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Metrix MX56C / BK Precision 5390 Multimeter Teardown

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Vince:
Thanks Robomeds for this post !

I am French and boy I struggled to tear my MX 55 apart... it took a chap from the other end of the world to tell me how to open "our" meters !  LOL  So thank you for detailing the procedure to put it to bits. Armed with this, I will make a new attempt at open it !  :) 

I agree with everything you said : great meter overall, I just love it and always wanted one when they were new 20/25 years ago and I was just a kid dreaming as I saw them tested in the electronic magazines of that day ! They were worth about a month of salary IIRC.

As for the history part, I read some thins wrong above, so I would like to correct.

It's easy to get lost because Metrix's life has been such a roller coaster throughout the years ! One day merging with similar companies to form a group, another day getting bought by a group or part of a group, then sold again to XYZ !  Hard to follow.

But to put things simply :

Chauvin-Arnoux and Metrix are both very old big French names in the instruments business, alwyas have pionnered the market, alwyas designed their stuff in house of course, and still do taody ! Their  R&D centers are still in France.

Chauvin-Arnoux was formed as far back as 1893, and Metrix in 1936, first famous for they vacuum tube test gear, as it was the technology of that era.

The two were then competitors, Chauvin-Arnoux always wanting to buy Metrix... which they eventually did later.

As for the ITT thing :

In 1964, Metrix got "affiliated" to ITT, nothing more.
In 1970, Metrix merged with a few subsidiaries owned by ITT, to form a group.
In 1979, the Metric products were rebranded " ITT Metrix "
In 1990, Metrix was the leading manufacturer in the whole of Europe, for analogue and digital multimeters.
In 1994, and of the ITT affiliation : Metrix got sold to a group of companies (mostly French and German).
... which had overambitious plans for Metrix, 3 years later, the group, and Metrix with it, bankrupted.
... that leads us then to 1997. This were Chauvin-Arnoux offered to buy Metrix, the judge in charge accepted... at long last chauvin(arnoux managed to buy his competitor Metrix !

The two French big names are now united in the same group.  Chauvin-Arnoux was/is clever enough to let Metrix design and build their own stuff, sold under their own Metrix brand, so nothing has changed. Metrix is still Metrix... except it's now out of trouble financially because Chauvin-Arnoux owns them.

So basically, it wall started a century ago with Chauvin-Arnoud, and a century later, after troubled times, it all ends with the same two brothers ! Except they are now joining forces, instead of competing against one another.

Here is a link to a PDF emanating from Metric, that describes the whole story, written in French obviously sorry, but I highlighted the main points above.

http://www.geode-annecylevieux.org/____assets/uploads/2012/01/METRIX.pdf

As for the chip driving the MX 50 series, I don't know its story. Obviously Metrix were not big enough to have their own manufacturing plant, so maybe they designed the chip in -house and just asked ITT to manufacture it for them... or did ITT already have this chip in their catalog and let Metrix use it to roll their MX50's around it.. or was it a joined effort / co-development, shared between ITT and Metrix... who knows really, not that it matters in the end... the result is a great meter ! :-D

As for BK et AEMC of who ever else, it was obiously  just a rebadged Metrix.


Anyway, sorry for the length... I came here as well to ask Robomed or anybody else... if there is a schematics or service manual available somewhere for the MX 50s series ?

.. because my MX 55, which is externally/cosmetically in like new.. must have something wrong deep down, as it acts weird and funny and pretty much every function but the current measurements.   I suspect most of the weirdness might be a problem with the input protection circuitry, but there is one misbehavior which I fear might not be as easy to diagnose : in the mV range, it screams at constantly. The "high-voltage"/danger alarm and icons, come on as soon as I set the switch/knob to the mV range, even though the test lead are not connected to anything. shorting the leads makes the alarm go off.

Anyway... would be great if I could have a schematic for this baby, to help me trouble-shoot the thing... because I just love this meter and although it is extremely sick at the moment, I will not give up on it without a good fight !  :)

No idea what kind of abuse the meter might have experienced to show so many problems... it was already like this when I bought it used, 3 years ago... the seller never mentioned anything about any kind of problem.... that's honesty for you ! :-/


Oh, before I go, for those interested, I attached a couple PDF files below. Commercial leaflets of the day, presenting the MX 53/4/5/6 series (in English) and another leaflet (in French) presenting the later/revised ' C ' version of this range of meters.

retiredcaps:

--- Quote from: Vince on August 26, 2016, 11:35:01 pm ---must have something wrong deep down, as it acts weird and funny and pretty much every function but the current measurements.   I suspect most of the weirdness might be a problem with the input protection circuitry,
--- End quote ---

Some questions.

Q1) If you measure an AA 1.5V cell, what does your meter show?
Q2) If you short out the probes with the meter in DCV mode, what does your meter show?
Q3) If you measure a 1k ohm resistor, what does your meter show?
Q4) If you short out the probes with the meter in ohms mode, what does your meter show?
Q5) Can you post clear focused pictures of your pcb, both sides?


--- Quote ---but there is one misbehavior which I fear might not be as easy to diagnose : in the mV range, it screams at constantly. The "high-voltage"/danger alarm and icons, come on as soon as I set the switch/knob to the mV range, even though the test lead are not connected to anything. shorting the leads makes the alarm go off.

--- End quote ---
Another member, modemhead, now has Robomed's BK 5390.   He can verify the above.

retiredcaps:
Using Robomed's photo,

Q6) RV1, RV2, RV3 and RV4 are your MOVs or varistors.  With another working meter, each should measure open circuit in ohms mode.  You can measure "in circuit".  Report your readings.
Q7) R2 is a PTC, thermistor.  It should read anywhere between 500 and 1500 ohms. You can measure "in circuit".  Report your reading.

ModemHead:
Away from home at the moment, but when I get back in a couple days I'll get out the 56 and see if I can learn enough about it to say something smart.

Robomeds:
Vince, thanks for the reply!  Honestly, posts like yours make it worth the effort.  I'm glad I checked the forum when I did.  I've been all but off the web for 2-3 weeks now due to the real world.  Kind of nice to see this and reply same day. 

I'm certainly looking forward to any comments/thoughts from those who replied to you already.  That's a good brain trust. 

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