^^^^^^
All of the squares have resistors... That one is nuts... To lazy to try to find the 220's.
I did exactly that to fix a digital oven display in our old home. It felt so satisfying to repair a $1000 wall oven with less than a dollars worth of caps!Bad capacitors are notorious on appliances (e.g. oven, range, microwave) displays.
Replace just those and you can usually read your display again!
Speaking about the venerable HP-8640B, mine is actually the military version called the AN/USM-323.
It is mostly like the civilian version, except that the military version can run on 400 Hz AC. Actually, the range of the AC frequency is 48-440Hz.
A ramification of the wide range of AC power frequency is that they could not use a regular fan. There's a whole extra board of electronics to run the brushless fan motor! Back then, there was no brushless fan with the miniaturized electronics built into the fan.
I tinker some. I recently built a GPS clock, shows both local and UTC within 40 ns, Keeps good time.
It surely does. Now, it needs to be put in an attractive enclosure.