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Old 6th Jul 2016, 11:54 pm   #1
threeseven
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Default Reading anomaly on old Philips DMM.

Was just measuring a 1.5 volt battery on this meter and noticed an odd reading anomaly.
It reads about 50 mv higher in reverse polarity than normal polarity of the test leads! Checked it on another meter and the reading was identical in forward or reverse polarity.
Any ideas on what could cause that?
For reference the meter is a PM2517E.
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Old 7th Jul 2016, 12:02 am   #2
TonyDuell
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Default Re: Reading anomaly on old Philips DMM.

Does it read exactly 0 if you short-circuit the test leads in voltage mode?

What happens if you measre a different voltage battery? Is the 50mV an offset (so that it reads 50mV more in one direction than the other no matter what the input voltage is), or a different sensitivity for the 2 polarities (so that, for example, if a 1.5V battery gives a 50mV error, then a 3V one would give a 100mV error.)
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Old 7th Jul 2016, 12:35 am   #3
threeseven
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Default Re: Reading anomaly on old Philips DMM.

Shorting the probes doesn't zero the display, it reads approx -122 mv, slightly fluctuating with probes apart, -40 mv, stable with probes shorted.
The difference on the battery should have been 80 mv, I had brain fade!
Just checked the difference at 3, 6 and 9 volts nominal on a bench PSU and the difference is as follows,

82 mv, 89 mv, and 92 mv, at 3, 6 and 9 volts respectively.
It isn't a big difference but 2 other DMM's read exactly the same at forward or reverse polarity, so I'm just intrigued as to what could cause it.
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Old 7th Jul 2016, 12:45 am   #4
mhennessy
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Default Re: Reading anomaly on old Philips DMM.

The usual cause of this is the integrating capacitor used by the ADC. If you check the datasheet of well-known ICs like the ICL7107, you'll see they recommend using polypropylene types for this...

However, I assume it did work correctly once? So Philips would have no-doubt fitted the correct type initially. But has it degraded in some way? Or is there some contamination on the PCB that is causing problems?
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Old 7th Jul 2016, 4:53 am   #5
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Default Re: Reading anomaly on old Philips DMM.

There's also offset voltage in amplifiers and comparators, and one that's not so well known: bias current flowing in imbalanced impedance presented to both inputs of an amplifier/comparator. There may be trimmers for some of these. I don't know that meter specifically.

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Old 7th Jul 2016, 9:50 am   #6
mhennessy
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Default Re: Reading anomaly on old Philips DMM.

I've just checked my PM2517X (same apart from LCD rather than LED readout), and it correctly reads 1.000V and -1.000V. With input terminals shorted, it reads -0.4mV.

Have you found the service data? I have a (fairly poor) copy if needed.
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Old 7th Jul 2016, 10:15 am   #7
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Default Re: Reading anomaly on old Philips DMM.

Thanks guys. The service data would be appreciated, thanks. I have the LCD version too, it's located elsewhere but I'll try it later. I was thinking along the lines of offset voltage trim but I've never looked inside one of these so don't know what adjustments are provided.
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Old 7th Jul 2016, 10:42 am   #8
mhennessy
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Default Re: Reading anomaly on old Philips DMM.

It's too big to attach here, so here's a link to it in my Dropbox: https://dl.dropboxusercontent.com/u/...Multimeter.pdf

Looking around, I probably found it here originally: https://archive.org/details/philips_Philips_PM2517

I also found this, which looks like interesting reading: http://www.extra.research.philips.co...978_79-181.pdf
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Old 7th Jul 2016, 12:57 pm   #9
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Default Re: Reading anomaly on old Philips DMM.

Many thanks for all that! The review is really interesting and I note from the image of the PCB that there are plenty of presets to adjust!
I do like using this meter so getting it back in calibration will nice.
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Old 7th Jul 2016, 10:33 pm   #10
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Default Re: Reading anomaly on old Philips DMM.

It might be a good idea to clean the metal parts of the test leads and their sockets in case some form of contamination is creating a battery effect.
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