How to Adjust DCV on DMM Circuit Board

[Electro Fan]

Hi,

I have a Radio Shack 22-812 that reads a bit low on both DC volts and  DC amps vs. my Fluke 179.  I'd like to try to make the RS align with the Fluke.  On the circuit board of the RS is an adjustment of some type for DCV (and also for ACV, 20uF, and 200nF). 

1. Any chance if I just adjust the DCV setting I can get the DC volts to read a few more millivolts?  (On the same circuit the Fluke reads 1.555 volts and the RS reads 1.549 volts)?
2. In looking at the DCV adjustment point it looks like some sort of very small POT that seems to have a pin size hole the size of a needle in the middle (it would have been a lot easier if it has been designed for small screw driver).  Any idea about what sort of tool is needed?  And is it just a matter of turning the setting slightly clockwise? - Or am I looking at the bottom of the adjustment point and even though it's labeled facing this direction maybe I need to remove the circuit board and adjust it from the other side?

Thanks for any info.  EF

[IanB]

Those meters tend to be slightly off in their factory adjustment.

I have adjusted the DCV pot, but I have not touched the others. You should find it will turn with a cross head screwdriver.

Set the meter on the DC V range and compare it to a trusted reference. Then carefully turn the pot by fractions at a time until the meter agrees with the reference. A reference voltage somewhere in the range of 5 to 10 V would be good.

[Electro Fan]

Thanks Ian, You were right; a small flat head screw driver did the trick.  With a bit of delicate back and forth it now seems to read the same voltage values as the Fluke at settings of 8.558 volts and 0.558 volts.

[Electro Fan]

fwiw, and I know this is pretty basic and preaching to the (experienced choir), having two DMMs that read the same is a major improvement in operations.

How common is it for multi-meters to have an adjustment setting?  If they don't, they should.  If they did it would probably make sense to get one good DMM (a Fluke or whatever) and then synchronize the lesser units with the ref unit.  I realize it depends on how accurate each DMM is for each type of measurement as to whether they could be synchronized overall and at a more basic level it would require the ability to have an adjustment for each type of measurement (unless some measurements rely on others?), but if users could somehow make their own fine tunes with the use of one reference device it would sure make the whole deal better.  Duh. 

[Wytnucls]

In the industrial world, meters are not adjusted unless readings fall outside of the stated accuracy tolerance specified by the manufacturer.
Regular calibration only requires the recording of deviations from an accurate standard over time.
Usually, cheap meters can only be adjusted via trimpots. More expensive meters can be adjusted either through trimpots or internal software. The best industrial meters can only be adjusted through software.

http://www.euramet.org/fileadmin/docs/Publications/calguides/EURAMET_cg-15__v_2.0_Guidelines_Calibration_Digital_Multimeters_01.pdf

[beeson76]

I just bought one of these Multimeters at Radio Shack for about 15.00 dollars.  Brought it home and tested it out in the ordinary electric outlet and it measured 145 volts.  Thought that was a tad bit too high so I got my Klein Multimeter and another cheap multimeter out and tried the same plug and they both tested at 124 volts.  So this multimeter was about 20 volts off.

I tried to adjust via the the little potentiometer and I could only get it down to 140.  Does that sound right.  I was able to adjust it the other way to 157.

Could I get any help and if this sounds right.  Is there another adjustment I can mess with.

[Wytnucls]

That would be way out of tolerance, even for a cheap multimeter. It is probably faulty. There should be a separate trimpot for Volt AC. Make sure you're tweaking the right one. Be extra careful playing around with 124V AC on the test leads, with the meter open. Rather get a replacement unit or buy something more reliable.

[alm]

You should use a heavy hammer to adjust this particular multimeter.

[IanB]

I just bought one of these Multimeters at Radio Shack for about 15.00 dollars.  Brought it home and tested it out in the ordinary electric outlet and it measured 145 volts.  Thought that was a tad bit too high so I got my Klein Multimeter and another cheap multimeter out and tried the same plug and they both tested at 124 volts.  So this multimeter was about 20 volts off.

I tried to adjust via the the little potentiometer and I could only get it down to 140.  Does that sound right.  I was able to adjust it the other way to 157.

Could I get any help and if this sounds right.  Is there another adjustment I can mess with.

Sadly the meter seems to be faulty. It should not read that badly. Can you exchange it?

[PA4TIM]

Before you "screw" it up  first check the battery.

Then check the maual about how much % and digits it is (5% and +/- 10 digit for instance. The 10 digits on a 2,5 digit meter is more a problem as on a 4.5 digit meter. and do the mayh. And remember, these are not true rms meters and our mains used to be a nicwe sinewave but that was long ago. So check frequeny range and crest factor for AC.