My first "pretty good" multimeter was a UNI-T UT61D.
Now it's discontinued and one can find a better one for a low price, but backin time it was a serious replacement for my 5 bucks cheap meter Main featuras are True RMS, 6000 count and autorange.
I still use it as second measurement/testing unit.
Last month I've notice it has a problem when using it in AC reading. As example trying to read 12V AC out from the sec of a transformer I get 3.3V, same reduced reading for my main supply, which in Italy is something around 230V.
First thing I've notice when I opened the meter it was a few components with some sign of corrosion, at least it seems to me that.
So simply resoldering three components solves the problem... but it did't last long. I thought the problem was related to cracked solder, but after a day the problem still exists.
I take to opportunity here to show you what's a genuine bodge high speed switching diode. When I opened the multimeter, I've noticed that the R9 does not seems a resistor. Indeed desoldering it, turns out that over the resistor someone in the farm soldered a high speed switching diode. It's marked C3, and I think it's a 1SS226, maybe to protect something, but I've not investigated further.
Back to the issue, as I told you I've not solved it by resoldering those three components. So I've to investigate. The meter it's based on Fortune Semiconductor FS9922. Although I can not find this meter schematic, I can find the FS9922 datasheet which is equiped with useful usage cases. I'm interested in the AC reading.
Turns out that datasheet show us how to connect the AD737 to FS9922. The AD737 is a True RMS to DC converter, and what's matter most it's the same IC used in by UNI-T in my meter.
I've resolder components around the AD737 and meter seems to works again, but it did't last long.
So, first of all I've hooked up cables to a few input and output of the AD737, to see if there's something wrong, but i did't catch anything.
I've thought it was the AD737, so I try replacing it. This is a long story too, I've buy a couple of AD737 from a seller, first time they sell me wrong IC. Ok, it may happens... they now ship me the right IC. At that point one month is passed cause it takes a few weeks for each shipping to be delivered. One the AD737 has beed delivered I've replaced it, but both does not works. So I've but two more, this time from a reputable seller. And this time replacing the AD737 makes the meter works again... but it did't last long. The first seller sends me fake IC. But even with the original one the meter does not work like it should.
To that point I though it was some components or the AD737, and I start thinking it was something related to heat, cause when I resolder things it works for a couple of hours.
I have made a post on the eevblog forum, and a user suggested me to look at the gain input stage of the AD737. I desoldered the resistors and checked it. Heating up and reading it.
Resistance does not differs that much when I heat that resistors.
Another thing I've notice is that just breathing on the back of the PCB a few times it's enough to make the PCB works for a few hours.
That turn on a light bulb for me... That must be something related to capacitors, cause the heat of the breath it's been dissipated in a few minutes, but the meter works for a few hours after that. So it must be something that chemically is renewed for hours just by a small amount of breath. And that should be an electrolitic.
So I've started investigation on the AD737 elecrolitics. I've first changed the one on the input signal, but it was not that one. So I changed the two of the power supply rails... and... it works! Even after a day it works.
That damn electrolitics cause the issue.
I takes a long to repair this meter and for sure it will be simpler and cheaper to replace it, but it's my first meter, and also repairing this gives me the opportunity to gain my knowledge on electronics.
Notes
- read risk disclaimer
- excuse my bad english