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JoeNelsens Web Site
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Date: 2-8-2017
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Number of Hours: 4.00
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Manual Reference:
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Brief Description: tach error exactly 2X
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1) Durring the taxi test run, I had borrowed an optical tach. It showed my JPI vintage instrument was exactly 2X high...double the optical tach reading...exactly. I thought for a moment, and sort of figured that the JPI was used to being connected to a magneto on either Contential or Lycoming engine ( most likely), and that the mags rotate at 1/2 crank speed and the electronics were set up for that, or would convert mag rpm to crank rpm. The insturment had worked fine on the bench several years ago and I had done nothing to it except install it. The AeroVee has two pieces of metal, one with a magnet and the other as a counterweight(??) opposite it. Since I am using a Dodge crankshaft sensor for the tach pickup, the sensor picked up two pulses per crank rev, hence the exactly double reading on the tach.
This picture shows the relationship of the crank sensor..Upper device...to the scope sensor (used to determine the rpm of the drillmotor on the right). They..the sensors.. are centered around a double ended magnet...north poles on each end. when they spin the crank sensor will pick up its signal and pass it to the tach on the very left. The scope sensor was used to sense the actual pulse train relationship to the speed of the motor. It was truning at 1840rmp. the tach would show 3680. that was exactly happenoing on the AeroVee.
To correct the situation without disturbing the installation on the sircraft, I decided to devide the incomming pulse train by 2, which should yield a tach reading of 1840 rmp, the same that was verrified by the scope. I used a DIP chip sn7474, a dual D-flipflop wired as a divide by 2 added to the input line inside the tach.After wiring and securing leads and stuff inside the tach, final tests, using the shown double pulse rotating magnet 'Tool" in the drill, I had exactly the correct RPM on a system outputting two pulses per rev.
The reason I went this way and not connecting to any part of the ignition coils was this tach is a seperate system and is unaffected by mag checks durr
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test setup
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