Keithley 199 behaves strangely in resistance mode

[Hrvoje_CRO]

Hello,

Well i have the following issues with my 199.
When in OHM mode i got under different ranges different readings. For example i've measured 10k resistor and sometimes it measures as it should, but if i measure it again it shows 23k. Also when i switch on autorange function under resistance mode, it does not works as it should. Display toggles between K and M letters when measuring 100k resistor. Lower resistance values are measured fine and accurate when autorange enabled, but if i manually change ranges it shows different values under different ranges...

So what could be the cause of the problem? All other functions works fine and with good accuracy.

Thank you in advance!

Regards,
Hrvoje

[Kleinstein]

The wrong reading in some ranges suggest a defect that mainly effects some ranges. A service manual for this DMM is available. The different ranges use different paths. If it is know which ranges work and which don't should give a good idea where the defect is.

Not working right in some ranges can also upset the auto-ranging.

[srb1954]

I also had a problem with erratic readings with my Keithley 199. Eventually it stopped giving any sort of sensible reading on some of the voltage ranges.

I tracked the fault down to a bad COTO reed relay on the range switching. I had to search all over the world to find a replacement relay as it seemed to be a custom or obsolete version that wasn't listed in the current COTO catalog. Replacing the relay fixed the problems with erratic voltage readings although the meter still does go berserk sometimes when powering up.

I don't know if a bad relay would affect the resistance ranges but I would check the contact resistance on all relays on the front end just to be sure.

[kawal]

From what you describing the issue could be internal to meter or interference coming from noisy environment (switching power supplies, Leds...).
Try turning off  all LEDs  around the area and repeat or use shielded cable to make measurement.  I  Have a 196 which uses inductors on input to filter out common mode interference. This is common and very efficient , making readings very stable as compared to 192 that did not have inductors on input.
Same approach is used in many other meters  like HP 3457A .......

From the 199  schematic https://doc.xdevs.com/docs/Keithley/199/199_901_01D.pdf it looks like the input has no inductor . The other issue is age of the meter , Check power supply for dried out caps. I had one 195 that had dried out caps and would make the meter unstable. The common mode inductor will not hurt.
  Also had one meter with dirty front/back switch which was causing errors on readings .  Cleaning the switch helped that meter.