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Old 13th Jul 2019, 7:02 pm   #164
john_newcombe
Hexode
 
Join Date: Apr 2019
Location: Skipton, North Yorkshire, UK.
Posts: 252
Default Re: Gemini 80-Bus System

Ah, for once I am slightly ahead, I did indeed check with a digital multimeter and sure enough D0 was connected to D1. By wandering around the board it was fairly easy to identify that the issue seemed to be under the CRTC controller socket. After lifting the socket slightly and much searching with a very powerful magnifying glass, there it was, a small sliver of copper from one track, touching the other. This was removed with a fine scalpel blade and hey presto I can see pulses on IORQ.

I replaced all the ICs and IVC Monitor and I now have a perfectly good IVC Video card, and to be honest I can't quite believe it.

My theory of how this all happened is that the original fault of the blown CPU and faulty IC31 is anyones guess, however, when I removed the CRTC to test it I suspect I either damaged the track slightly beneath the socket or disturbed some previous damage. Oops

At this point I must extend my thanks to Neal, John B, Julie, SiriusHardware and others who chipped in. Without you guys I would have given up long ago. Many many thanks.

Photos of a working Gemini are at the bottom of the page... https://glasstty.com/wiki/index.php/Gemini_IVC_Problems

Time to get the SCSI working, which means I am back on the hunt for a Gemini CP/M Bios with Winchester support....
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