Help. I am trying to bring a Defender MPU back from the dead and I am stuck.
It first started with power issues. I replaced the power and video headers (they were really corroded) and the fliter capacitors. Once the power was stable the next issue was the watchdog line not being reset and the CPU reset line pulsing:
After replacing 4 7474s and the 74LS139 I got it to this point:
After some fiddling around I found one of the 74153 was bad and replaced it. The RAM test seemed to indicate a couple of bad RAMs so I replaced them and it now boots, sort of:
It seems to pass the POST and then hangs. Also the video is spaced out and there is the tear at the bottom.
If anybody has seen this before and can point me in the right direction, that would be a big help. I have check most of the components in the video circuit with an HP Logic Comparator and they all seem fine, so I am stumped at the moment.
ken
It first started with power issues. I replaced the power and video headers (they were really corroded) and the fliter capacitors. Once the power was stable the next issue was the watchdog line not being reset and the CPU reset line pulsing:
After replacing 4 7474s and the 74LS139 I got it to this point:
After some fiddling around I found one of the 74153 was bad and replaced it. The RAM test seemed to indicate a couple of bad RAMs so I replaced them and it now boots, sort of:
It seems to pass the POST and then hangs. Also the video is spaced out and there is the tear at the bottom.
If anybody has seen this before and can point me in the right direction, that would be a big help. I have check most of the components in the video circuit with an HP Logic Comparator and they all seem fine, so I am stumped at the moment.
ken


