Good news, 3458A is fixed and calibrated
. ADC drift checked, input filter changed just in case, upgraded to option 001 which it didn't have. Will post pics when it arrives from Germany
Eventually I sent it to Adrian (ap) @ ab-precision.de who took good care of it. He was very professional and a pleasure to work with, I'll post the cal results later.
Also thanks to TiN for al the good help, rumor has it that he never sleeps as he will reply messages at any time
So the problem was in the Gate Array from A3, which was replaced, and this is what was happening:
When A5 does not receive any bits from A3 at startup it will throw an ISOLATOR FAILURE error. However in this case it did receive some lonely bits. This fooled A5 CPU which did wait for more data, until a timer restarted the communication process again. This continued for 5 retries after which another timer did cause the reset of the equipment with no errors displayed.