Bench DMM Calibration

Those pots may be span and zero, with separate span pots for different ranges. If the calibration is so far off that the meter is useless, some playing around ....er.... experimentation.... may reveal what's what.

Short the leads and see what the meter reads. Connect a known stable voltage and see what the meter reads. Pick a pot and give it a quarter turn and see what happens with the voltage applied and with the leads shorted. Did the reading with voltage change? Did the shorted reading change? If the shorted reading stayed about the same but the reading with voltage changed, it's probably the span pot. If the shorted reading and the reading with voltage changed by about the same amount (readings offset about equally), it's probably the zero pot. If neither reading changed, turn it back to where it was and repeat with a different pot.

The span pot can be considered as a multiplier. The shorted readings and readings with voltage will each change by some percentage. The zero pot can be considered as controlling offset. Shorted and with voltage change by some fixed amount.

If you can determine span and zero pots, adjusting them will be an iterative process. Adjust span to get voltage to read right. Adjust zero to get zero to read right. Repeat, hopefully getting closer each time.

The pots may be very sensitive, to the point where putting a screw driver in the pot and thinking about turning it makes a difference (veteran of aligning 14 track FM tape recorders...very tedious at times!).

Morning Popcorn and Thank you very much for that.

At the moment I'm reluctant to do any 'experimentation' for 2 or 3 reasons .. .. .. .

Firstly, your diagnosis procedure reads as quite straightforward, but with my ham-fisted luck nothing ever goes quite to plan;
Secondly and perhaps the most serious, the three pots are situated directly opposite the mains input, just about an adjusters length apart and the likelihood of connecting mains voltage to a pot adjusting screw is very real. There is a battery power option although part of it has been removed, I would want to reinstate and use that in preference to mains.
Finally, at the outset I contacted several local Calibration Services but received little response; however, I have now had an email from one who profess the ability to calibrate it for me, so I'm going to pursue that avenue first.

Meanwhile, Thanks again for your help, much appreciated

MM

Update : I'm in the £1 seats !

The aforementioned emailer who offered calibration turned up here at 9am this morning, took my DMM off to his van and returned 20 minutes later with a calibrated unit and a bill for £25.

Understandably I suppose, he declined to tell me which of the presets does what, but he did point out that the presets only affect the display and not all are voltage related. Although I never use it, there is a Frequency Counter on board so that may be involved.

Either way, I have my favourite DMM doing what it does best with some accuracy again.

£1 seats !!

MM

And Finally .. .. .. LOL

Out of curiosity, I opened the top of my DMM to see if there was any evidence of the calibration method .. .. .. .
The 3 Presets have been marked with a felt pen .. .. ' + ' ; ' 0 ' ; ' Hz ' ; which seems to support Popcorns analysis very well .. .. .

Useful information for the future perhaps .. .. .

MM

Sometimes I do make a good guess

Too bad they didn't add that to the silk screen.

For The Popcorn said:

Too bad they didn't add that to the silk screen.

Click to expand...

They may well have done so .. .. the 3 presets are poking through a hole in a tin screen - PCB is not visible without some dismantling .. .. .

MM

You should of taken before and after adjustment photos.

Glad your meter is back in business. I suspect those pots are for what the cal. guy said they were for. I have a mastech 20000 count 5 digit 8218 which from the start was off by 3mv (checked against Fluke 732a voltage standard) when new (should have been +-.05% accurate. Opened the meter and no pots at all! All calibration is by software and has to be done by cal. services with a relationship with Mastech. If you like paying for cal. service, maybe Mastech is for you...not for me. Next handheld I get will be Fluke.