Another Metrahit Pro with leaked batteries - repair attempt

[AlphaBravo]

Hey everyone!

I'm new here and honestly new to this. But I bring a similar challenge to some already detailed in this forum. Essentially I have bought a used Metrahit Pro (M242A - nowadays called AM Pro) that seems to be manufactured around 2007. Still has the original sticker from GM suggesting a calibration somewhere in 2009. The previous owner posted some pictures of the multimeter working and claimed it seemed to work before it shipped. This is half true because it did indeed turn on and show something on the screen, but was always throwing the over voltage warning, it was constantly beeping and no matter what I did I always saw OL on the screen. Only for frequency measurement could I get a 0.000Hz value without connecting the multimeter anywhere.
In any case, I went for the most obvious first step and checked the batteries. These had leaked. So I opened up the box and found the PCB to only have a couple of affected areas, hopefully hinting at an easy fix (see picture 1st picture for affected areas).

That is what brought me here. Upon opening it I proceeded as follows:

• Observed that a small area around each battery terminal was slightly corroded (areas 1 and 2)
• Cleaned them with vinegar and demineralized water (had no isopropyl alcohol at home)
• Tested again and observed the same symptoms
• Checked in more detail below the metal shielding and found 2 additional small spots of corrosion (3 and 4). One had a big "white cloud" of corrosion and the other was more of a dark spot (see image for detailed area). White cloud observed near the two caps (ceramic + tentalum). Black spot to the left of it
• Removed shield, cleaned spots in the same way, tested again and determined the over voltage warning had gone away. Still only getting OL though.
• Took the unit to work where I have a proper electronics lab, and took the board under a microscope. Area 4 has more obvious corrosion damage to some components pads + vias (check images below)
• In order to fix this spot properly I intend to use an eraser to be able to measure continuity across traces and vias more easily so I removed a total of 4 components (highlighted in an image below)
• Because the "white cloud" of corrosion had been quite big, I wanted to check how the pads under the two affected caps looked like so I proceed to remove those two as well. The pad of the ceramic one fell apart as soon as it was de-soldered. This was the pad on the side where the "cloud" seemed to be created from.

My next step is to proceed with the eraser as planned and make sure the most affected area gets the love it needs. I am also obviously in the need for a ceramic cap. Would anyone have any idea of the part number for this? The only IC I removed reads C4475Z3 on top. It also has some corrosion to a couple of its pins (pin 1 and 2 to be more precise).

Also, I noticed two caps seemed to have been soldered in parallel as highlighted in one of the pictures below. Was this the work of a past fix or a hot-fix by GM? I'm assuming so far that no trouble is coming from there.

Cheers in advance for all the help!

[BILLPOD]

Good Morning AlphaBravo,  were the batteries that leaked Duracell brand

[AlphaBravo]

Good morning BILLPOD! In this case they were Energizer

[AlphaBravo]

More images I could not get on the above post

[shakalnokturn]

This mostly looks like a case of fixing the broken vias, hopefully they won't be leaving any buried tracks dead-ended.
If you're thinking of replacing the large grey capacitor, I don't think it's ceramic but rather a film capacitor. Try measuring its value in circuit at the nearby vias, the risk is that you won't be able to measure anything once it's desoldered.
I'd leave the piggybacked capacitors alone for the time being, could be factory modification for frequency compensation.

Is the DMM IC (the one with the sticker) a BTC-xxx similar to those used by Brymen?

[AlphaBravo]

Hi shakalnokturn!

Thanks for taking the time to take a look and for your insight. Yes, I am hoping for no dead ends in the inner PCB layers.
And I need to replace the large grey capacitor as one of it's pads fell off when I removed it (in hindsight I should have tried to measure it first). Also, it is likely a film capacitor as opposed to my first guess.

As for your last question, I can't confirm without removing the sticker.

I am now waiting on a kind soul to try and measure this capacitor's value so that I can buy the same/similar. In the meantime, I would try to repair any broken vias/traces I find

[shakalnokturn]

I'm guessing a little here but would assume that the film capacitors are used for DC blocking on AC sections of the signal path.
The offending one could be on the frequency measurement section input.
If you have no luck getting information from other owners, there's a fair chance that if it's the same size as the other 3 of the same type, it's the same value.

[AlphaBravo]

I will take a look at that tomorrow. Could be the case, that it is indeed used for DC blocking.
Unfortunately, this cap seems slimmer than the other so unlikely they share the same value. So I will patiently wait on other owners to share what they know

UPDATE: I already used a small eraser on area 4 and a couple vias look nasty. Tomorrow I will start measuring continuity in this area to check what might no longer be connected. Wish me luck

[AlphaBravo]

I am a step closer to having completed this repair, I hope. Measuring continuity yesterday I found one via that was so corroded that it broke the connection between two point on (fortunately only) the surface layers. So I drilled this via out with a 1 mm drill and used an enameled wire to repair this connection successfully. Pictures attached.

Then, in an effort to test the multimeter, I found another damaged component, this time a diode. I might have damaged it while removing without noticing. I could live with the missing cap for the power on test, but I feel reluctant do to so without this diode. I was hoping I could get a few pointers on how best to identify something that could replace it 1 to 1 or at least be equivalent. The footprint seems to be something around 0805 (2012m). See picture 12. The missing diode on the top side seems to be exactly the same as the one that is to it's right (highlighted with the arrow).

So in general, I hope I am correct in saying I am 1 capacitor and 1 diode away of a "good as new" multimeter. Again, hoping one of you kind souls can measure the capacitance of the missing cap.

Tentatively searched on Mouser filtering for what I believe is known and narrowed it down fewer possibilities. Measuring the current cap to be around 1.4/1.5mm in height and have (most likely) a 1913 (4833 metric) packages size.
https://www.mouser.de/c/passive-components/capacitors/film-capacitors/?height=1.4%20mm~~1.5%20mm&package%20%2F%20case=1913%20%284833%20metric%29&termination%20style=SMD%2FSMT&instock=y&rp=passive-components%2Fcapacitors%2Ffilm-capacitors%7C~Height

Also tried to narrow down available diode alternatives based on what I know, but I am less confident here.
https://www.mouser.de/c/semiconductors/discrete-semiconductors/diodes-rectifiers/?mounting%20style=SMD%2FSMT&package%20%2F%20case=0805~~0805%20%282012%20metric%29&instock=y&rp=semiconductors%2Fdiscrete-semiconductors%2Fdiodes-rectifiers%7C~Package%20%2F%20Case

[shakalnokturn]

You're doing a decent job on this, I'm sorry you're not getting more input.
Contrary to a lot of new-comers you're keeping the topic alive. Many seem to post, not know there is a new post notification option and the topic just gets abandoned shortly after it is started, leaving the experienced people with the impression it isn't worth the bother answering a first post ever.

Some manufacturers have the CD4148WS that ressembles your dropped diode. So nothing exotic...
What function on the selector switch does your crusted film cap. link to?

[AlphaBravo]

No need to apologize shakalnokturn. I take what I can get
The diode you shared does seem to fit! Many thanks for the suggestion. I'll look into it and try to find an equivalent one around the office.
As for the function linked to the cap: it is the continuity/diode measurement function. Let's see if I can get this last piece of the puzzle.

[hefferlump]

Hi,
I have a Metrahit AM Pro (M242A) that developed a fault in the same manner that you describe. It could be that the batteries leaking and the subsequent damage was not causal of the fault you describe - I can easily understand leaving the multimeter closed away in a draw when it stops working!

Something that might be helpful - on the device I have, the IC that is covered by the black and white label in the photographs you have uploaded (which has "3-603-415-01" or similar written on it) is a Fortune Semiconductor  [FS9704B][/https://www.ic-fortune.com/upload/Download/FS970X-DS-44_EN.pdf] - a DMM analog front end chip. As I was working through trying to source the fault, I figured I'd check the analog inputs to that IC, check the voltage reference and hope to chase the fault out. From the datasheet, pin 34 is the voltage reference for the internal ADC for example.

Unfortunately, the device suddenly started to work again... I noticed there's a AD 5231B, a digital potentiometer in the analogue front end. Makes me suspicious of a firmware issue. I had left the batteries out of the device for a considerable period of time however so I had assumed any state would have been reset.

[AlphaBravo]

Hi,

First of all, I'd like to apologize for the hiatus in updates since March.

Since then, I got contacted by a little bird which was kind enough to inform me that the components I had mentioned on reply #8 were the following:

• Diode is a TS4148-0805
• Capacitor is a LDECC2470KA5N00 with 47nF

This same bird left some warnings as well:
"Be very careful when soldering these caps. The melt extremely fast. Use leaded solder and turn your iron down juuust until the solder melts."

Thank you Mr. Bird for your kind hints!!

I duly applied these fixes but without any success with respect to the reported behavior (original post). On the other hand, I must admit these fixes made the multimeter more stable. By this I mean, the initial behavior is always reproduceable and not sometimes hidden by power or other unknown faults.

As to what hefferlump just wrote:
I hope my device also starts working again just as suddenly, but I am not too hopeful.
I'll attempt the approach you suggest and come back here with any conclusions/fixes that I might come across.