Wilson Lamb wrote: > A few minutes experimenting with a series resistor and > capacitor on the circuit board would get it done. Ideally, one would use an > oscilloscope to see when the filtered pickup was providing just the right > trigger puilses. > > I'd try winding the pickup wire around the spark wire a few inches outside > the cap. That MIGHT increase the desired pulse and suppress the others a > little.
When I first bought the thing, I had the same results. I called the manufacturer, and was given his usual tweaks, which involved using different numbers of turns of the pickup wire wrapped around the chosen sparkplug wire, then various different resistors in the system, and I eventually asked why I couldn't just use a potentiometer and dial it around until I got results, and he said that ought to work. It didn't. On one extreme it was all over the place (as usual) and at the other, the signal died altogether and the tach reverted to hourmeter status. A capacitor in there somewhere probably would have helped, but he didn't offer that solution. Instead he sent me a whole 'nuther unit (which he later billed me for, after I'd installed it) that had a filter network already built in. He calls it the -X. I think as cheap as filters are, maybe he should include them in all his units, but apparently you have to be at the end of your rope to receive one, and then you have to pay for it. Anyway, the new unit was no better than the old one with regards to wild and random fluctuations anywhere from half to five times actual RPM, despite another hour of tweaking with the pickup wire, etc. I finally decided that it must have to do with the pickup receiving EMI from the other wires, so I shielded it with aluminum foil and grounded the shield, and now it works. You may be right that it's a capacitive thing, but what I did works, and that's good enough for me. If I build a new panel, that thing will probably go in the trash, but for now I hate to leave a hole in my panel that was specifically designed for the thing (http://home.hiwaay.net/~langford/kpanel.html). And like I said, it now seems to work better than the EIS's tachometer, although I should mention that I haven't called EIS for suggestions on how to fix the problem with theirs bouncing around and not reading correctly, despite proper and careful installation. The last time I posted this story I sent a copy of it to the manufacturer, and got a nice note from them last week explaining that "The basic Tiny-Tach was designed to be an inexpensive method for tracking RPM and hours on small one and two cylinder engines -- basically Briggs and Stratton and Koher type lawn mower engines. Because of its low cost, it spread through the marketplace and is now being used on a wide variety of applications for which it was never intended. It was not meant to be used on multi-cylinder engines, cars, airplanes, etc." Oddly, their web site doesn't mention this limitation, and Aircraft Spruce, Wicks, and Great Plains are selling them with no mention of that limitation, and with no mention of the X-version. I don't know of very many aircraft installations that are single cylinder, but I'm sure there are a few. To be fair, the email also said that if I thought I'd been cheated, they'd refund my money if I returned the unit, but the one I've got works, the previous one was disfigured in several ways (regarding the way the wiring ended up), and their warranty clearly doesn't cover dead batteries, which is what's wrong with the first one, so I'll pass on that offer. I don't think I'll be buying another one, but if you have one already, this might be the answer to getting it working, and spiral wound sparkplug wires may also be part (or all) of the problem. Since I've had to buy three, I now have about $135 in my one Tiny Tach that works, and am facing the prospect of having to buy another one in three years when the potted-in battery croaks . Actually, I guess I have even more in it, because I had to buy a $50 optical tach to help me figure out which one was right, the EIS or the Tiny Tach! After my "fix", the optical tach and Tiny Tach agree perfectly, as if they have the same electronic guts in them. Aircraft Spruce's catalog says the battery is good for 8 years, Tiny Tach's site says "more than five", and my first one lasted a little more than three in the comfort of my basement, never having counted a single rev. Of course you could argue that it sat on the shelf at Wicks for several years before being shipped to me, but I seriously doubt that. I'll probably make a new panel when this TT croaks and buy something that I know will work, and runs on ship's power with battery backup... Mark Langford, Huntsville, AL N56ML at hiwaay.net see KR2S project N56ML at http://home.hiwaay.net/~langford
