[Coco] Gold Plating the Tin Card Edge Fingers of Old Coco Paks, MPIs, etc.

[Gene Heskett]

On Saturday 13 April 2013 21:32:04 Kip Koon did opine:

> Hi Everyone,
> 
> I'm having great difficulty keeping my Coco 3 compatible MPI
> operational. It worked fine yesterday.  I disassembled my Coco 3 setup
> to test some other cocos and when I set my Coco 3 setup back up with
> the Coco 3 MPI later on, it was a no go.  The Coco 3 will not boot up,
> even with no paks plugged into the MPI.  It'd just the Coco 3 and an
> empty MPI.  I looked at the card edge contacts and they are awful!
> Yuck!  No wonder it isn't working!  How do I revive those card edge
> fingers plated with tin.  It is possible to have them gold plated
> somehow without completely removing everything from the PCB? Any ideas
> would be greatly appreciated.  Thanks in advance.
> 
> Kip
> 
BTDT Kip. BTTS and wore it out too.

What I did was find an old gold plated pc card with the same connector 
spacing, ISA seems to fit, sawed the card edge connector off, then 
laboriously verified there were no remaining shorts from any pad to any 
other pad, there were, and I wound up setting in on a grinding stone and 
removing 4 or 5 thou at a time while maintaining the back edge straight, 
until there were no more shorts.  Then I sawed about 5/16" off the MPI's 
card edge pattern & carefully sanded it till I could hold the two together 
with only very small gaps visible.

Then I had been sold some super strong solder, claimed strength was 50k 
psi, for too much, $150 lb, but I had already done some glasses frame 
repairs with it and it sure was stronger than regular electronic solders.  
I put a very small drop of superglue on the ends of it and superglued it 
into position.

Then, using the finest tip I had for my soldering iron, I bridged each gap 
with a dot of this solder that was just barely the width of each runner.  
Both sides of course.  This solder doesn't seem to oxidize near as badly 
with time like most solders do.  I also cleaned the junk lead from the 
grounding ears on each end of the connector and recoated those with this 
solder.

20+ years later I have not had to warm up a single one of those dots of 
solder.  It Just Works(TM)

Mine is one of the first cream colored ones, old pcb, with a coco3 logic 
kit in it AND quite a few 12 gauge jumpers on the bottom of it to better 
bond the ground pattern, it was so broken up that a timing glitch in the 
r/w directions to the '245's on the board was causing a 30 nanosecond long 
ground bounce of almost 3 volts due to more than one device trying to drive 
the buss at the same time but in opposite directions.  So it was drawing 
several hundred milliamperes during this overlap period.  Lots of 
additional jumpers to tie it together, and a couple more .1uf mylar caps to 
the 5 volt rail from the stiffened ground, and the nearly 3 volt bounce was 
reduced to about 600 millivolts, not enough now to noise up the edges of 
the signals, which were a mess in "polite terms" before.  I should have 
taken the time to find the timing error, but this was quicker.  FWIW, I'd 
suspect that all of the older MPI's do it.

Independently of that, I pulled 3 of the 4 pullup resistors for the pin 8's 
on the card sockets, located along the front of the board, and tied all pin 
8's together with a jumper wire, so no IRQ's have ever gotten lost again, 
and, all the power supply parts were removed because my whole rig is 
powered from an old old XT power supply.

No heat to speak of now, I can throw a furniture blanket over it with a 
thermometer directly over the disto 2 meg kit, turn off the monitor and 
come back the next day and the thermometer is 2F above room temps.  

Whats not to like?

Cheers, Gene
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