Yet another 50$ multimeter, Ohmeron mt488e

[rollatorwieltje]

I had to swap the batteries on my multimeter, and as a regular viewer of Daves blog I decided to take a quick look inside and post the results.



These multimeters are offered over here at several dump stores for roughly 50$. I mainly bought it (a few years ago) for it's large display.
Basic specs:
40mV/400mV/4V/40V/400V/1000V DC
40mV/400mV/4V/40V/400V/750V  AC
400µA/4mA/40mA/400mA/20A DC
400µA/4mA/40mA/400mA/20A AC
400?/4k?/40k?/400k?/4M?/40M?
40nF/400nF/4µF/40µF/200µF
100Hz - 30MHz
-40°C - 1000°C
Diode tester
continuity beeper
2 off positions

The manual can be found here:
Manual


4 self tapping screws hold it together. There's a seperate battery area, but the cover is also held on by a self tapping screw.
A spring connects the rear shield to the main board.


Board layout. Notice that the terminals are on a seperate board and are full tubes. Interestingly for every resistor and capacitor the value is printed on the silkscreen. The chip is a FS9922-DMM3.
A quick look through the datasheet reveals that this chip actually supports RS232 data output. I don't see RS232 on the display when I try to enable it by holding the REL button for 2 seconds, but Auto Power Off does turn off. The LCD doesn't seem to have the RS232 "segment" (when you turn the meter on all the segments go black for a second). There is a segment for hfe though, but this model has no transistor tester.


The through-hole resistors on the left are placed quite poorly. the blue and brownish one on the left are almost touching.


There appears to be no real fuse inside. The 20A lead has no fuse as stated on the front panel.
A little surprise here, the signals are actually carried by the standoffs / screws, including the 20A range (the shunt is on the main board). I have never seen anything like that. Not sure if it's brilliant or insanely stupid.

Not really much to say about it. It works, no surprise there. Component layout looks quite ok to me, although the through hole component placing could use some attention. Thumbs down for not gluing the battery leads in place, I just don't like wires being soldered directly on the board without mechanically fixing them in place. Thumbs up for the large clear display. The stand is just one of those 90 deg flimsy plastic tab type, it sits just a little too flat for my taste. However, it doesn't rock sideways and the mode selection can be easily operated.
Using standoffs as conductors... 

I couldn't really be bothered to thoroughly check the accuracy of the different modes. I tried some genuine onehunglow 56k 5% resistors and they all measured well within spec. Voltage looks ok as well, similar to my Graupner Ultramat 12 battery charger and a (cheap) Turnigy high current measurement tool within 0.01V. That's good enough for me, I mainly use it for R/C related stuff.

I guess it's now time to get the soldering iron and start hacking a serial port on it

[Mint.]

Nice one!