Tuesday, July 26, 2016

Memory defect repaired, full 16K words working on the 1130 again


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
The signal runs from Slot G3 pin D06, over the backplane to slot D5 pin B03. In slot D5 is a jumper connector block that should connect pin B03 to pin B12. From there, the signal runs from pin B12 up through the core array to the diode pack. There are three 'runs' where the break may be occurring, which I will now trace to the failing component.

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
The 3467 cards were replaced in C3 through L3 positions, everything tidied up and closed, then I did a test. Using the storage load and storage display CE switch functions, I successfully wrote and read every address in the full 16K of memory. Problem fixed!

No comments:

Post a Comment