Short on 7100/66AV logic board

[RetroTechGuy]

I got a hold of a Power Macintosh 7100/66AV a while back with some serious corrosion around many of the capacitors. I think @Kai Robinson has posted about the same kind of board with similar damage.

I got the capacitors off without any visible damage to the pads and I scraped off the corrosion layer with a fiberglass pencil, as seen in the picture.

My problem now is that there is continuity between the capacitor pads (positive and negative) on the following seven pads (out of 26 pads): C13, C26, C43, C49, C58, C63, C65. I've marked these with some green on the board and encircled in red in the picture.

I can't seem to find any schematics of this board - does anyone know what these capacitors have in common and how the positive of all of these capacitor pads could be shorted to ground?

I realize the scraping off of the the corrosion probably caused this but I cant seem to find an actual broken trace. Also one of them C26 is in an area entirely without damage while there is no shorting to ground on some of the pads in areas most affected.

 

[trag]

What resistance are you seeing between the pads? There are normal conductive paths through the MB compents between the GND and Power layers. So you can't expect an open (infinite resistance) between those pads. Typical resistance is actually surprisingly low. But I can't remember if that's in the neighborhood of 50 ohms or 500 ohms or what.

 

[RetroTechGuy]

What resistance are you seeing between the pads? There are normal conductive paths through the MB compents between the GND and Power layers. So you can't expect an open (infinite resistance) between those pads. Typical resistance is actually surprisingly low. But I can't remember if that's in the neighborhood of 50 ohms or 500 ohms or what.

Thanks, I’ve tested now and there is 16 ohms resistance between positive and negative of all encircled pads. Resistance between other pads without continuity between positive and ground vary widely, C62 for instance has 337 ohms and C35 has 542 ohms. Some show values in mega ohms.

 

[trag]

That does not sound out of line for transistor leakage in such a system. 5V / 16 ohms = ~1/3 amp.

I suspect the ones that show 16 ohms are bypass caps which sit directly across GND and the 5V rail and the others are used in a different type of circuit somehow, such as audio.

I don't have access to a 7100, but it would be great if someone else could just unplug the PS and measure the resistance across one of the 5V and one of the GND pins in the power supply connector of a 7100 board.

 

[RetroTechGuy]

That does not sound out of line for transistor leakage in such a system. 5V / 16 ohms = ~1/3 amp.

I suspect the ones that show 16 ohms are bypass caps which sit directly across GND and the 5V rail and the others are used in a different type of circuit somehow, such as audio.

I don't have access to a 7100, but it would be great if someone else could just unplug the PS and measure the resistance across one of the 5V and one of the GND pins in the power supply connector of a 7100 board.

Thanks! Yes, it would be great if someone could measure that. Do you have yours accessible @Initialise_Disk ?

I might already feel emboldened enough to solder the 26 caps and hope for the best…

 

[trag]

Mine is packed away in a box in the attic somewhere. It needs to come down for cap replacement, but I'm not going in the attic until Summer is well over with (central Texas).

 

[luminescentsimian]

I don't have access to a 7100, but it would be great if someone else could just unplug the PS and measure the resistance across one of the 5V and one of the GND pins in the power supply connector of a 7100 board.

I just verified these measurements on a working, not recapped, 7100/66. I get <1 Ohm between the grounds (5,6,7) and ~13.5 Ohm between a ground and the +5V pins (2,3,4). The other pins (1,8,9,10), are open

 

[RetroTechGuy]

I just verified these measurements on a working, not recapped, 7100/66. I get <1 Ohm between the grounds (5,6,7) and ~13.5 Ohm between a ground and the +5V pins (2,3,4). The other pins (1,8,9,10), are open

Excellent, thanks! That sounds promising - I'll try to get round to getting new caps on there to try it out.