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Old 06-25-2017, 02:59 PM
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VintagePC VintagePC is offline
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Neat trick: Ringing a flyback with only a scope

I found out today you can ring test a flyback with nothing more than a scope (and maybe a handful of capacitors of different values).

All you need is a scope that has a square wave output of some kind of at least 10vpp.

My Tek 453 has a "gate" output that goes high when channels are triggered.

Wire it up to one of the horizontal primaries of your flyback. and connect your probe in parallel. (In my case this was a little complicated since I used the A gate to pulse the flyback, and the B channel to observe the ringing. It's easier if you have a 10vpp cal output or some such as you don't need to force A to trigger in addition to your monitor channel).

If you did it right you should see a pulse on your probe. Now dig through your capacitors and bridge the channel with a typical TV type film cap - 0.1 to 0.01 µF.

You should get a nice ring (see attached). If you dont, try a few values or a different terminal, it depends on the resonant frequency of your fly. If it won't ring, you've probably got a shorted winding.

Best done with a known good fly first to get a feel for it - Once you get a ring... try shorting another winding, such as the HV rectifier filament (just like a single shorted turn)... and you'll see severe damping of the oscillations.

Pretty neat, eh?

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File Type: jpg ring.jpg (38.1 KB, 55 views)
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Old 06-26-2017, 02:06 AM
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An old trick but a good one. You don't even need 10V, just turn up the gain of the scope to suit whatever voltage you have. The probe calibration square wave that most scopes have is pretty good.
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Old 06-26-2017, 12:52 PM
kf4rca kf4rca is offline
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You can also uses the A gate for testing coaxial cable. You loop the pulse output thru the vertical input with a tee. Put a 100 ohm pot on the end of the line. Rock the pot back and forth and observe the waveform. You will see the reflected pulse go up and down, but when it flattens out, a match is achieved. Now remove the pot and measure the resistance which is equal to the impedance of the coax.
This method also shows also shows discontinuities such as splices and kinks that would create a reflection.
In the old days, TV engineers would use this method to "look" at the transmission line going up to the antenna, particularly after a nasty lightning hit.
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Old 06-26-2017, 08:19 PM
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That's pretty cool too!

I'd imagine there are many more such "tricks" that have been lost to the sands of time as techs have retired and more modern gear has taken over...
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Old 06-27-2017, 11:58 AM
kf4rca kf4rca is offline
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The coax pinger later evolved into the TDR (time domain reflectometer) which included a calibrated time base and compensation for various cable's velocity factor. (Ex.: fused disc cable is different than solid dielectric.) TDR's are used extensively by CATV companies to locate faults especially underground.
The TDR has since evolved into the OTDR (optical time domain reflectometer) which is used for Fiber "cable".
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