7100/80AV motherboard - tracing/grounding rotten cap

[Byrd]

Hi all,

I recently picked up a 7100/80AV reported as working (FWB Jackhammer and Radius Telecast cards in tow), took it home and found extensive battery corrosion inside - predominantly with case damage but also extending along one side of the motherboard.  It was reportedly in storage and going by the leaking was stored vertically so the damage could have been worse. 

Cleaning up the corrosion revealed the main damage around the closest RAM slot, and at C58 where a 16V/47uF capacitor lived, now eaten away.  I realise it'll need a full recap (and that RAM slot might need attention but looks OK cleaned), however wanted to see if I could get some life out of the board by replacing the C58 16V/47uF capacitor initially.  I've a donor 7100 chassis and hope to get one good working machine from two.

I have been able to repair the +ve pad at this site, but the -ve pad is destroyed.  I'm not great at fixing traces; could I ground this cap with a wire running to a nearby cap, in this case C56 16V/47uF? Otherwise what is the common rule here when grounding caps in a non-standard location?

Thanks in advance

JB

 

[trag]

You want a low resistance route from the capacitor to the approximate former location so that bypass isn't delayed (time constant, RC).   So, connect to the ground plane as close as possible and use a relatively "thick" wire. if you can't connect back to the exact same spot.   For the 47uF caps, they're more regional, rather than high frequency local, so some distance is not a big deal.   It's the smaller capacitance tiny caps that are really evening out the higher frequency power swings.

Also, on every 7100 I've worked on, the heat sink grease has dried out and stopped doing its job.   I recommend cleaning the CPU and heat sink (carefully, there is a risk of cracking the ceramic (plastic?) CPU package if careless), and replacing the heat sink grease with fresh material.

 

[Byrd]

Thanks as always, trag

I resoldered a tantalum cap in place and it displays the same symptoms as before, no video and a short PC speaker style sound when powered on.  And yes, thermal paste was replaced and the board cleaned to within an inch of its life

I have a spare 7100/80 board, uncapped, which exhibits a static sound when turned on, nothing else.  Looks like I'll be recapping that one next, with fingers crossed.  Lots and lots of 16V/47uF capacitors on these boards, I purchased some Type D tantalums for $12/100 to trial in this restoration.

JB