Adjusting an old multimeter, Velleman DVM890

I've had this old multimeter for quite a long time (can't remember; maybe since around beginning of 2000). Nowadays I also have other multimeters that are of better quality. But I wanted to adjust it, because it isn't that accurate.

I'm not talking about calibrating, because this meter is too cheaply built, so it cannot be calibrated.

The meter had readings that differed noticeably in DC voltage, AC voltage, temperature and frequency in comparison with better quality meters.

 

I tried hard looking for some information on this meter, but the manufacturer doesn't tell anything and their support forum was quiet (I commented on another post in their forum that was asking the same thing).

So I opened the meter up and found six trimmer resistors. All trimmers are labeled VR1-VR6, so it's easy to find them. VR1 and VR5 are hidden below the display on my specimen. It seems that on later models, VR5 is just below the power switch and only VR1 is hidden.

I began adjusting them in order.

 

I figured out the following:

VR1 DC voltage
VR2 AC voltage
VR3 Capacitance
VR4 Temperature
VR5 Zero adjust?
VR6 Frequency

VR1, DC voltage, is very tricky to adjust. You have to remove the display when adjusting the trimmer and put it back each time.

I was able to do it by holding the display in place with my hand and testing between adjusting the trimmer. DC voltage became quite OK.

AC voltage (VR2) is not possible to adjust so that both low and high voltage ranges are correct. You have to select which one is more important.

Frequency (VR6) is now spot on. 

I wasn't able to adjust the temperature (VR4) any higher. It shows 12 °C at 23 °C with the trimmer turned all the way. The same K-thermocouple sensor displayed correctly on another meter. The temperature functionality must be broken somehow, but I didn't investigate more.

Capacitance (VR3) was a tiny bit off, but I was able to adjust it.

Resistance is quite OK and you can't adjust it on this meter. The accuracy depends on the accuracy of reference resistors R5 to R10 that are also voltage dividers (which later became apparent from a schematic, see below).

My references when adjusting was an AD584 voltage reference chip, Brymen BM867s DVM, GW Instek 8251A bench meter and Siglent SDG810 signal generator. 

I later found out that this meter is a rebranded Mastech MY-64. The schematic for this meter is available on the Internet. Hopefully nobody minds even if I post the schematic. I think it is the responsibility of the manufacturer to make specifications and adjustments available to the user of the meter (at least as long as the instrument is built the way that it is possible to adjust).

 

It is not worth bothering more with this meter. As long as it works it can be useful for throwing in your toolbox and measuring simple stuff (except for the temp now). It is also quite bad (accuracy and functions) compared to cheap meters today (2021). If I bought a cheap multimeter today instead of this one, the ANENG AN870 would be a very good candidate.

References:

Velleman product page

Velleman support forum question