I began by clipping onto the diode packs to test connectivity from the G3 card pins D06 and B13. All diodes are good, no open or short situations. I then clipped leads to the inputs of the diode packs - for the Read Driver (RD) and Write Gate (WG) signals - and tested continuity back to card slot G3 pins for those signals. The WG line had continuity but the RD line was open!
|Diode board on core array, diode packs I tested circled|
In any case, I can compensate for this by running a wire to jump over the bad run. This is good news, other than the question of whether this failure portends future failures in similar components that may be more serious to track down and repair.
Specifically, if it is the backplane, I have already had jumpers over failed traces in a couple of locations on each of the two memory backplanes, thus this will be another in a string. The earlier trace failures happened several decades ago.
The earlier failures could have been a manufacturing defect although if it had been under IBM service contract they would have replaced the backplane. That means it was more likely something that developed in later years, before the machine was decommissioned but after it went off maintenance contract.
After tracing, the defect was where I suspected - lack of continuity from pin D06 of socket G3 to pin B03 of socket D5, meaning a failed trace on the backplane. I tacked a jumper wire on the back of the connector block at D5, pin B03 and inserted the jumper onto pin D06 of socket G3.
|Lack of continuity between these two pins on backplane|
|Jumper restoring continuity of signal|