MODCOMP II Revisited... It's running again!!!

[Lyle Bickley]

On Tuesday 10 July 2007 12:04, Lyle Bickley wrote:

> Factoid: Many TI chips produced during 1975-77 period had very thin "legs"
> and poor tinning. When they were wave soldered, the solder offered them
> protection. However, when they were socketed (as in the MODCOMP) they had
> very little protection from the environment. They would rust and eventually
> fail.
>
--snip--

I've now replaced all the "nasty" TI chips in the MODCOMP II CPU planes and, 
as needed, in the Memory planes.

The total number of chips replaced was about 297 (I may have missed a few).

I'd been testing with short hand-written diagnostics as I went along - and 
could see the system getting "better and better" as I proceded (with a few 
"relapses').

I can now load a linking loader and diagnostics via the loader. Memory diags 
run w/o failure (including tests of "protect" mode) - YEAH!!!!!
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Note: Here are some thoughts for anyone deciding to take on a massive project 
like this:

1) Inventory your chip requirements. Add enough chips for spares and fails 
(not all new/pull chips are going to work).

2) Obtain all the chips you'll need up front from your "stash" and buying as 
required. Bag or box them in numeric order near the system. In my case, I 
needed to acquire a lot of chips - as I had very few 74H and 74S series in 
stock - and the "fast" MODCOMP II needed scads of them.

3) I only replaced about a dozen chips at a time before running a basic 
diagnostic. It turns out that replacing too many chips at once can create 
diagnostic nightmares.

4) I used fine tweezers or a safety pin to remove broken legs from the chip 
sockets. I then used a strong magnet (Neodymium) to make sure I got all the 
legs, etc. of the broken chips picked up and away from the planes.

5) I make it a practice to have a log for every system I restore. In the case 
of replacing so many chips, it is even more essential to log every chip you 
replace and every "hand" diagnostic written and each result.

	(a) Whenever I replaced a chip, I noted the location and chip type replaced
	(b) Whenever I could "skip" replacement (It had been done previously, etc.)
	    I noted the fact with a "circle" around the location.

Besides documenting my progress, the above was very helpful when testing - as 
I knew immediately which chips were likely "suspects". 

5) Take plenty of breaks. Replacing hundreds of chips requires a steady hand 
and a lot of patience.

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Regards,
Lyle
-- 
Lyle Bickley
Bickley Consulting West Inc.
Mountain View, CA
http://bickleywest.com

"Black holes are where God is dividing by zero"