Quote:
Originally Posted by G6Tanuki
In times-past I've found old EGA/VGA computer-monitors to be a good source of LOPTs: you may need to fiddle around with the capacitors that tune the LOPT to resonance.
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The lopt and the associated yoke inductance is tuned to "a resonant frequency" (by the capacitor on its primary) so that a cycle of this resonance is typically in the range of 50kHz to 60kHz vicinity.
A half a cycle should be in the order of roughly about 10uS to 12uS duration.
The tuning capacitor, if increased in size will simply lengthen this time (the flyback time) and lower the peak voltage on the horizontal output transistor's collector.
Or if the capacitor is made smaller the flyback time will shorten and the peak voltage will be higher.
This is because a fixed amount of magnetic field energy from the lopty & yoke (for any picture width) is transferred into the capacitor as electric field energy which peaks at 1/4 cycle of the flyback oscillation, which is half way into the flyback period, and the energy stored in the capacitor is CV^2/2, so if the capacitance is lower, the peak voltage is naturally higher.
The main thing to watch out for is that the capacitor is not too small in value and the peak voltage too high or it will take out the horizontal output transistor (HOT) by exceeding its collector voltage.
So if you are experimenting with non standard loptys for some set, it is best to temporarily double up on the pre-existing capacitor value, it will make the flyback time long and encroach on picture time, but then you can slowly reduce the value in steps until the flyback time just does not encroach on active H picture time, and the peak voltage on the HOT will be as low as it can be.
If you have a failed lopty due to a single failed EHT rectifier (leaky or shorted) inside the molded lopty assembly it can work to leave that as it is and string another EHT rectifier in series with the output. Doesn't work for voltage multiplier types.