Step function broken
Testing revealed that the STEP key did not produce any responses in any of its operating modes. This was disappointing because the 9946 IC on the keyboard PCB that generated the outputs for STEP and REC PROG had already been replaced and the relevant outputs had been verified as good.
Repeat testing showed that the keyboard output for STEP was indeed working.
Testing STEP with Diagnostic Microcode
The keytest diagnostic code is able to report the STEP key and running this code showed no response to STEP but correct reporting of the Learn/Run buttons and Fl/Sc.
This was very helpful as all of the inputs above are gated onto the B-bus when B-bus source is set to 2. Since STEP was being generated correctly at the keyboard, but not appearing in the B-bus when other related inputs were appearing, this provided a clear indication of the place to look.
Faulty 9946 in B-bus Gating
The signals above are gated onto the B-bus with a 9946 IC connected in wire-OR mode. Instrumenting this IC with the MSO logic analyzer showed that the 2-input NAND gate responsible for gating STEP onto the B-bus was stuck.
Replacing this 9946 with 74H01 and appropriate pullup restored STEP function