Industrial motherboard burned capacitpr on negative (-5V) power supply rail

Hello!

I'm working on motherboard from old CNC machine.
Markings on the motherboard are on the following picture. The schematic is most likely not available.


I found a burned tantalum capacitor, but the PCB board is also burned in the vicinity of this capacitor:


And the pictures of the same area of the board after cleaning:


The input for negative (-5V) power rail is on the header above the burned area. It comes from -5V LDO regulator (MC7905CT), which is located on another power supply board.

I think that the -5V power rail is interrupted somewhere on the burned area. The resistance to ground on this power rail is out of range of my meter (more than 40MΩ). I also tried to inject -5V from the laboratory power supply, but there is no power consumption on this power rail.

Any idea, on which component the -5V power rail is connected?

Hello!
I've checked schematic for motherboard based on PGA370 platform (Gigabyte GA-6VEM REV:1.0), similar to yours board. On this motherboard, -5V rail is used for ISA slots. There are few ISA cards which are using -5V line, but this line is used very rarely.
Check if you have continuity from power supply -5V line to pin B5 in ISA slot.

Thank you for your response!

Unfortunately I don't see any ISA slots on this board.
Here is a picture of connector board, which is connected to this motherboard:



Any further idea?

Oops... It's time to buy a new glasses for me I think...
In case there are no physical ISA slots on the board, the only idea that I have at the moment is trying to find a -5V rail using multimeter on continuity mode (with "beep" speaker) with one multimeter probe on -5V line and second probe checking pin by pin on those grey "IDE look-like" sockets on motherboard. If there you can't find -5V rail, try to probe pins on edge connector at the bottom of motherboard (near CPU socket).

You are right!
I found the connection to -5V rail on one pin of "Gold fingers". There are no connections to all other connectors (including "IDE look-like" connectors).



I will solder new tantalum capacitors, but I'm afraid there is a continuity (greater or lesser resistance) between the inner layers of the board caused by the burned laminate under the damaged capacitor, which is probably irreparable on multi-layer boards.

Yes, burned laminate may be a problem. I don't know how many layers are in this board (maybe six or more), but burned laminate with its resistance may cause further damage of board. The bad thing is that burning goes through all layers (you can see it at the opposite side of the board, where traces and vias have contact with copper from inner layers). It can be very hard to remove all burned laminate and rebuild all traces and vias with proper isolation of every next layer (using UV hardened soldermask for example). Lack of boardview and any documentation of the board doesn't make it any easier. Layers should be reconstructed one after another. As example of that repair, search out for Apple famous "long screw damage" repair.

After the repair the board still does not work completely proper, but it is better than before.
I'm pretty sure that the cause is a burned laminate, which cause resistance between inner layers of the board.
I think it is unfortunately irreparable without detailed documentation.

However, the main question of this topic was solved.
@xamledyD thank you very much for your help!