EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: iblackford on January 26, 2022, 05:54:02 am
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Hi All,
If my posts so far haven't given it away, I'm working through getting some of my old DMM's functional again. I've got a cosmetically nice HHM31 that is having issues.
This meter uses the UM7108F, but I haven't been able to find any schematics for it, only datasheets for the individual parts.
Symptoms:
-Ohms mode doesn't register or is way way off. Unusable.
-Volts mode, if I put in ~12V on the 20V setting, I get a reading of about -19V. If I keep increasing the voltage, the display voltage actually goes down to -17V, -16V, etc...
Things I have checked:
The UM7108F only has one differential ADC input, so all preconditioning and mode selection needs to be sorted before going into the ADC inputs.
1) VREF is ~86mV. Datasheet says this should be 100mV, so it's a bit off. I can adjust it with a pot nearby, but I never achieve 100mV when adjusting it.
2) Vin(differential) is ~-.169V when inputing ~12V, and it tracks down to -.1593V as I increase the voltage to ~15V (this behaviour seems to track the LCD behaviour)
3) Vin- and Vin+ on the UM7108 have 100k in series on their inputs. There is no voltage drop across these resistors, suggesting that the ADC inputs are not leaky.
I could spin my wheels and check other things, but I'd love some guidance on what to check next. Schematics would be greatly appreciated also!
This is an ok meter when it's working, and I'd like to get it back that way!
Thanks, Ivan
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Measure the voltage of the built-in voltage reference: "V+" (pin 9) to "Analog Common" (pin 46).
Must be 3±0.2 V.
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I checked it, that's 2.940V. On the Datasheet this is listed as the supply voltage, not a Vref, does that sound right?
Thanks, Ivan
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After doing a little more sluething, I found an analog switch, a CD4053, had gone leaky on the channel that was feeding the ADC:
(https://i.imgur.com/gmGDsQl.jpg)
manually bridging the channel restores normal operation! Now I just have to get that part on order and i have a working meter again!
Thanks, Ivan