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

On Tuesday 10 July 2007 12:04, Lyle Bickley wrote: --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!!!!! ---------------------------------------------------------------------------- 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. ----------------------------------------------------------------------------- Regards, Lyle -- Lyle Bickley Bickley Consulting West Inc. Mountain View, CA http://bickleywest.com "Black holes are where God is dividing by zero"