I was digging through my junk box, when I came across one of those failed projects most of us are no doubt familiar with.
A few years back, I picked up a faulty multimeter for next to nothing (I probably paid a pound for it). The meter in question had suffered the usual fate of such things, someone had left the batteries in and their caustic contents had eaten away the battery box contacts.
I vaguely recall cleaning them up when I first looked at the thing, only to find that even with the crud cleaned off, the meter still wouldn't do anything other than issue a string of plaintive bleeps. The display remained stubbornly blank.
Well to cut a long story short, when I picked it out of the junk box yesterday, it had a very interesting wispy black fern like pattern across the display. It certainly didn't look like that when it landed in the junk box all those years back.
Black wispy ferns... Very odd... so well worth investigating obviously.
I brought it in to the house from my store come workshop (where the average winter temperature hovers around freezing), and examined it under a magnifier. The black pattern (which sadly I didn't photograph), looked like a black frost fern pattern, so I decided to rip the thing apart and warm up the display to see what would happen. Sure enough the pattern started to break up. It didn't vanish completely. A few little black dots remained, but the display almost returned to normal, (still blank, of course).
I also discovered several faults on the meter. Various wires simply fell off the board, due to poor solder (early lead free stuff no doubt). Further intrigued, I set about soldering these back in to the correct locations.
Some minor microsurgery later, it was time to test it... but not before removing the battery contacts, and tinning them heavily with (leaded) solder.
A quick rummage in my component bins turned up a couple of suitable 1.5V silver oxide button cells (no doubt several years past their use by date).
I popped them in and the original beep-beep-beep... but no display... problem was still there, so I ripped it all apart again and examined it carefully with a hand lens.
The gold plated contacts for the LCD display zebra strip and the mode switch had a strange thin white coating of crud on them, so I broke out an ink eraser and gave them a good clean and re-assembled.
This time the thing sprang in to life.
But... does it actually work, after all these years?
When compared with the reading for a cheap modern, and fairly accurate Chinese meter, it compares pretty well, to within 2 or 3% on most readings, so I'll call that fixed... now all I need to do is find the battery box cover.. it must be in that junk box somewhere..
I wonder what else in the junk box might be worth a second stab at repairing...