Well the unit arrived and sure enough error 625 is present. Outer condition is excellent, opened it up and removed the logic board.
My heart sank when I saw four 8 pin devices with pretty grotty looking soldering. Turned out they were the opto isolators for the data between processors. Gave the soldering a touch up and cleaned the mess of old flux off the board. Nothing else visually but someone obviously had a go. Put the card back in and powered it up. Supplies are all present, battery voltage at 2.82V Will fit a replacement once I can lay my hands on a Panasonic BR2477A/VAN in the UK. Digi-key lists them but can't transport them! Outgoing processor 5V_ER supply present, both the 3.8MHz and 12MHz clocks are running the 5V_ER power on reset goes high, low then back to high. The main processor sends some data via the opto and buffer to the outgoing processor but no reply back from the outgoing processor. While in the chassis it is difficult to probe the outgoing processor for activity on all the pins. Think I have to get the card out and under power to test further. I'm hoping it's not the outgoing processor itself U305-PROG PART LOWER LEVEL 1821-1876 Manufacturers part number 34970-88803. Don't know how easy it will be to obtain one? Are Keysight likely to provide components rather than complete assemblies? Get another go at it next week sometime hopefully something else holding the outgoing processor in this state?
5V_BP is switching and the IO_RST to the GPIB controller is happening so U305 can't be completely dead as it switches both of those, processor must be running. Just not sending data back to the main processor?
Was thinking perhaps an interupt from either of the io devices may be holding things up?
With GPIB interface selected
UART interupt is low and remains low, GPIB interupt goes from low to high on power up. Which inturn sends the common interupt line low to high on power up?
When the RS232 is selected UART interupt goes low to high GPIB interupt goes low to high and Common interupt goes high to low.
Does that sound about right?