Metrix MX22 multimeter

[mike38]

Hi,

I am working as a teacher in french high school and we are using Metrix MX22 multimeter for electronic practical courses.
After one of these lessons, 6 of them got a problem. Fuse are ok, but on ohm-meter even if you connect directly the cables, it does not measure anything. On voltmeter, the value is oscillating and absolutely nonsense.

I try to open it, but do  not find any obvious failure. Does anyone got a schematic or repair manual, or any advice to find the problem, I hope all of them felt with the same issue (I am confident with that, as they have the same symptom and brake just simultaneaously after the same course).  battery and screen are ok.

Thanks for any help

Mike

[radhaz]

Have you contacted Metrix support?
http://www.chauvin-arnoux.com/fr/support

[Jeroen3]

It has the exact opposite connections of any other meter on the market, did you notice?
I've tripped the OCP on my supply so many times with this.

[retiredcaps]

I try to open it, but do  not find any obvious failure.

Clear focused pictures of the pcb, both sides, would help us make suggestions on what to test.

[mike38]

Thanks for your answers:

- I contact metrix/chauvin arnoux they tols me they cannot give me schematics
- sure the connection is odd, why did they change the usual order?
- I did some photos, hope it can help

Mike

[retiredcaps]

After one of these lessons, 6 of them got a problem.

What lessons were they doing?  It will help us if we know for example if they were measuring mains voltage (220AVC).  Once you give more info, I can suggest some tests that will require a working multimeter.

[mike38]

Sorry for late answer, I was trying to have more informations.

It seems that the students where measuring voltage on an electrical heating resistance to determine the energy needed to boil water.
the voltage should be less than 220V but I do not know exactly what they may have done wrong...

Hope it can help.

Thanks

[retiredcaps]

Here are a couple of simple tests you can measure.  You can measure them "in circuit".

1) There are 4 MOVs (varistors) circled in red.  With a working multimeter, all 4 should measure open circuit in ohms/resistance mode.  Report your readings.

2) The PTC is circled in yellow.  With a working multimeter, it should measure around 1k ohm. Report your readings.

[mike38]

Hi,

Thanks for help

1) all 4 are open circuit, the ohm-meter give me infinite resistance (a working fluke 179)
2) I measure 1.5 kOhm

Seems correct...

[Jeroen3]

Does dc voltage range still works? Try measure a battery.
Does it output current when measuring ohms? Use another validated meter for this in amps mode.

[mij59]

In the second picture,  below the two blue pots,  there are several thin traces with marks on them from the range selector, maybe they are cut by the range selector or by arcing.

[mike38]

Does dc voltage range still works? Try measure a battery.
Does it output current when measuring ohms? Use another validated meter for this in amps mode.

no and no 

There is no change on screen when I try to mesure voltage, it oscillate around  0mV

No current on the other multimeter when in ohm mode. 

[ModemHead]

The symptoms and checks you've done so far suggest that something in series with the input has failed open.  The PTC is OK, so be sure to check those serial chains of resistors branching off from the positive input jack.  There appears to be two sets, one on the top and one on the bottom.

The evidence of damage from the rotary switch contacts going across those masked traces is indeed ugly!

BTW, nice photos.

[mike38]

Ok I did some more checks:

- I tested the resistors on both sides after positive input, and they seem all correct. I see no open circuit from input until rotary switch.
on one side they are all 1,47kOhm, on the other 57Ohm

- I check the traces on the rotary and the ones where the mask is removed. I found no trace cut and identify no short cut.

Mike

[ModemHead]

Without a schematic, it's difficult to give any more really specific advice, so I can just toss out some thoughts that may or may not help.

The CyrusTek ES51988 datasheet will provide some clues, if you haven't already gotten it.  Just note that the designers of this meter may not have followed the datasheet exactly.

But one thing that is reasonable to expect, is that in DCV mode, the V+ jack should be connected to pin 32 (VR1) through a 10meg resistor.  The 10meg may actually be more than one resistor in series.  Also, it looks like pin 16 (OVH) is the source of test current for the ohms range, which you have verified is missing, so maybe the failure is something common to both paths.

Given the situation and the number of simultaneous failures, I'll speculate that mains voltage was applied while the meters were in ohms mode, and they didn't quite survive the overload.  Hopefully it did not fry the ES51988 ASIC.

[mike38]

thanks for the datasheet.

I can confirm that the VR1 is connected to + jack with 10MOhm, so no cuts there...
There is also connection to OVH to amp jack (390 kOhm) but nothing to mA jack, is it expected?
and 14MOhm to V+ input jack.

I fear it is the IC who fried.... Is it programmable or I can just try to solder a new one (if I can find....)

[ModemHead]

I would expect a fairly low impedance connection from OVH to the V+ jack with the switch in the ohms position.  The datasheet application example circuit shows a 100 ohm resistor plus a 1.5K PTC.  This connection has to be low impedance in order to provide the ohms test current without dropping too much of the limited supply voltage.

The ES51988 ASIC is not programmable.  It should be replaceable if you can find a low-quantity source, which might be difficult.  Frankly, on a device like this, such a repair may not be cost-effective.

I took another look at your pictures and I see something a bit unique.  It appears that the COM jack goes straight to one of the fuses, making that fuse important for all modes.  (On most DMM designs, the fuses are only involved for current measurement.)  If you concur with my observations, you might want to double-check the fuse, the holder, and the surrounding bits.