Recalibrating a Sanwa CD770 DMM

[Aphelion]

Hello, I'm trying to recalibrate my 7 year old meter (a Sanwa CD770, my very first DMM ). I can't seem to find a schematic online and on the manual it says nothing about recalibrating the meter.

Apparently, the volts range is WAY off about 10% (just confirmed this with my professor's fluke 725  ), and I'm not sure about the ohms ranges. I'll confirm this with the uni-t ut61e that is going to arrive 2 weeks from now.

There are some trim pots on the pcb of the meter, is it ok to try to adjust them one by one to identify what trim pot adjusts what function? I don't want to kill my meter thoroughly as I still want to use it.

Here are some pics of the multimeter and its pcb.

[Aphelion]

some more pics of the DMM

[ModemHead]

There are some trim pots on the pcb of the meter, is it ok to try to adjust them one by one to identify what trim pot adjusts what function? I don't want to kill my meter thoroughly as I still want to use it.

Fiddling with the pots without a calibration procedure (and the equipment to do it) is a good way to make it completely useless.

If you must, just use a loose resistor to bridge the pot terminals to see what it affects.  Use a resistor value the same order of magnitude as the pot.  And be aware that sometimes the wiper is tied to one of the ends.  I see four pots.  There's probably a DCV pot and an ACV pot.  Maybe one for capacitance.  Not sure what the fourth might be.  Adjustments for resistance and current are fairly rare.  The DCV pot will surely affect both AC and DC volts.

[retiredcaps]

Two ways to find out which pot does what.

One, use freeze spray on the pot and see if that affects the reading. Alternatively, take a tip of the soldering iron and hold it close, but not touching, the pot.

Two, download the FS9711 datasheet and look at which pin is the DCV.

[coromonadalix]

take a note of the ohms values between each legs of the trimpots   just in case ...  tried to find the fs9711_lp3 schematics but always revet to the fs9721_lp3

found this one ; https://elektrotanya.com/multimeter_vc97_sch.pdf/download.html     

from a vichy vc97 ??  but the lp1 version

[Aphelion]

From what I saw from coromonadalix's posted schematic, ill try out adjusting VR1 to VR3 as in the pin it says REF0 and and REF1 since OPIN seems like the odd one out (ill mark what was the original positions carefully with an x-acto knife).

Ill probe it first if it matches the layout on my DMM

[Aphelion]

There is no DCV pin or ACV pin in the schematic, only REF0, REF1 and OPIN. Maybe REF0 and REF1 is ACV and DCV?.

I jotted down the results from my profs fluke 725, so ill use that as a baseline. I still have a AC clamp meter here that is still in spec, so ill use that for now.

[MosherIV]

Have you contacted Sanwa to find out how much the service manual costs?

They should be available (calibration companies need them).

As Modemhead said, it is not just the pots, you will also need accurate DC and AC volt source at adjustable reference points (typically near full scale of each range) and the same for current and Ohms. Calibration companies have special equipment for this.

[Aphelion]

Have you contacted Sanwa to find out how much the service manual costs?

They should be available (calibration companies need them).

As Modemhead said, it is not just the pots, you will also need accurate DC and AC volt source at adjustable reference points (typically near the full scale of each range) and the same for current and Ohms. Calibration companies have special equipment for this.

I have contacted them for the service manual, thank you for the suggestion. Just have to wait for their reply before I mess with my meter

Essentially I just need this meter to be accurate for below 20v DC and mains AC (if possible, since this is going to be my backup).

[coromonadalix]

if you measure all the trimpots between the 3 pins has a safety you can play with them afterwards, i  did that in the past, you just put them back to their values if not the good ones ...

[Aphelion]

Well, asking for the service manual got nowhere and the distributor in my country didn't even reply . I put my power supply to close to stable 5v, 10v and, 18v output and used another meter as a reference to calibrate my Sanwa.

Turns out VR1 adjusted the DC volts readout and not affected AC volts, ohms, resistance, duty cycle and capacitance readings (not sure about current, I didn't check yet). I tuned the pot until the volts range is close to my other meter and the results have been good. Thanks to those who helped.