Today I uncovered a multimeter I forgot I had. A Micronta 22-207. Specification wise, it’s a pretty nice VOM.
I acquired this meter several years ago from a gentleman I used to work with. The only issue he said it had was that it “didn’t work in Amps.”
Apparently he had placed the selector switch in the 10A position, stuck the probes in a 110VAC wall receptacle, and enjoyed a loud pop and some smoke. I remember asking why. He said he wanted to measure the Amps in his house. My reply was probably something along the lines of “Oh.”
Anyway, I bought it from him thinking it could be repaired. I asked one of the techs in the lab to take a look at it. He found one burnt/exploded resistor. He didn’t have an exact replacement for the broken .91 Ohm resistor so he wired a couple of values he did have in series to get .94 Ohms. Close enough. I used it once or twice then moved across country a couple of times.
So back to the present. I found the thing while looking for another thing and I decided to add it to my bench. The first thought I had was that I hoped I had removed the batteries before storing it away.
I had not.
Leaving batteries in a device while it’s in storage is a dumb thing to do. Somewhat akin to setting a multimeter to the 10A setting and sticking the probes into a 110V wall receptacle.
I did, however, manage to get the battery compartment and battery contacts cleaned up. I removed the contacts and put them in a bowl of white vinegar for about 15 minutes. The corrosion was gone. They were still rusted looking but I was hopeful that a little 800 grit wet/dry emery cloth might finish the job, so I set them aside.
I needed to replace the blown resistor bodge mentioned above so I begin to clean up the solder pads after pulling out the resistors. During that process I noticed more corrosion on the selector switch, so I pulled it apart.
No problem. I’ll go to work on the PCB contacts with a little vinegar and a cotton swab while the contact fingers have a vinegar soak. I started by removing the two screws that held the group of 3 contact fingers to the housing. That didn’t go so well.
I know this is an anti-climatic ending but I need to give some thought on how I’ll replace these contact fingers. Maybe laser cutting if I can find a place that cuts metal and that can cut the type of metal that would be appropriate for these contacts. They look like they could be beryllium-copper. I don’t know if anyone works with that material anymore. I certainly doubt anyone will laser cut it. Maybe some 1/2 hard nickel-silver.
I’ll update if a solution presents itself. For now it is getting packed up (no batteries) and placed back on the shelf.