Previously I reported that I am now getting corrupted video out of the DECstation 220. I have spent the last couple of weeks trying to understand the problem, checking the tracks to all the chips in the battery-damaged area, on both the original and the spare motherboards. I have also double-checked the tracks and vias in the damaged area to see if I missed any bad tracks. I have not found any new problems. In doing so, I have partially reverse engineered the schematic using Eagle. The partial Eagle schematic is here.
I decided to put the board back in the machine, after re-fixing one of the repair wires that had come loose. You can see my repairs in the photo below. As you can see, I am not very good at it, the repairs seem to work, but they are not neat and have poor mechanical strength. If anyone would like to offer any tips on how to do it better I would love to hear them.
I still get the corrupted video, but this time I get some clear beep codes, 1 long, 3 short, 1 long. However, every manufacturer used different beep codes, and I have not been able to find the specifics for this board. However, in searching for the beep codes I discovered that this machine is actually a re-badged Olivetti M250-E, and I found a pocket service guide here. This at least told me what the jumpers are.
I tried to do a bit more analysis of why the Paradise PVGA1A chip does not seem to write to the video memory. One possible thing that came to light is the EMEM input (pin 21), I found that this is high, but the trace on my scope for this signal is strangely faint, which makes me think there is something odd about it. This signal comes from a buffer whose input is tied to zero, the enable input of the buffer is also faint on my scope, but generally low. I have a feeling that the buffer enable is supposed to be low most of the time so that another source of EMEM can be used, but I can’t find that source and it looks like the signal could be floating as a result, which may explain why the signal is faint on the scope, but I am not at all sure.
I also found that MWRN (Pin 32) on the PVGA1A was never active. I could not trace its source either. I am not sure if this could be another reason why the PVGA1A never writes to the video memory.
One jumper is interesting as it disables the onboard VGA. Which made me think that if I installed an ISA VGA card on the riser, I might be able to get some video out of it. I am pretty sure I have an ISA VGA card, but I have not been able to find any yet. When I do I will give it a go, but for now I am going to have to put the machine away, as I don’t know how to diagnose the VGA problem any further.