DMM Question Keithley 199

[MDM3D]

Hello I  garbage picked a Keithley 199 without the scanner card . The Amps, ohms both 2 wire and 4 wire read correctly, and the 300VDC and 30VDC ranges read correctly. However the 3VDC range tends to float to ~ -2.5VDC with no leads attached and the 300mVDC range reads overflow. BUT!! If I provide a voltage source(variable DC bench Supply) across the leads then it measures correctly to the best of my abilities(A Fluke 115 DMM). The 4 VAC ranges appear to read correctly.

Is it normal for this offset to appear when no leads are attached? EVERY other dmm I have used goes to zero when no leads are attached.

 I think their is a small "leak" because if I try to measure the ~-2.5VDC with the fluke then the error changes to ~10to20 mVDC, the fluke dmm has to low of impedance and is drawing down the offset voltage.  I have checked the power supplies and they are within spec and tried to poke at the mux JFET switches using debug mode and they appear to be doing what they are told. Any help would be greatly appreciated. I am happy to supply more info I just really don't know what would be relevant.

Is there any good way to find the leak without shotgunning parts? I really don't want to do that because there are a lot of matched pair transistors in this 199 and I really would not like to have to match my own and probably ruin the dmm calibration. and a goofy teflon spacer/lug with 6 T0-92(JFETS?) attached to it that look like an absolute nightmare to disassemble.

Thanks in advance for any help. It looks to be a nice meter especially if i can get my hands on a scanner card.

[Smith]

Doesn't sound like a problem to me. Lots if meters float, especially in the lower voltage settings. The Keithley 199 does too (I have one). It's the price you pay for high impedance inputs. When you get to the 30/300V range, the input impedance lowers to 10M, wich is enough to prevent it from floating.

[Dr. Frank]

I strongly recommend to read the manual!

Ordinary DMMs have 10M impedance on all DCV ranges.

Bench DMMs usually only have 10M in 1kV and 100V due to the internal 100:1 divider, but the 10V , 1V and 100mV ranges have none, so these are really high Ohm, only pA input bias.
The KEI199 equivalently has 300V and 30V ranges with 10M and 11M, and 3V and 300mV are > 1GOhm  ( see datasheet.

Therefore, it's completely normal that at this high input impedance, the readings starts floating.
At first, the input amplifier picks up any stray radiation.
Secondly, the amplifier integrates its own bias current into the input capacitor.

If you apply a bigger, known capacitor of say 100nF parallel to the input, you may calculate this bias current from the linear voltage change.

Frank

[MDM3D]

Awesome! I love it when I am the problem that needs fixing. A good rap on the head and the repair work is complete. 

[tooki]

Yep, totally normal. It should zero if you short the inputs.

As an aside, I'm jealous! I'd been looking for a 199 for a while, on account of the large LED display. (I already had a 197, which is non-backlit LCD.) Ended up getting a 2015, which is lovely with a VFD display, but still not as big display digits!

[Smith]

I know. I have multiple Keithley  2k series meters, but I still preffer the Keithley 199.

[tooki]

I know. I have multiple Keithley  2k series meters, but I still preffer the Keithley 199.

ugh... must... not... buy... more... test... gear....: http://www.tutti.ch/bern/tv-audio/audio-hi-fi/angebote/nixie-roehren-digital-multimeter-dmm-7005-systron-donner_14389777.htm

[macboy]

I know. I have multiple Keithley  2k series meters, but I still preffer the Keithley 199.

Same here - my two 199 meters get powered up at least ten times as often any of my 2001 meters.

MDM3D:
This is a fantastic meter than you scored. Thanks for saving it from impending destruction.
BTW, are you certain is doesn't have the scanner card, or is it just missing the connector boards? Those frequently go missing even on meters with the scanner.