5170 ISA memory fault then short circuit

[SpidersWeb]

Just after some thoughts - I need to remove the motherboard yet.

So usually once a month I walk around my vintage room and power every single machine up to keep them healthy. Occasionally one doesn't, in this case my home built 5170. It's a type 3 motherboard, 8Mhz 512KB, with an AST Ramvantage providing the bonus 128KB + 1920KB of XMS. This current setup has been running fine for at least six months now.

So when i powered it up, the memory gets to 1024KB and fails - parity error 2. That (if I'm thinking correctly) would be a failure in bank 1 (the second set of 18 61256's) but then it got weird.

I run Checkit, and it tests the extended memory from 0100000 (I might've missed a 0) a segment at a time until it hits 0160000 then I suffer a hardware lockup.
I repeat the process a few times removing other cards from the machine and making sure all the ICs were firmly socketed (they were, all flat), after a while I start getting a RAM error at 1088KB and Checkit starts crashing at segment 0170000

I removed all cards, switched video, powered up and then the power supply stopped after 5 seconds.

Removed all cards, power supply stops after 5 seconds. (NB: PSU is uses a 200W board I pulled from a 486 machine - not IBM standard)

Disconnected motherboard, PSU runs fine.

Check resistance, and the -5V line has 0.03 ohms.

a) has anyone taken note of what caps are on what line? e.g. where to look for a short first?
b) is it remotely possible the shorting cap caused my AST Ramvantage to suddenly fail, or should I be looking elsewhere for this fault? Just seems a bit odd for two faults to suddenly appear

 

[H-A-L-9000]

Powering on/off may have offended that capacitor.

 

[Chuck(G)]

-5V should not be used for anything on the 5170. Lift the -5 PSU lead from the motherboard connector and try it.

 

[modem7]

-5V should not be used for anything on the 5170.

Confirmed.
The circuit diagram for the 5170 motherboard (types 1, 2 and 3) reveal nothing on the -5V line, not even a filter capacitor.
The -5V line runs straight from the P9 connector to the expansion bus connectors.

 

[SpidersWeb]

Cheers for the replies. I somewhat wrote that post while being yelled at to come to bed and have only just now had time to jump back in.
(work/wedding in a few weeks/gardens etc etc blah)

Confirmed.
The circuit diagram for the 5170 motherboard (types 1, 2 and 3) reveal nothing on the -5V line, not even a filter capacitor.
The -5V line runs straight from the P9 connector to the expansion bus connectors.

This made me retest it, and yes -5 has nothing on it at all - no reading on the multimeter at all but I was certain that wire was showing 0 ohms the other night.
My only theory is somehow a line got shorted, or being late at night I'd fitted the power connector off by one pin or similar :S

The machine also powers up again now (with the same VGA + AST RAM card fitted) but still stopping at 1024KB so I'll need to look at that next. Hoping it's the RAM card itself.