I recently adopted a four channel 100 MHz 54501A.
I don't have any pictures of its guts yet but this thing is
incredibly for being just shy of 28 years old (assuming that the date stamped on the inside of the case back that reads "OCT 21 1989" is indeed the manufacturing date). Judging by the calibration sticker dated 8/29/2003 it came from a DaimlerChrysler facility in Huntsville Alabama. I've only taken a cursory look inside the unit, just enough to feel comfortable enough to briefly power it up.
As expected the DS1235Y NVRAM battery is dead so that's the first order of business. I can replace the old Dallas part with, a new DS1235(Y) or,
according to Wuerstchenhund[1], a
Cypress STK16C88 nvSRAM[2]. I've also heard that there's an FRAM part that can be used (maybe the FM16W08) but there's
conflicting information[3] on its suitability. The DS1235(Y) and STK16C88 parts are similar in price on eBay (new old stock), US$15 or less from US sellers.
From my reading the other things that I should look into replacing are the PSU caps and the Schaffner mains filter. I'm just having a difficult time deciding how far to go with this. While a HP 54500 scope is perfectly usable I plan to replace it with something a bit more modern that doesn't tip the scales at 10 kg (22 lbs) like a DS1054Z when I can.
So, while it's a bit old it's not a total boat anchor and since I spent exactly zero dollars for the scope and three 100 MHz HP probes I'd say it was a decent deal.
[1]
https://www.eevblog.com/forum/repair/hp-54501a-troubleshooting-repair/msg213749/#msg213749[2]
www.cypress.com/file/111946[3]
https://www.eevblog.com/forum/testgear/hp-54501a-restoration-and-calibration/msg939719/#msg939719