A,B, and D PC/16em boards are working but C never does.

A,B, and D PC/16em digiboards are working but C never does. The system has been working fine for decades but now decides to only recognize 3 of the 4 boards. Switching around the boards, changing cables, and even switching out the original C with other spares changes nothing. It is always the same 3 letters that work no matter how they are linked. This is on a SCO Unix (3.4.2) Pentium machine.

Riyadh White Water
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It turns out there were 2 bad power supplies. The initial tests showed the expected 5 volts. Since we only changed 1 item for each test to prevent confusion, at least 1 of the bad power supplies was always connected. Later, when retested under load, the voltage would drop and it became obvious they were bad. Replacing the bad power supplies has fixed the issue.