Replacing failed powersupplies on qbus PDP-11s

Since I'm not aware of your electronics/troubleshooting knowledge, I thought I'd make mention of one of the known BA23/BA123 issues. When the boxes were first introduced, they were shipped with backplane power cables that had a major design flaw. They used IDC-style connectors on each end of what was basically a heavy-gauge ribbon cable. The Insulation Displacement Connectors tended to add significant resistance to the connection, and couldn't carry their rated loads. Depending on the load, the connectors would either discolor from the heat. . . . start to melt, or, in extreme cases, start on fire. An FCO was issued, and the "proper" power cables have individual wires, and crimped-on pins. I don't think it would be unreasonable to speculate that, if your particular power supply had an old-style connector at some point (which chose to melt) that the solder joints for the header could have been compromised from the heat. Check the circuit board where the header is soldered, to make sure there aren't any cold or bridged solder joints. A shorted pin could prevent the power supply from coming up. . . If you have the old ribbon-style power connectors, I would recommend replacing them immediately. I "rolled my own" with parts readily available at MCM Electronics. I'm sure suitable connectors could also be obtained from Jameco, Digikey, etc. T