OK so I replaced the EEPROMS, programmed with known good firmware v1.0
Whilst waiting for the chips, I realigned the HP1349A display with it's internal testcard. It's working great now, bright and focussed.
I guess one good thing about a 35 year old machine that's been presumed dead on a shelf for 18 years, is that the display is only 17 years old :-)
When I power on, with the new firmware, I get the same, one-line message as before.
E S@M G@IL!1=PQPQPQPQPQPQPQPG>m
I had guessed this was a checksum error message, thrown up by the boot loader?, but now it's focussed and readable, the message itself seems corrupted?
Pressing "Preset", I get a small "HP" logo, then the one line message again.
So the CPU appears to be alive and trying to run something...