I found the issue. The A58 M/A/D/S unit was not powered by the A59 Source Interface Board due to a burnt resistor which was open.
The burnt resistor is a 2.2 Ohm, 3W, wire wound resistor and is on the input on the LM117 linear voltage regulator. VIN=15V, VOUT=9V.
The only reason I can see to have such a resistor on the input-side of the linear regulator is to filter out high-frequency components in the power rail before using the 9V to feed the amplifier in the A58 unit. The resistor must be used as an inductor.
An interesting finding was that the A58 M/A/D/S draws 1.3A from the 9V VOUT from the LM117. Given ohms law, 3.7W should be dissipated by the 3W resistor. It seems odd that that extra 24% of power could have caused the resistor to fail. I ordered a 5W replacement resistor, just in case.
Anyways, for now, I have replaced with a piece of solid wire and the instrument is able to do basic measurements. The attached image shows a simple (uncalibrated) S11 measurement of my 18 GHz cable going into my 22 GHz spectrum analyzer. I will next continue checking it out via detailed self-tests and do any needed adjustments.
/John.