Okay, instead of the linear PS, I'm going to connect a modern SMPS and see what happens. This will rule out a PS issue.
Next I'm going to replace all the tantalums on the board and see what happens then.
As a last resort, I'm going to start cutting traces.
I'll post my findings here when I'm done.
Here are two pics showing the DC voltage on the 5V rail with power off and then with power on. When power is turned on, the voltage jumps to 1.7V momentarily and then drops to .56V
A continuity test shows no connection between 5V and 12V and 12V with -5V. But looking at the picture you quoted from me above, it certainly looks like the blue connector was plugged in wrong. It might have been and that would be on me and being careless. However, after seeing your comment I...
From a visual inspection, the connector pins line up with the edge pads. I also did a continuity test on adjacent pins and there is no shorts between any of the pins.
All four electrolytic caps on the main board have been replaced with new ones. About four tantalums have been replaced on one...
Without the PCB plugged in: 52 Kohm
With the PCB plugged in: 100 ohms
I ran through all the items in the checklist found here and everything checked out fine.
The +5V edge trace goes to pin 21 on the PSU
The two GND edge traces goes to pin 20 and 24 on the PSU and the 3rd GND wire goes to the...
This is the approach I took on a short I had on another board. There was a short to ground and by cutting traces I was able to narrow it down by one half of the board, then by one quadrant, then by one row. The only downfall is repairing all the cut traces afterwards.
The resistance between 5V and GND with the board unplugged is: 62 Kohm
Unfortunately, I do not have a variac but I do have a thermal camera. When the main PCB is plugged in, the main board does not light up anywhere, but the power PCB does by the four resistors next to the +5V adjustment pot...
I'm trying to diagnose the main PCB on my Midway Space Invaders. The linear PS is working fine and the +5, +12 and -5V rails are all within spec. As soon as I connect the harness to the main PCB, the 5V rail drops into the mV range. The +12 and -5V rails are fine.
From the Midway 8080 Test...