Posted by David Sherman on Wednesday, October 13, 2004 at 10:38PM :
In Reply to: Sun Tach box repair posted by Kevin Sullivan on Wednesday, October 13, 2004 at 2:05PM :
I don't doubt that any kind of mercury batteries are impossible to get nowadays now that even the mercury in thermometers and fluorescent light bulbs is considered deadly. About the only thing you're allowed to do with mercury these days is put it in your teeth. When I was a kid, I would salvage the mercury out of old 10 volt smoke-alarm batteries by breaking them open, putting the innards in a bent glass tube and heating it with a propane torch to boil off the mercury, most of which would then condense in the cool part of the tube. I'd probably still be alive today if I handn't fooled around with that stuff.
Anyway, the main reason they used mercury batteries was that they held their voltage for a long time at low current draw. The clocks in computers also need to have a battery that will last for 10 years or so, and they are mostly lithium batteries, which weren't available back when mercury was popular. I would replace the mercury cells with the lithium cells they use on computer motherboards. They're not very expensive and are readily available at computer shops, Radio Shack, etc. Lithium cells put out about 3V per cell. I don't exactly what the mercury cells put out, but it was probably about half that, so I would first try one lithium cell for every two mercury cells. Of course you'll clean all the gunk out of the works at the same time. For maximum life, it would probably be good to put in the biggest lithium cell that will fit. If you don't want to waste money if the tach is really dead, try it with regular alkaline or carbon/zinc batteries first. Make sure you get the polarity right. If it's not marked on the circuit board and there isn't one red wire (+) and one black wire (-), check the markings on the mercury cells before you remove them.
If you absolutely can't get it working and you decide to junk it, send it to me and I'll take a crack at it just for my own amusement when I get some spare time. No promises, but if it works I'll send it back to you, and if I come up with an easy and good quality fix, I may go into the sun-tach upgrading business, or at least publish instructions.