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

[Gene Heskett]

On Monday 15 April 2013 19:35:31 Kip Koon did opine:

> Gene,
> Your solution gave me an idea.  Would the Silver Solder from a Jeweler's
> repair shop be a good alternative to cutting off Gold Card Edge contacts
> from old PC motherboards? 

No clue, no real knowledge of the alloys jewelers use since they are 
generally a pretty tight lipped bunch.

If its the same basic silver solder alloy the refrigeration people use, I 
doubt if my fawncy iron can get that hot, and in any event the traces would 
boil right off the board under that sort of heat.

> Is Silver solder able to be soldered cool enough on a PCB to replace the 
> tin-lead coating?

Generally no, this particular alloy was maybe 3% silver is all, and while 
it would slow down the formation of the lead oxide over time, on a 10 year 
time scale, probably not enough to make a real diff.  IIRC this stuff was 
melting about 75F warmer than the std eutectic (lowest melting point), 
commonly called 63/37% formula.  Some add a bit of copper which slows its 
attack on soldering iron bits somewhat, but 2% seems about all they can 
leach out of making it up in copper pots, using the copper as a sacrificial 
metal.

The brand name of the stuff I bought from a traveling salesman was 
Rhotainium or Rohtainium.  It did work well indeed, but it was also $150 
for two 1/2 lb spools of it.  I suspect that salesman ate well that 
evening. :)

> My mind is swirling
> with ideas. I wonder if I can get a Jeweler to do this or if I can buy
> some silver solder and do it myself.  Do Jewelers use a torch or a
> soldering iron?

What I've seen is usually a smallish butane torch.  But it may have been 
burning the 1000F hotter MAP gas for all I know.

> My sister has been in the Jewelry business for years,
> so I'll have to ask her. I wonder if they could Gold Solder it also. 
> Still thinking.  Thanks for all the responses so far.  I appreciate the
> help.
> Kip
> 
> -----Original Message-----
> From: coco-bounces at maltedmedia.com [mailto:coco-bounces at maltedmedia.com]
> On Behalf Of Gene Heskett
> Sent: Saturday, April 13, 2013 10:11 PM
> To: coco at maltedmedia.com
> Subject: Re: [Coco] Gold Plating the Tin Card Edge Fingers of Old Coco
> Paks, MPIs, etc.
> 
> 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


Cheers, Gene
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