Hi!
Is there anyone out there who knows which is which of the potentiometers on
dvm890’s pcb so I can calibrate it?
velleman.eu/products/view/?c … n&id=18686
Kindest Regards Kristoffer
Hi!
Sad that nobody replied to you (I know this post is old). I have the same meter and tried looking everywhere for some information about this, but it is non-existent.
I can’t do a real calibration, but as the meter is clearly out of spec compared to my other meters, I wanted to adjust it.
Anyway I thought it valuable to post the information I figured out from my own DVM890.
VR1 DC voltage
VR2 AC voltage
VR3 Capacitance
VR4 Temperature
VR5 ?
VR6 Frequency
All trim pots are labeled if you look carefully (VR6 is the one just below the display). VR1 and VR5 are hidden behind the display, so you need to remove the display in order to adjust them. This makes it tricky. I was able to adjust VR1 by holding the display in place with my hand and testing between adjustments. I didn’t yet figure out what VR5 does (didn’t touch it).
The meter was out of spec in DC voltage, AC voltage, temperature and frequency. I was able to adjust DC voltage quite well. AC voltage is not possible to adjust so that both low and high voltages are correct. Frequency is now spot on. Temperature is way off and can’t be adjusted, it shows 12 °C at 23 °C with the trim pot turned all the way. Don’t know the issue with that. The same K thermocouple probe shows correctly on another meter. Capacitance was a tiny bit off. Resistance was OK, so I didn’t try if that would have been VR5.
My references were an AD584 DC voltage reference chip, Brymen BM867s DVM, GW Instek 8251A bench meter and Siglent SDG810 signal generator.
OK, I found out that this meter is a rebrand of Mastech MY-64. The schematic of that meter is available on the internet. I’m attaching it here as well.
VR5 is possibly zero adjustment (“in LO” on the 7106 chip).
Resistance measurement accuracy depends on precision and stability of the reference resistors, probably R5 to R10 (that are also voltage dividers).
It’s not worth bothering more with this meter, as it is quite bad compared to the DMMs available today. However, there is some value as a really basic meter if the information to adjust it would be more readily available. Something for manufacturers to think about!
Ok, i did a calibration and sold it and bought a Fluke instead😀
Kindest Regards Kristoffer
Hi,
It’s been broken for 10+ years and never read voltage right. Resistor R55 is reading low at 8k instead of 13k. Turning the VR1 pot as far as it would go wouldn’t let me go higher than 6ish volts on a 12.35v source ( verified by another meter ). I replaced the VR1 201 pot with a 10k pot (3296 - 103 BAOTER).
fwiw turning the pot to it’s side so it faces the right makes calibration way easier… turned it until it matched the same 12.35 on my other meter. I still need to calibrate other stuff, but I really just wanted the voltage and a win for now. It’s my first multimeter, given to me by a dear friend.
Thank you Ledman and Johanh both very very much, made the account just for this. Hope your both well.