Wanting some help with a PDP-8/a

Bryan Longram Driverless at protonmail.com
Mon May 11 16:09:47 CDT 2020

I acquired a non-functional PDP-8/A several months ago and in that time I've replaced all the outwardly damaged parts and have gotten the machine to power on with no issues. However the problem I've been having now that I can't quite seem to pin down is that every time I power the machine the address field is displayed as 07777 and the value field is displayed as 7777. Attempting to change addresses or the value of the address doesn't work as I can enter the value just fine but upon entering the Load Address button it defaults back to being all sevens. I've made sure the pins on the CPU board are set so it should start at address 0, the advanced options are also all turned off and as far as I can tell there's nothing out of the ordinary with the pin settings on the IO board.

I'm fairly certain at this point that the issue is with the Programmer's Panel but I'm not 100% sure.

I've done some further testing using advice I got from users over at the Vintage Computer Forum and SR and LSR work as intended, I can enter a value into SR and then view it at a later time. Something interesting I did find was while checking over the CPU I was looking at the switches on it and while S1-1 was set as expected S1-7 was also on which is the CPU autostart disabled feature.

Upon powering the computer back on after turning S1-7 off both the ADDRS and DISP displays show a single 0. I thought I was on the right track though now the panel doesn't work as if the panel lock was toggle even though it's not, not even the read functions and SR are working. Switching S1-7 back on reverts the problem back to what it was before and far as I know all the voltages on the power supply are correct. Any help would be appreciated, even if it's just a push in the right direction.

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