> -----Original Message----- > From: Rob Jarratt [mailto:robert.jarr...@ntlworld.com] > Sent: 13 August 2017 20:31 > To: General Discussion: On-Topic and Off-Topic Posts (cctalk@classiccmp.org) > <cctalk@classiccmp.org> > Subject: DECstation 220. Another Impasse > > I have reached another impasse in my attempts to get my DECstation 220 to > work. I have posted the details on my blog here: https://robs-old- > computers.com/2017/08/13/decstation-220-update/ > > If anyone has any suggestions I would be interested to hear them. >
I have looked at this problem a little more. I have two motherboards, neither of which work, but one at least produces a corrupted video pattern. The one that works best appears not to be writing to the video memory. When I look at the EMEM pin on the Paradise PVGA1A chip I can see a signal but the scope shows a trace that is very faint. When I look at the same pin on the other motherboard, I get a nice clear bright trace on the scope, using all the same settings on the scope. This pin is driven by a non-inverting buffer (74LS126). The input side of the buffer is tied to 0V, the enable signal comes from a custom gate array. Comparing the buffer's enable signal on the two boards I see the same dimming effect on the board with the corrupted video pattern, and no dimming on the other board. I have checked the other pins on both the buffer and the gate array and I don't see anything suspicious. I am thinking of speculatively replacing the 74LS126 because I can go and buy replacement parts for it, I can't replace the gate array (although I could conceivably swap the part on the two boards). Can anyone offer any suggestions as to the best course of action? Thanks Rob
