a long time hobby of mine has been old cars. Although I like nearly all makes from any era of the automobile, the ones in my driveway are pretty similar ('64 Olds, '64 Dart, '63 Dodge 330 wagon, and the one outlier, a '86 BMW 325es). Most of my initiative to work on them and go to junkyards was long ago while I was in my 20s. I recently was able to re-organize my sprawling spare parts collection and I came across an old tachometer that I probably found at a fleamarket maybe 25 years ago. I thought it might look cool in the station wagon so I opened it up to replace the lead wires.
I ended up cleaning it up and replacing 1 capacitor as well as the 57 bulb. the 57 bulb is in series with the circuit, and it was blown. The case was riveted together, so no telling how long ago someone took it out of a car because it stopped working. the cap I replaced was questionable and bulging, but the bulb was the only failed component.there are 3 solid state devices,, two NPNs and one device with only 2 leads so I figure it to be diode, or a zener diode, I'm not sure. there are numbers and letters on all three, but only the 2N2926 was one I could find data on. the others are a GE NPN 4J25E2 (not sure about the "5" or the "E") and a 4C1137.. I'm not sure about the "4" in that..., thats the diode type device.
I don't quite understand the engineering behind putting components in series that have different amp draws. Like the 57 bulb and the transistors. the bulb has a .24A draw and the transistors certainly don't draw that. maybe that diode will draw what the transistors don't?
here are pictures, and a schematic I drew out. Check out the name scratched into the inside cover. I don't know if this was a heathkit type thing or not. the only way anyone could scratch their name like this would be if it was completely disassembled, plus the solder joints are just component to component with leads going through holes in the board. It's also a 6cyl/8cyl combo tach, which in the world of 60s era tachs wasn't one of the premium ones (like a Sun or a SW). Maybe this was an electronics class project? or maybe "POWRTACH" was a company sold through JC Whitney type catalogs.. I did see two google search images for ebay auctions long past, but no other info.
oh, and last but not least!! it works! I guessed the trigger input came off the (-) coil post only because that's about the only place to get a low voltage pulse on an old car. There was a chance there was another unit involved (like an old Sun) but I certainly didn't have it and there was only one way to find out.
Topic: vintage tachometer solid state (Read 9002 times)