[mapesdhs]

Hi all!

A followup to my previous thread about fixing an IP6 for Personal IRIS; I'm now trying to repair a GR1.5 base board (in final assembly it will have a VB and Z fitted, but for the moment it's the base board which needs fixing). I have a number of GR1.5s but none of them work properly. The least bad is a board which shows vertical lines (these are high res images so one can zoom in):

http://www.sgidepot.co.uk/misc/P1190506sl.jpg
http://www.sgidepot.co.uk/misc/P1190507sl.jpg

Here's a reasonably high res image of a GR1.5 (not the one I'm working on btw):

http://www.sgidepot.co.uk/misc/P1190508sl.jpg

The gfx architecture is described in the technical report, Chapter 3:

http://www.sgidepot.co.uk/pitechrep.html

I started doing some diode mode testing on another board to get a baseline idea of what some of the ICs should show in terms of readings (already checked all caps and resistors), but I thought I'd post here to ask: given the display output, anyone have a notion as to what the most likely origin of the bad picture probably is? A VRAM IC? Support IC to the VRAM? XMAP? Static RAM between the XMAPs and DACs? Just pondering where I should start so as to have the best chance of locating the fault sooner rather than later, ie. minimise the time wasted searching less likely fault locations. I don't think it's the RE2 chip since three different RE2s show the same output.

The VRAM is present as 12 rows and 5 columns of HM53461ZP-12. Nearby there are 4x CD74ACT646EN, below these are 4x CD74AC174E and one CD74AC157E. Many other ICs aswell but these are the main ones which support the VRAM. Still reading up on what the other ICs are for.

I'm now thinking perhaps one of the XMAPs is bad, so that's my next target for diode mode testing. So far I've taken measurements of every memory IC and all the ICs mentioned above; comparing to other boards didn't show anything conclusive. Problem is, until I have a fully working board I can't be certain what normal readings should be, so I'm gambling that multiple other boards are unlikely to all have the same fault and thus by cross checking several I can hopefully infer what should be normal readings. Ultimately I plan on buying a great many NOS spares and refurbing the other boards to fully working order. In time I'll add all the info to my site.

Thoughts welcome!

Ian.

PS. Uber thanks to jan-jaap who's already helped a lot with my Personal IRIS woes. 8)