Calibration procedure for Extech 22-816 multimeter for VDC and VAC readings

[cfkerchner]

Hoping someone can help me. I need the calibration procedure for Extech 22-816 True RMS multimeter for VDC and VAC readings. This is an older model.  I bought it on eBay stated by the seller as new.  It looks new but it apparently needs calibration (in my opinion) as it appears to be reading slightly out of spec on the high side. Probably from years of setting around in some surplus sales warehouse, etc. For example it reads 5.07 VDC on a 5.00 VDC reference source. It also is reading about 1.90 VAC on the high side for a reference 120 VAC reference voltage. I have sent an email to Extech support via their website asking for the calibration procedure for their model 22-816 a day or so ago. They have not replied thus far. I have taken the multimeter partially apart. See attached image.  I see on the PC board (6) six very small surface mount adjustable potentiometers: VR1, VR2, VR3, VR4, VR6, and VR7.  I did not see a VR5. Possibly that one is on the other side of the board to which I have not accessed as of yet.  Or maybe this model did not have a VR5. Thank you in advance to anyone who can help me.

[coromonadalix]

take a close picture of the main cpu could help maybe finding datasheets for it, a 4000 count dmm chip should be available ...

the only thing in mind,  measures and note all the trimpots values between the three legs before moving them, measure a low voltage value  ex: 20 volts 19 volts dc, try one trimpot after the other to see a value increasing or decreasing ... do it for the ac, ohms, milli amps  etc ....

[cfkerchner]

Thank you for the trial and error tip. Attached are some more pictures which show the (6) six potentiometers that I can see thus far: VR1, VR2, VR3, VR4, VR6, VR7.  As I mentioned in my initial post I don't see any VR5.  As for your suggested trial and error tip,  I think I will wait a few days hoping that someone can identify for me the specific pots to adjust for VDC and VAC calibration. A couple of these pots are for the temperature reading settings. I don't want to monkey around with the wrong ones and then have to calibrate those, lets say, using ice water and boiling water, etc.  But I do like your idea about making the potentiometer readings first. If I have to try your suggested way to find the correct pots to turn by trial and error, I will follow your suggested tip.  That was a clever tip. Thanks. See more pictures attached.

[coromonadalix]

please write us the markings of the main cpu  i cant see nothing

[cfkerchner]

Looking at the picture attached, the large rectangular many pinned processor IC is marked Silan SC9711, which is a multimeter circuit DMM IC per my Google search on that number.  See: http://pdf1.alldatasheet.com/datasheet-pdf/view/301471/SILAN/SC9711.html  There is a pin reference dot on the lower left corner of the large IC which you can see looking at the picture.  To the left and slightly lower is the 8 pin IC which is an Analog Devices AD736 True RMS to DC converter per my Google search on that number.  See: http://www.analog.com/media/en/technical-documentation/data-sheets/AD736.pdf  There is a pin 1 reference dot on the the 8 pin IC which you can see viewed on the picture. VR1 is to the left and just below the 8 pin IC.  VR2 is below several vertically mounted resistors below the right lower corner of the Silan SC9711 IC. VR3 is located above the row of 3 electrolytic capacitors which are just above the 8 pin IC in the picture, and below the relatively large black beeper/sounder part in the upper left of the picture.

Comment:  In my looking at the pinout details and Typical Application Circuit for the SC9711 on about page 10 of the datasheet it would appear to me that Extech has switched the naming of potentiometers VR1 and VR3 on the printed circuit board when compared to the SC9711 datasheet. That is on the datasheet VR1 is shown going to the pins for SC9711 that VR3 inside my meter goes to on my meter's printed circuit board.  Either that or I'm looking at things and counting pins wrong. Let me know what you think about that comment. See picture below.

[retiredcaps]

Rather than trial and error, see

https://www.eevblog.com/forum/testgear/adjustment-of-metrahit-18s-and-18c/msg736118/#msg736118

Conversely, someone else suggested using the heat from the tip of the soldering iron to warm up the various pots to see what changes.

[cfkerchner]

Well, after studying the SC9711 multimeter processor IC data sheet and comparing the pinouts to the relevant connections in my Extech 22-816 True RMS Multimeter, I deduced the trimmer resistors I needed to adjust. They were labeled on the printed circuit board in my Extech 22-816 True RMS multimeter as VR1 and VR3. See attached picture. I then deduced also that VR1 was likely the VAC adjustment since it was closest to AD736 True RMS AC to DC converter 8 pin IC. Next I measured the trimmer resistors existing resistance settings so that I could put things back the way they were if need be, and then did the trial and error as suggested earlier to confirm things, i.e., which one changed the VDC calibration and which one changed the VAC calibration. As I noted in my prior comments the trimmer resistors on the SC9711 typical circuit schematic are labeled with different VR numbers from the labels on the PC board in my multimeter. In my multimeter the trimmer resistor labeled VR3 calibrated the VDC and the trimmer resistor labeled VR1 calibrated the VAC.  And again, that makes sense for this meter's printed circuit board layout in that the VR1 trimmer resistor is located very close to the AD736 True RMS AC to DC converter 8 pin IC. Using another Extech 22-816 True RMS Multimeter that I've had for many years (and it was known to me be acceptably accurate in my use over the years) I calibrated my VDC and VAC for the Extech 22-816 which I had recently purchased on eBay. Now both multimeters report the same readings for the same reference voltages.  To get the VDC reading even more accurately calibrated (for both meters) I have ordered a precision DC voltage reference source from http://www.voltagestandard.com   When I get that I will then adjust both units using the precision 5.00000 VDC source as a reference. The VDC reading is the most important to me when I troubleshoot my computer power supply and motherboard, etc.   I'm not as concerned about the absolute accuracy of the VAC reading so long as both meters give the same reading for a 120VAC wall outlet at my workbench, and of course for the VDC reading for a 5VDC source, which they now both do. All is well now. Now I can keep one multimeter on my work bench in the cellar and the other one I can carry around in my travels as needed to help family and friends out with troubleshooting things, as I do sometimes. 

Thank you to all who have replied to my query and all the tips and help you provided.

P.S.  I now see that Google search makes this thread the number one top line hit for the term "calibration procedure Extech 22-816"     Hopefully this thread will help others as I was not able to find a single thing online before via Google searching re DIY calibrating the Extech True RMS 22-816 Multimeter. Extech support still has not replied to my email requesting the calibration procedure.   

[cfkerchner]

RetiredCaps - Thanks for replying. That cooling spray on the trimmer resistors to see which readings they affect and change idea is a good tip. I'll remember that one for the future. However, overnight and this a.m. I figured things out. The comments from "coromonadalix" prodded me to research the IC chips used and their pinouts and thus ultimately figuring things out. See my prior posting: https://www.eevblog.com/forum/testgear/need-calibration-procedure-for-extech-22-816-for-vdc-and-vac-readings/msg1352766/#msg1352766  Thanks again to all on this thread.

[retiredcaps]

I'm glad you got it working.  BTW, Extech has had CEM Instruments make a lot of their meters in the past and current models.  edit: The large tallish exaggerated lcd digits is a giveway of usual CEM components/design.

Yours is likely a variation of this series.  Basically, Extech tells CEM to make the outside green and orange and change some of the labels around.

http://www.cem-instruments.com/en/Product/detail/id/971

On your pcb motherboard, you might find something silkscreen like DT-92x?

[cfkerchner]

RetiredCaps:  Yikes. I find that Chinese company's URL link you provided has a website server which is extremely slow in transferring data to display their various pages. Or maybe they are pumping out way too many images with very high resolution and file size. Whatever, their pages are loading very slow and thus the site is not user friendly, imo.  But, yes from what I could see before giving up waiting for their servers to pump out the entire image their meters do look a lot like the Extechs I own. Thus as you say they make them for sale under whatever brand label you want put on them if you are willing to order a sufficient quantity. As of now, two True RMS multimeters are all I need.    Thanks for the link.