I tracked them down in the schematic (just updated the text to reflect it) - the two in the analog section are actually just precision matched resistor networks! The one in the digital section is an unobtainium microprocessor
made by a company nobody's ever heard of, which specialized in MPUs for T&M applications. (I can't remember the name anymore) made by HP themselves, as free_electron pointed out - don't recall where I read that but I don't think it's true.
There are definitely some quirks. The digital board in particular looks like it was laid out rather carelessly - all sorts of pointless kinks and twists in traces, etc. The black jumper wire you can see in the
picture of the MPU installed in its socket? There's a perfectly good space on the back of the board to run a trace for it. (Possibly it's there to allow disconnecting for service, but why a long wire with crimped connectors when a two-pin header-style jumper will do?) Parts of it are clearly designed with servicing in mind, but others are totally not - for instance, most of the connectors are directly soldered/crimped. Removing the GPIB port to get the digital board out was interesting.... the ribbon cable is mounted into the IDC connector and soldered directly to the board. I had to work the connector itself out through the back panel cutout (it was that or desolder twenty-something wires).
Of course, if you don't need to fully
remove the boards, both of them are mounted in with flexible plastic hinges. Just unscrew on one side and fold out.
And I'm curious about that triax connector on the rear panel. It's directly paralled with a standard coax connector on the front panel, and the guard is undriven, so what's the point?
Unless it was a junk box replacement by a clueless person. That is a possibility - there was one screw missing from the chassis, so I don't think I am the first one inside this.
I don't think I'm going to have to dig far into troubleshooting this one. I think it's just some stuck keys. But does anyone have access to a schematic that is actually legible? Agilent's copy is ridiculously smudged, as usual. (How the hell does the company that made it not have access to decent-looking schematics??)