HELP with settings on old B&K scope!

[fastline]

I am testing a magnetic pulse coil on a motorcycle application. Book spec is 400-500ohms. I am testing at 110ohms. No sparky of course. I like to verify she no worky.

The scope is a old 1477 dual channel. When I have the auto trigger on, I only see a spike in one direction and voltage 10x what it is with it off. I should see a pretty sinusoidal wave form and with trigger off, I see a decent sine wave.

My probe hase 1x, 10x, ref. Ref and 1x both show 250ohms through the leads. I should not need 10x since optimal voltage is typically around 2V RMS. I really need to make sure I am on with my settings here. Can someone help me select proper settings?

 

[KeepItSimpleStupid]

You failed to mention whether or not we are talking points, capacitor and an ignition coil or some sort of electronic triggering.

The transformer usually has a primary and a secondary with one side tied together. The transformer primary is excited when the points open. That voltage will be quite high and more like an exponential.

The sinusoid your seeing is probably the line frequency from stray pickup.

At the very least, you don't know how to use a scope. Usually there is a tang with the word cal near it. This is a square wave and it's used to set the probe compensation. When you move to the x10 position, a screw should be exposed. This is adjusted such that the calibrate wave has square edges. Compensation at x1 is not possible.

If you have resistance readings of both the primary and secondary, usually they are a good indicator if that far off.

 

[fastline]

I was testing a pickup coil. They are just a pole with magnet wire wrapped around them. The flywheel passes by and induces an AC wave that the ignition box senses for the firing sequence. I tested a known good machine and the wave form looked exactly the same. I then connected the function generator and was generating a nice sine wave. I also learned that there is a short in my leads thus the odd performance.

No, I have never calibrated a 10X probe so I will have to get on that!

Still not quite sure why I only see voltage in one direction on a pickup coil. I would think it would show one complete cycle each event.

 

[KeepItSimpleStupid]

Still not quite sure why I only see voltage in one direction on a pickup coil. I would think it would show one complete cycle each event.

Wikipedia does mention a sine wave output though: https://en.wikipedia.org/wiki/Variable_reluctance_sensor

 

[fastline]

Not enthused about the non-definitive test results but going with my gut I guess and replacing it. Test at 100 ohms and book is 400-50ohms. Book also indicates 2V performance peak to peak. My test show no more than 1V but an odd wave. I am assuming two bits of wire are welded together in coil causing only a partial short. I hope I am not wrong....

 

[dr pepper]

Testing a coil with a meter at dc is only a rough guide, even different meters might give diffrent readings, however your reading is a 1/4, thats enough to suspect the pickup.
It would take a lot of windings to short to drop that much.
Compare it with a new one, if its way different thats the issue, of course this does involve actually buying a new one.
I was thinking of testing it with a 9v battery and lifting a washer but doing that might magnetize it.

 

[fastline]

I happened to have it's brother (engine) in 500 pieces on the bench so I was able to test its coils...spot on with book spec. I usually don't get puckered if values are slightly off but this one seems far enough out to replace just on that alone.

In operation, the coil is never powered so I would hate to damage it in testing. They are not exactly easy to get to on most modern 4 strokes. Usually reliable so they bury them.

 

[dr pepper]

There you are then.

I spose you could demagnetize them by sticking the whole lot in a coil fed with ac.