So far, I have replaced all pin headers and connectors on the power board, recapped the power board and sound board for good measure, replaced a blown fuse on the audio board, and cleaned all other pin connectors and socketed chips. The -5v and 12v are now very close to their intended value, but +5v is still only at 4.8v or so. I'll get new caps for the MPU board and see if that fixes the voltage.
Anyway, the problem is still happening. Is there a good way to test the CPU or the 6821, or do I just need to install new ones to see if it helps?
I haven't tuned into this project for awhile.
I've done a few more experiments with power on these games. the caps will cause the voltage to drop either a little bit or a lot -- they're 34+ year old parts now, that'll happen as they dry out. the factory Williams headers aren't much help either. in changing both out on the Defender at work though, I noticed the voltage was still low. while the juice was sufficient for Defender, the Stargate I dropped in it for testing didn't appreciate it at all. it wound up being the power plug in addition to all those other factors listed. I don't know if they just get dirty or what, but those IDCs won't have the trifurcon pins that grab the headers for better surface contact anyway.
I think the Defender power supplies were factory jumpered to output 5.00V, either that or it was a misconfiguration on the pair of Defender and Stargate ones at the shop -- I ultimately installed adjustment pots on those since I had a ton of 10k's laying around and just wanted the linears working. the Joust/Robotron/Sinistar ones at work put out 5.10V. the adjustment pots can be dangerous, I frown upon them because they can walk, and they can walk far, north of 6V. if you can get it at 5.10V, just leave it be.
you will have to measure at the grey and black wires on the DC output plug to see what it's putting out on the +5 line. don't be concerned with +12 or -5 -- if you rebuilt the power supply, it's the best you can do, unless you have a failing bridge rectifier (BR2 and BR3 respectively) if you have anything above 5.00V, then you shouldn't be losing that much, especially with new headers and plugs.
if you're at or below 5.00V, go to this link, it's for the Sinistar drawing set:
http://arcarc.xmission.com/PDF_Arcade_Williams/Sinistar_Drawing_Set_(16P-3004-103_Feb_1983).pdf
if you go to page 10 of the drawing set (12 in Adobe Reader, I think) it has instructions on how to boost the +5 in the corner. removing R10 will put less resistance on the voltage to the regulator, thus increasing the output.
if this doesn't help, then I have another ancient secret that should work.