Yep, that says there's a short from -12V to Ground. All you need to do is find the cause of the short...

If all the tants are removed then I'd start by checking any socket for bent or mangled pins, or anything in the sockets that might be causing a short. Also look closely at the front and back to see if any small ceramic capacitors look like they've (or any traces nearby) got hot. It's rare (as Deunan said) but ceramics can fail short.

Page 154 of the service manual ( http://www.nj7p.org/Computers/IBM%20PC/work/PS2_Model_25.pdf ) says:

The + 12 V dc and -12 V dc are used for powering the Electronic Industries Association (EIA) drivers for the serial port.
All four power levels are bused across the two system expansion slots.

So also have a look around the DS1488 (I think I see that on the photo I found) that I think is the line driver for the serial ports and should have -12V on pin 1.

Good well lit photos of the front and back of your board might help someone spot if there's anything that looks odd, and might help trace where -12V is routed.

snufkin - Thanks for the link! I'll get to snooping around as soon as I can - I'll also try to get some good close pictures of the board as well and post them up.

A decent PCB photo would help I think. This is IBM and they were pretty quick to adopt SMD parts. Just because all THT tantalum were removed doesn't mean there aren't any SMD ones hidden somewhere - like on the backside for example.
Short on -12V line is otherwise very unusual since not much really uses it except serial ports and (not always) DAC on the video card.

Deunan - I've had a heck of a time trying to get a good picture without reflections, etc. but I'll see if I can get a good pic if I take it outside if the sun comes out tomorrow!

Well, the sun was finally out long enough for me to get some pictures! Hope these will work.

I haven't traced the whole thing yet, but it might be useful to try removing L11 (near C111) and then seeing if the short has disappeared on either C111 or C100. It looks like the -12V supply goes through L11 before going to the outer pins of C111, so removing L11 will disconnect the capacitor pads from the -12V supply. If the short is after the inductor then C111 will still be shorted, but C100 should be ok. If the short is before the inductor then C111 will be ok, but C100 still be shorted.

If the short is after L11, then there's a ceramic capacitor on the back, C110 (near C111) that I think is connected to -12V, might be worth removing that and seeing if the short goes away. I can't immediately see what the -12V is used for around that area.
If the short is before L11 then there's the MC1488 chip U4.

I'm now very nearly certain that all of the 3 pin capacitors are centre positive, and the outer pins are always connected on the PCB. Doesn't really help until we find that short on -12V.

snufkin - sounds good - I'll try to get this done over the weekend if possible! Thank you!

On the back just above the PS/2 ports I see a dark spot that I didn't see before (near R65)- I don't see anything on the other side, so I wonder if something between the layers got fried?
I just checked the board - that spot isn't there, it must have been one of my fingers making a shadow!

>UPDATE< Stuff has gotten super busy and I haven't had time to get this done yet. I'm still going to do it!