I picked up an ancient (~1975) 5 1/2 digit DMM on eBay (Systron Donner 7205).
Oh, you got that one! I was watching it for days too, undecided if I should buy it or not, then I thought... nah.. I don't need another 5-1/2 digit multimeter, I wanted my next multimeter to have more resolution. So I passed on it, but I just love anything Panaplex, it was what caught my eye. That, and the fact that the seller had the manual with the schematics, too, made it less risky. However, I wanted more resolution on my next meter so I ultimately let it go to someone else.. that was you. Nice to see someone here got it. So I'm definitely curious how this comes along for you. Do keep us informed!
Regarding the schematics, there are no 7205 schematics on the net. Do you have the ability (and time) to scan the manual with schematics? It would be a great service to the net-at-large to have it at the BAMA site and /or at ko4bb.com
What should a DMM like this read for voltage with the inputs floating? After disconnecting from a test 5 volts, the reading gradually drops toward zero, but incredibly slowly (~300 microvolts/second). It's like there's a capacitor in there that's slowly discharging.
Is that normal on a DMM of this vintage? The input impedance is speced at "> 1000 MOhm".
As answered already, yes, the slowly counting down numbers in the DC range with leads floating is normal for any high-impedance input DMM. As long as it's stable with a real-voltage, there's nothing to worry about (aside from accuracy!) You can bring your hands closer to the leads and it will pick up a charge from your hand, and bump the numbers up
It also gives crazy voltage readings when it's first powered up (no leads attached). But it goes close to 0 when I short the leads (needs a calibration).
Other than that it seems to mostly work, altho it's a bit "flaky" - sometimes it works great, other times does odd things. I think the giant multi-pole button switches in it need cleaning, but haven't figured out how to get at the wipers. I tried 100% ethanol (drops from a syringe) + vigorous working of the wipers, but it didn't seem to help much.
It's probably going to need a good cleaning, and check the power supply rails and caps ESR if you can. They will be old by now, might need replacement.
I've seen the other site, but do post some pics of yours, please.
Congratulations, it looks like a great meter to own!