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Diagnosing PDP 11 h742a 15V failure

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  Diagnosing PDP 11 h742a 15V failure by Bill Degnan - 01/23/2017 11:27
The backplane lost the +/- 15V signal while I was testing the drive controller.

The power supply is hard to get to in the rack. Pictured is the h742a power supply disconnected from the rack but not the computer. I positioned two cinder blocks on each side of the rack with a strong board between them to form a table. Unbolted from the rack I was able to move the h742a 90 degrees to sit it on the board with the power connections intact. Now the unit is much easier to access. Click image for larger view.


I removed the power control board from two h742s including the unit to be repaired. It's always good to be able to compare and contrast a working board with a non-working board. Pictured is the inbound 15V signal connector ("IN") coming into the board, the fuse, and the outbound port P9 (OUT) that is routed to the h745 voltage regulator. Click image for larger view including both control cards side by side.


I knew that the likely issue was a blown fuse, why? I had previously installed a simple PC power splitter cable into the 15V power circuit. With this I could measure the signals on both sides of the fuse. Basically it's like an extension cable with a "Y" output. The "to P9" end connects to the other end of the P9 Molex connector, and the "for test" connector (with black electrical tape to cover when not in use) can be used to measure the voltage coming from the h742a *before the fuse*. The connector at the bottom of the photo attaches to the Molex connector coming out of the control board. Click image for larger view.


The splitter is installed. Click image for larger view.


With the splitter in place I could check the h742a fuse without opening the power supply enclosure by simply probing the test connector hanging down from the power supply. Turns out, yes it was the fuse. I must have caused this when testing the 15V signal coming into the M7762 RL11 control board. I still needed to pull the control board and set up the cinderblocks, etc but if the fuse had not blown, then I would have first turned my attention to the h745 voltage regulator.

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