So far, the system seems to be working (although 15 years out of calibration). The CRT is in fantastic shape, the source port is good, the inputs are good, all the buttons are working, etc....
The initial failure mode was that after it was powered on, it would just beep. Nothing on the display. I took the covers off to look for obvious things but found nothing. The inside was rather clean - even the fan was barely dusty. The service manual outlines the boot sequence but says nothing about a constant beeping. Considering it would not boot up - my first look was at the CPU card. While it was beeping at me, I noticed a reset button that re-boots the digital section of the system. When I pressed it, the result was the same as doing a power cycle but with one difference - the CRT was warmed up. When I reset the digital section with the CRT warm, I could see the error message pop up for about 1/2 a second before the screen blanked and the beeping started. This error message (in the pics above) clearly pointed to a specific ROM chip on a specific board. At this point, I thought the repair was over. When @TheSteve posted the link to the site where the ROM images were posted, it started looking much better.
I ordered a TL866A programmer and a dozen EEPROMs to replace the (
28pin DIP 256k PROM's. They arrived yesterday and I was successful in getting the EEPROMs programmed and proceeded to remove the originals off the board. I do not have a good through-hole de-soldering solution that can deal with these chips so I had to fake it. The original plan was to simply cut the legs and trash the chips, but that idea was abandoned because I wanted to be able to easily read the chips and compare them to the new ones. I only have a Sold-a-pult for de-soldering which does not work very well so I used my messy air-blast technique. Essentially - I melt the joint and use a fine tip compressed air gun to blow the solder out. It used Kapton tape to contain the splatter and it totally frees the chips with no whiskers that can pull pads off the board. You can see in the pics that it is a bit of a mess but zero damage to the board. Easy to clean with IPA. 224 pads - no problem.
After the PCB was cleaned up, I installed sockets for the new EEPROMs and plugged them all in. The system booted up, passed all the checks and it looks like we are in business.
Thanks @TheSteve for the tip - that was critical.
The instrument will be helpful in testing control loop response at low frequencies.