wrote in message
...
Yo Jim -- and anyone else who may be interested.

I spend an hour last night tracking down some noises in my furnace
blower, using my usual piece of aquarium hose stuck in my ear g, and
I wondered if there might be a market for an advanced kind of
mechanic's stethoscope -- something that ought to be a piece of cake
for someone like you.

Maybe something like this is on the market. If so, forget it. If not,
consider this:

I had two noises, from different sources, and the interference between
them made it all but impossible to find the origins of the noises. One
was some mechanical interference between the centrifugal fan and its
housing, at one end of the armature shaft; the other was vibration
resulting from stickiness in the centrifugal throw-out switch (a
fail-safe switch that prevents the gas valve from opening), at the
other end of the shaft. I finally took the whole thing apart and found
both problems, but it wasn't easy to find them when the motor wasn't
running.

So, I wondered about the idea of making a small, cheap,
battery-powered amplifier, with a mike and a headphone jack, that
contained a couple of active, adjustable audio filters, one high-pass
and another low-pass. Made a notching filter or bandpass filter to
make it slick.

I don't play with engines much these days, but I can recall times when
such a device would have made quick work of tracking down engine
noises.

There it is. If you make it and sell it, the idea is yours. Watch out
for patents.

--
Ed Huntress

====================

That's how a knock sensor works. It selectively amplifies the
resonance frequency of the engine block and retards the spark if the
level is above a threshhold, or allows it to slowly advance if the
level is below it. Thus it constantly adapts spark timing to the
fuel's octane and the power demand.

I first saw them on the mid-70's Buick turbo V6.

-jsw