To verify 5V at the CPU -- measure directly across capacitor C1 near J1.

But YeOldPinPlayer had the link to what you should be following.
Disconnect all displays - verify display 60V/42V outputs. Then install displays one at a time and re-verify voltages (turn machine off when pulling/installing plugs!). This should hopefully narrow things down.

For System 80 --
J2 on the power supply requires a 6 pin plug with one keying pin at position 3
Couple options --
Molex 09-50-8061 with key 15-04-0297
or
Generic CS156-06-LR with key 15-04-0219

Contacts -- probably just go with 08-52-0113 for both plugs.

At CPU end -- 09-01-6051 (obsolete).
I have some remaining but there is a new Molex alternate for this one. I'll stock this one eventually.
For contacts -- 08-52-0072

For some of these - the connector bodies can be reused even if the original was an IDC type plug. Doesn't make much sense for the header type plugs (e.g. CS156) as the replacements are inexpensive. But for the edge connectors - we may need to go this route in the near future.

https://www.greatplainselectronics.com/categories.asp?cat=37

Ed

Badly worded. I meant 08-52-0113 for either 09-50-8061 or CS156-06-LR.
08-52-0072 for 09-01-6051 edge connector.

Trifurcon has two extra tabs on the sides of the connectors that help center the center the post within the contact area and provide a bit of assistance in vibration prone environments.

For any of these connectors - you can also just go with 08-52-0072 flat contacts.

It works. But doesn't look pretty. Not any real choice as these connectors have pretty much become 'unobtainium'.

I don't see why the AMP versions won't work. Haven't ever tried it, though.
These are also obsolete in larger sizes and getting harder to find. I have some 24's left:
https://www.greatplainselectronics.com/products.asp?cat=128

Bifurcated contacts -- split down middle. Supposedly for use with surface areas that may not be perfectly flat (such as edge connectors).