Micronta 22-191 multimeter teardown

And here is the second part of the $1 multimeter deal from the last Hamfest. Together with Micronta 22-201U analog meter reviewed here I picked up this 3.5 digit digital one of a similar age. The poor thing looks like it had a rather difficult life!

Updated 11/2016

 

 

2 AA, not some 9V. And a spare fuse holder

A bit of corrosion on the battery contacts

A bit of corrosion on the battery contacts

One screw and a few clips hold things together:

Nicely shielded back cover

Nicely shielded back cover

Case front- buzzer, shielding

Case front- buzzer, shielding

Not much to say safety wise- no board slots, MOVs etc:

Hand taped traces?

Hand taped traces?

We may have found a datecode? 1982

We may have found a datecode? 1982

After cleaning and drying, the meter powered up, but the display segments were very flaky:

If I wiggled the flex things got better. So I took the display off to see what I can do. Big mistake! The paper-thin flex just peeled off the board and now we have a display that is not connected at all! On the other hand we have another datecode confirmation 82-1:OLYMPUS DIGITAL CAMERA OLYMPUS DIGITAL CAMERA OLYMPUS DIGITAL CAMERA

Not sure this damage is reversible- I suppose if somebody sells the conductive tape like this, but what are the odds..

 

Update 11/2016

Well, it appears things are not that bad. After reading about 3M Z-tape, I got a hold of some. Mine came from Sparkfun, and  has since been discontinued. Adafruit has a similar product  and also a nice page with details on how it works, but it’s basically a double sticky tape with vertical conductive channels. Kinda like Zstrip used with segmented LCDs but on a much smaller scale.

3M 9703 Z-tape

Next step was to clean the junk off the board. Flex side proved to be trickier- I had to carefully dissolve some of the paint on the back to get to more conductive ink traces. After that it was all basically peel and stick:

Applying Ztape to the board

Next is the pasting part- remove tape liner, align traces (or what’s left of them) and press:

Old flex aligned with the pads

And finally add a piece of ESD foam to keep things under  pressure for better contact

Some ESD foam for even pressure

Some ESD foam for even pressure

Reinstall the two screws holding the display, and we now have a readable LCD!

It's alive!

It’s alive!

And now for some vintage views:

1975 datecode on this wirewound resistor

One way to open solder mask

OLYMPUS DIGITAL CAMERA OLYMPUS DIGITAL CAMERA OLYMPUS DIGITAL CAMERA OLYMPUS DIGITAL CAMERA OLYMPUS DIGITAL CAMERA OLYMPUS DIGITAL CAMERAOLYMPUS DIGITAL CAMERA OLYMPUS DIGITAL CAMERA OLYMPUS DIGITAL CAMERA

 

Schematic diagram:

Schematic courtesy user manual

Schematic courtesy user manual

Resources

 

3 thoughts on “Micronta 22-191 multimeter teardown

  1. Pingback: Multimeter Teardowns: the Micronta 22-191 & 22-201U « Adafruit Industries – Makers, hackers, artists, designers and engineers!

  2. A good old multimeter, i have one and adjusted the onboard reference resistors to make it more accurate, and re-working the mode switch so it does not include the fuse in the resistance ranges.
    It helps to get a lower zero ohms on the 200 ohm scale, 0.2 ohms stray is typical with real copper probes.
    Safety depends on the user, i have never had even a hint of problems in 30 years of using this multimeter 😀
    Perhaps its luck 😀

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