Obscure French AOIP MN5127 bench DMM teardown

[shakalnokturn]

I picked this one up a few months ago, only just got round to uploading some photos.
My first bench meter with GPIB. (Now I wish I had grabbed that NI PCI GPIB card for 60€ while it was around...)

I remember using AOIP in high school, never really thought much of it but the name really has an interesting history.
Although I have no proof yet, I believe the Metrix MX591 (just as rare!) uses identical hardware.

Link to the high res. photos for those who want to peek in the box:
https://photos.app.goo.gl/NmBQasifS2mNP1B5A

Also attached memory dumps.

[tmlab]

Hello,
I have the same unit but with differents front panel colors and a grey case. It doesn't power up and has many damages. Do you have service manual or any schematics?
Thanks

[shakalnokturn]

Hello,

Sorry no schematic or manual, you could try getting in touch with http://electropuces.pagesperso-orange.fr/ he has one unit for sale and he's been doing TE repairs for a while now so if you have a little luck...
Then there's the Metrix option, you could also try asking Chauvin Arnoux for documentation on the MX591 as it does seem to be a clone.
(Please share if you get anywhere with the documentation.)

Does yours have bad physical damage? If so, start by the obvious things such as broken pins or torn tracks under the transformers.
There are 2 jumpers on the main PCB that look like they are for mains voltage selection, make sure they are both in the right place.

Of course you can check a few other things like power supply (there are a few tantalums on the main PCB) main IC's oscillator and reset, I²C bus activity...

If you're still stuck once you've been through the basics and have a few specific relevant points to measure I can open my meter and check them.

[tmlab]

I join pictures of mine.
There are two main problems:
1: The display was mechanicaly damaged. I tried to glue it but it doesn't work, there are bad contact between LCD and electrical rubber (I don't know how to explain in english ).
2: The multimeter seems not start with the GPIB board unplugged. The Display displays " --- ". I made cross tests with another MN5127 one year ago and my multimeter with another GPIB board and another display board works fine!
I will focus on the GPIB board and try to contact AOIP or your proposals.

[Neomys Sapiens]

I remember using AOIP in high school, never really thought much of it but the name really has an interesting history.

Indeed, I read up on them too. I happen to have a 5000V isolation tester from them.

[shakalnokturn]

User manual, Cal procedure and schematics now available here:
http://eninz.free.fr/AOIP_MN5127/User_Cal_Schematic_AOIP_MN5127.pdf

Thanks to tmlab for asking and to AOIP for having.

[shakalnokturn]

Partly in reply to tmlab.

I've re-opened my MN5127 (I was getting busy on a Hameg 203-6 but easily get sidetracked, I just enjoy cluttering-up with works in progress...)
Here too main PCB will not run without GPIB connected, it just displays "---" too. I can't really see the reason for that either, as the manual does say that GPIB board is optional.

I tried looping Tx and Rx on main PCB (J6, 4-5) same display.
Tried removing NVRAM from socket on GPIB, that does not stop the DMM from working. So that more or less rules out my idea on a low NVRAM battery causing problems.

I had a quick look at the bus, (no captures, didn't pull the DSO out) the main seems to query GPIB on power-up, GPIB responds briefly.
After that main transmits data regularly to GPIB (likely measurement data for DAC on GPIB to feed the analog output), communication in the GPIB->main direction is silent (Rx remains high).

Your burnt tracks on primary side are quite surprising, any chance it was hit by a lightening spike? Unless it was saved from a H.V. test lab (the military have such a place in the south west of France) Do you remember any arc marks across the isolation before cleaning (around opto's maybe?)

Try focusing on the CI10 (GPIB µC), reset sounds good, but beware of it as /RES power on reset is based on a tantalum drop and these can get very "scratchy" with age.
Check oscillator, then that data reaches pin 12. (You'll have to power-cycle, main PCB only tries once on power-up, it drops Tx if there is no response from GPIB.)
There's only if CI10 is fried that you're going to have a lot of trouble saving the DMM.

[lionelkjh]

Good evening; I have the same multimeter AOIP MN5127, but I miss the transformer TR1 (blue); can you give me its characteristics? (secondary power / voltage). Thank you for your dedication!

[tmlab]

Hello,
No specs marking on the TR1 transformer. Only 1 marking: "EBERLE BV 3433". I measure 43VAC on the output of the transformer.
4 pins on the primary, 5 pins on the secondary. No other informations.