3-4 years ago, one of the broken 3458A's I bought had a bad A3 board. Given the recent repairs and valuable info from TiN I thought I would give it another shot. All of the voltage checks were performed years ago, but I went ahead and did them again. This was a donor board as I had acquired a replacement from Agilent.
The board had a 202: Slave Test Convergence error. Based on TiN's recommendation, I went through the ZR_LO and ZR_HI inputs from the A9 reference board. Then I went through the +12ref, -12ref, and +5ref to confirm they were working.
I checked the Reset lines and clock to the 8051 (U220). That looked good.
TiN had mentioned the TARM command during his repair that I didn't previously try. Using the command, I watched the outputs of pin 6 (OUT) briefly change on U110,U111,U112, and U140. So far so good.
When I got to U142, I could see the negative transition on the input, but the output never changed from +4.2V. After testing it a couple of times, I decided to replace U142 and fired up the 3458A.
The meter came up and was reading DC.
I ran the self-test and ACAL DC successfully.
The EL2018CN was sourced from UTSource .
Thank you TiN and all the others that have added to this thread.
. Next step, repair defective A2 board with EL2039 from the same seller.