Well, the Tek TDS644A I picked up works fine with no errors, contrary to what the seller claimed. It passes all tests multiple times with no issue at all, so I have a working unit on my hands... So boring!
I've ripped the firmware out for archival purposes and uploaded to Tekwiki, and enabled the expanded memory option too, and I have to replace a missing foot.
I think I'll check the PSU for any Rifas and the one cap for the standby power in the PSU that is
always high ESR and replace them if present (Presence of both depend on the particular PSU variant).
It barely needs a spit shine even, this unit 'just works'... I think I should be able to make a return on costs in the region of an order of magnitude I think. Then, I'll buy more!
Also, I poked at the HP 5445A Frequency Counter today too. I stole the display cover panel and a front panel potentiometer that is still nice and clicky for my unit, then replaced a capacitor in the PSU that seems to be a weak point and is a common failure.
Looks like this one is all working well too, just need to clean and reinstall all the button caps, and find a replacement for the display cover panel and it can be sold on too.