I have been doing more work on the DECstation 220. As I commented in my previous post, it is only putting out a corrupted pattern to the screen because it is not writing anything to the video memory, apparently because the EMEM signal to the PVGA1A chip is incorrect.
I have found what I think is the reason for the EMEM signal looking wrong. At first I found that a couple of pins on one of the custom gate arrays seemed shorted together. This would have been a disaster as those gate arrays cannot be replaced with standard parts. But then I discovered that they were connected to the Chips and Technologies 82C206, and I realised that a lot of the pins on the 82C206 appear to be shorted together. I suspect that the chip may have got damaged as a result of powering it on when other parts were damaged by the battery leak. This chip can at least be replaced.
However, as it is a surface-mounted PLCC84, it will take some doing to replace it. I have a spare board where the 82C206 seems to be OK, but it does not beep or produce a video output of any kind. I would like to use the partially working original to get the spare working better, and it is best to do that before removing the 82C206. This is because I could damage something else in removing the 82C206, and as the original board gets further into the boot sequence than the spare does, it will help to have something to compare against while trying to fix the spare.
So I have switched my attention to the spare for now. I have been trying to find out why it won’t produce any video output at all. I have found that there is an Inmos G176 which outputs the colour palette and that this output is also sent to a set of 3 comparators (on an LM339), these comparators drive the monitor sense signal that goes to the PVGA1A. However I have found that the comparators are not producing valid output, the input signals are varying above and below the reference voltage, but the outputs never change. So I will have to change the LM339. I don’t have any so I am going to have to order some.
I am not totally convinced this will fix the spare board, it may yet be failing earlier in the boot sequence, we shall see when I replace the LM339.