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| Keithley 196 AC Voltage Measurements Incorrect |
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| RPP:
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 |
| Kleinstein:
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. |
| coromonadalix:
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 |
| RPP:
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 |
| Kleinstein:
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. |
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