One of my Rev 7 ][+ bench boards recently started doing somethign rather odd: When a (known good) Disk ][ card is installed in Slot 7, the system works as expected, but if the same card (tested on other machines) in installed in slot 6 or a lower slot, the system exhibits random errors and crashes. The video output becomes distorted (dims, flickers, oscillates), and/or it tries to boot and crashes, or it simply hangs with a wavy video output.
I only noticed this because I had to load CP/M for a test, and CP/M needs to run from slot 6...
When CP/M does load (fluke), it gives errors, glitches the screen (erratic/pulsating pixels or partial raster lines) randomly crashes to monitor, has exhibited the behaviour of simulating a constant repeating 'V' keypress, and has shown 'CP/M v2.10 4K Version', which of course to my knowledge is not extant, and is probably a video output error. I figured I would mention that, as it all seems to point to address line faults, in my opinion, but I am open to other opinions.
I'm pretty sure this was caused by a faulty board shorting something:
As I recall, there are two 73LS138s that handle address bus decoding, so I was going to swap those out. I figured I'd ask if anyone has any other component suggestions, as this is quite odd. It seems as if memory addresses are being overwritten, again pointing to those 138s, but IDK why slot 7 would work normally, unless it is simply because it has priority.
P.S. A VIDEX in slot 3 works normally. This fault occurs with the Disk ][ controller as the sole card in the system, i slots lower tha 7, and mixing it with other cards. I am guessing that the action of the autostart itself is a issue with the memory map?
I will try those 138s tomorrow, but if any of you have a clever notion in the interim, let me know. ;)
I was going to turn my attention to a Rev 4 board with no video, but this is annoying as it is my primary test bench board, so I need to either resolve it, or pull another and clean it.