I do have one of these meters for a long time, and recently purchased another one. I was surprised it not only looked differently, but also behaved differently (clicky keys, annoying beeps after every keypress). Both of them have the “Auto power off” feature, which really annoys me, sice they beep and turn off frequently on my bench. I decided to look around for a good meter and found out, that many meters on the market have the same chip, and was curious if there is a way to improve me meter, that is, turn off some of it’s features.
As I was looking around, I quickly found the Vichy VC97/VC99 meters, and got interested. They are really cheap ($31 on ebay), can measure current, and have relatively large display. That’s exactly what I need (really, I just want to monitor current to my battery being slowly charged over a week or so). I digged around to find out if the auto power off can be turned off (don’t know yet), and noticed some people were modifying them to output data to RS232. Datalogging meter for $31?
As I read the articles, it seems a lot of meters may have this capability. Both my MT-1232 meters have the RS232 segment on a display, and it turns on on powerup for a while. I decided to investigate further and take them apart.
Older MT-1232 / rebranded Victor 81CD
The first meter is my older one. I had it apart before, since it fell into a bucket of water. Since then, the screws are a bit rusty, but no damage inside. Also, the accessible side of the PCB does not contain much: few trim pots, MOVs, 10A current shunt, buzzer and a fuse. You have to unscrew a total of 6 screws to get to the other side, where all the fun is. Although the DMM IC is an epoxy blob, the silkscreen says some interesting stuff: the IC is FS9721LP1, and the meter is actually Victor 81CD, presumably manufatured in 2008 (has it been that long?). A quick google reveals, that you can still buy the 81D, only in yellow holster.
It seems pins RS232 is active as long as 84 is pulled low, and 64 is the TX pin at 2400 baud. Unforunately, the datasheet notes that they are “No Use” (really, exactly like that) on the LP1. Well, that’s a bummer. Still I soldered wires to the pins and tried, just in case the LP3 was cheaper in Shenzhen that day in the week. Nope, it was not.
At least the datasheet notes that in order to turn the auto turn off off, you have to press and hold “SELECT” button on power on. This is enough to make the meter twice more useful than before!
The rest of the PCB looks just fine. There is some input protection, the binding posts are strain relieved, and even the switch contacts are still looking good after those years and the water inside :-).
The second meter is completely different story. Although you can access almost all the circuitry immediately after removing the backside, the silkscreen does not say anything about the OEM or IC. Bummer. I don’t really want to spend time doing more research on this.
The construction on the other hand catches my eye. It a lot worse than the older model. The battery contacts are a bit funny, but they work. The input protection is there. But the fuse is a bit funny. Upon touching it, it is completely free on one end, as the holder is bent. I patched that, but the fuse contact is surface mounted, even though the other one is though-hole. Upon more careful inspection, the post for 10A measurement, which the fuse is part of, is not really soldered on this side and the soldering on the front side is not much better. I patched that too, but will never use it for a serious voltage or current.
Both meters work. They measure voltage on my bench for some time now, even current (mostly on mA range).
The older version is much better in every sense. The meter feels good (for the money), does not buzz on keypress, the LCD has nice and large numbers, and as far as I can tell, it is fairly accurate. After the teardown, I learned how to turn off the auto power off, and since then, it happily monitors current on my bench. I would recommend buying it, especially if you need a cheap meter without datalogging.
The newer version is a lot worse. After every keypress, a buzzer beeps and you can’t disconnect it, since that removes the continuity tester feature. It has backlight, but it’s poor and does not really compensate the smaller numbers. The rightmost key is also split in two halves, one for the hold and the second for backlight. The keys are clicky and hard to press, the AC/DC switch is mechanical switch (and beeps). The mechanical construction is really bad, I won’t ever use it for any large current or voltage measurement. And it turns off after 15 minutes of inactivity. I would recommend buying $10 DT830 instead, it’s much more useful :-).