Keithley 196 AC Voltage Measurements Incorrect - Page 1
Forum section: electronics / testgear

author: RPP
author_rank: Contributor
timestamp: June 03, 2022, 07:28:29 pm
content: I picked up a clean Keithley 196 last week for $50, which i thought was a pretty good price, working or not. Turns out the DC and resistance readings are spot on, but the AC readings are off. With no leads attached and with leads shorted, it reads -15.4xxx VAC. I took a look inside and tested voltage rails - everything looks OK. I did a quick check of input AC voltage vs Keithley reading and it shows that the readings are linear, but consistently low (see graph below).I'm guessing this is just a calibration issue, but would appreciate some feedback before I dig any deeper.Rich

author: Kleinstein
author_rank: Super Contributor
timestamp: June 03, 2022, 08:13:32 pm
content: negative AC readings look a bit odd. If it is an issue with the calibration, it is way off - like a memory failure or user error in the cal procedure. The output of the RMS chip should be a god test point to look at - this may be a little off, but should not be off that much.Another point to look at is the amplified AC signal - this should be relatively OK handling a square wave and obviously not oscillating.

author: coromonadalix
author_rank: Super Contributor
timestamp: June 03, 2022, 09:04:01 pm
content: lucky you schematics are available, you have to follow the measurements paths in the meter ... if the supply lanes are good, progress slowly, dont change parts for nothing

author: RPP
author_rank: Contributor
timestamp: June 04, 2022, 04:21:39 am
content: When I cracked it open, i found the calibration switch 'turned on' and the red and green wires going from the input jacks to the L3 inductor were reversed. I suppose that means someone was in there without really knowing what they were doing. . . . maybe a mis-executed calibration makes the most sense at this point. Or having the inputs reversed and sending AC voltage through the wrong path fried something, although nothning measures bad in the front end of the circuit. I'll have to find an AC source to try the calibration as a next step, I think.Anyone ever used an SMPS transformer in reverse to boost an AC signal from a signal generator?Rich

author: Kleinstein
author_rank: Super Contributor
timestamp: June 04, 2022, 07:11:25 am
content: I have uses a more normal (~ 3 VA iron core) transformer to boot the output of a function generator. It worked OK to at least some 25 kHz.A frist point to check would be how the other (lower) ranges behave. The 3 V and 300 mV range use quite a different amplifier path.Even with swaped leads to the terminals there should be no easy way to cause damage, as the DMM should be reasonable protected against using the wrong inputs. The error also does not look like a bad input. It looks more like totally off calibration. One could check for a loose shield or similar that could have causes extra hum and this way a totally off calibration.It would still make sense to check the output of the RMS converter (pin 14 of U27) with a shorted input or measuring AC current with open inputs. This should be reasonable close to 0 V.

author: RPP
author_rank: Contributor
timestamp: June 04, 2022, 01:31:05 pm
content: Thanks KleinStein. I'll take some readings on U27 and run some tests on the lower AC ranges this weekend.Rich

author: RPP
author_rank: Contributor
timestamp: June 05, 2022, 04:21:02 am
content: Some progress. The AC readings on the 300 mV, 3 V and 30 V all read fairly close to actual, so it's just the 300 v range. I measured pin 14 on U27 with the leads shorted and got 0 VAC (using pin 3 for ground). With the leads open, it also measured 0 VAC. So it would seem it just needs a calibration. Anything else I should check?Rich

author: Kleinstein
author_rank: Super Contributor
timestamp: June 05, 2022, 07:34:03 am
content: If U27 pin 14 reads near zero and the DMM a totally off, this really looks like bad calibration constants. There could be a slight offset or background from noise, but this should not be much. Subtracting is tricky with with AC anyway, especially if there are 2 parts.If the DMM allows separate calibration of the 300 V AC range it is fine. These cal constants are known bad, so nothing to loose there.If the calibration could only be done as a whole it would be more tricky to somehow save most of the old cal constants.I don't know this meter much (it is a long time ago I used one), but some meters allow to read and save the cal constants via GPIB, just in case something goes wrong with the calibration.

author: RPP
author_rank: Contributor
timestamp: June 05, 2022, 02:18:11 pm
content: Looks like i can calibrate just the 300 VAC range, so I'll cobble together some crude voltage sources for the 30VAC and 300VAC reference points needed. It won't be super accurate but at least I can determine if everything works properly. If it does hold the new calibration constants, I'll probably send it in for a proper calibration. Looks like i can get it done for $80 + shipping, so it would be worth it to have an accurate DMM on the bench. Thanks for your help.Rich

author: RPP
author_rank: Contributor
timestamp: June 05, 2022, 05:55:52 pm
content: Calibration complete. The 196 is extremely easy to calibrate - I only did the 300 VAC range and it allows you to input the actual voltages you are using for high an low points so I cranked up about 140V at 500 Hz which is the most i could get out of a 3VAC/120VAC transformer wired in reverse to my function generator. For the low point I used 20 VAC. In both cases, I just entered the actual reading from my Tek scope which is calibrated. Works perfectly now.Rich