Ancient 5 1/2 digit DMM - voltage with floating inputs?

[Dave92F1]

I picked up an ancient (~1975) 5 1/2 digit DMM on eBay (Systron Donner 7205).

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".

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.

 

[duke37]

You seem to have found an excellent meter. An input impedance of 1000MΩ is very high and the scope will take very little current from the circuit you are measuring.

There will be an input capacitor which will be the capacitance of the input device (transistor or valve?). My calculations show that this is very small. When you disconnect it from the 5V it will discharge this capacitance through the 1000MΩ.

It gives crazy voltage readings on power up since the input will be measuring the charge in the air. Try brushing your cat nearby!

You can get switch cleaner to have another go at the switches.

 

[Dave92F1]

Thanks; I suspected as much, but none of my other DMMs do that (not even the Fluke 867B, tho that's only 4.5 digits).

I really bought it for the Panaplex display (neon orange gas discharge). If I can get it working as a reliable DMM, that's a bonus.

I just ordered some DeOxIt from Amazon - people seem to think that's good stuff.

 

[shrtrnd]

Like duke37 said, if it has a tube or tubes, I'd suspect them first. It would be wise to clean contacts, but it sounds to me like if it's not using vacuum tubes, a capacitor is leaking and needs to be replaced.
Good luck with the project.

 

[Dave92F1]

No tubes; it's all solid state. This was a super-high-end thing in 1975 (probably $3000 or so in 1975 dollars - you could buy a nice car for that much). I paid $50 for it.