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!