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The SCC seems the most likely problem. This is a Zilog Z8530CS according to the schematic diagram (page 217), A1U209.

Pictures

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Fixing the issue

The machine seems to work fine, but the pod selftest fails.

I entered a simulate program to test the pod (the DTE test as described in the service manual for the 18179 pod around page 350). This sends data and toggles the rts and dtr lines. Running this program shows dtr nicely flashing on and off- but dtr remains space.

Following the schematic diagram for the CPU card we see that _DTRA_ (negated) leaves from pin 16 of the scc. The oscilloscope confirms that there is an inverse signal there.

Probing the same signal on the pod print (the small print containing the input opamps) on U600 pin 13 shows no pulse. Conclusion: the cable is bad. Checking continuity on the cable indeed showed a single pin not being connected (pin 26)..

Re-pressing the cable solved the issue, and the test now passes.

Time to put the whole thing together again…