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Last night, I removed the 505, 540B and Glitch RAM boards from my C8 case hoping to transfer them to a C4 case. I decided not to go through with it after realizing that the D-13 board is an uncomfortable (though possible) fit in the C4 case. So, I plugged the boards back into the C8, powered on, got the normal junk screen and flipped out when hitting the reset/break button produced:

0000 2D

My gut reaction was to keep pressing the reset button... because. After a few seconds, I noticed that the H/D/M was appearing at the top of the screen for a split-second only to be replaced by the 0000. I left the system on, walked away, came back maybe ten minutes later, hit the reset button and - poof - H/D/M was the only thing on the screen. Walked away again for a good hour leaving the machine on. Returned, powered off, walked away for a few moments and then powered up, hit the reset and encountered 0000 XX again.

Gosub "My gut reaction" and after a while, I could get H/D/M again. Plus, after powering off overnight and powering on this morning, H/D/M showed up.

Any thoughts on what component or misconfiguration could have caused these symptoms which now seem to have disappeared? I know they will return.....

I can't say I can tell what is wrong with your system, however I have had similar problems ultimately caused by an over-voltage condition on one power supply in a C4P. There are two power supplies in the C4P one for each for two of the four backplane slots, one of them was 5.8V or more for some time. I was having weird problems with the system even after correcting the voltages. Finally I replaced them with a modern switcher, but I continued to have random failures. It would work for 10s of minutes sometimes, and then glitch out. Sometime reboot worked, sometimes not. Sometimes it would fail after only a few seconds. H/D/M didn't always work. Hitting it with cold spray or heating wasn't conclusive. Logic probe didn't show anything obvious. I tried replacing 8T26's since video memory was sometimes corrupting but in the end replacing the 6502 CPU solved all the problems. It was so nice to have a stable system again!
I have also had bits fail in 1702s on a 500CPU. I have had bits fail in 2316 mask ROMs too. The other failures, TTL chips popping & releasing magic smoke, or shorted capacitors were pretty obvious.

So check the C4P voltages... perhaps they caused a problem. At least the 6502 is socketed if you happen to have another one around. Good Luck!

Something must be working for you to get the H/D/M? prompt, and the monitor display.

In the monitor (0000 xx) can you type numbers? How about typing D or M on the H/D/M? screen?

Did you try disconnecting the keyboard?

I’ll check the PSU’s on the C4P. As for the issues when the boards were re-installed in the C8, I found the issue: one of the 8 (48-pin) connectors on the bus has a fault. Moving the 505 board to another connector fixed everything. Let’s see if I can get things going with the C4.

It’s good practice to DeOxit the 48 pin bus connectors and pins. They are getting kinda old.