything seems to be working now. I think I have to do the same for all the burned traces, just to be on the safe side for the future.