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Old 11-13-2019, 02:45 PM
dtvmcdonald's Avatar
dtvmcdonald dtvmcdonald is offline
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Question about flybacks circuit changes

This is a general question, though the context is the CT-100 and 21CT55.

What happens to the operation of the "typical" flyback circuit, like those
or just the garden 1950s large BYW set, when things get disconnected?
By that I mean ... does the (AC, pulse) voltage go up or down, does the
average horizontal output tube cathode current go up of down, if you ...

1) remove HV rectifier cap

2) use an HV rectifier tube with blown filament

3) unplug damper

4) unplug damper but wire in a resistor to get B+ to the horizontal output tube

5) disconnect yoke

6) vary the load (i.e. brightness)

7) lower (but not raise) the horizontal output tube screen voltage

8) various combinations such as 3) and one of the others?

The idea is ... what's likely (or not) to harm the horizontal output tube
or the flyback, or cause arcing?

Of course one can experiment using a variable B+ voltage, starting low enough to be sure of no harm (presumably no lower than the screen voltage)

It may be that nobody knows, except for things known to cause disaster


Doug McDonald

Last edited by dtvmcdonald; 11-13-2019 at 02:56 PM.
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Old 11-13-2019, 03:29 PM
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Electronic M Electronic M is offline
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1.) and 2.) (Assuming filament opens and doesn't short to plate or it's self) HV AC on flyback shoots up (possibly by as much as 10-20 kV), winding insulation survives a few sec to a few hours depending on flyback, then insulation breaks down, windings short, fuses open or things catch fire... assuming 0 human intervention at any point halting the process.

3.) Flyback and plate current in the output near 0 and no sweep. Output grid 2 current very high as it becomes the effective plate of the tube. A few minutes to by and the tube may be damaged. Flyback probably won't be harmed.

4.) Dunno.

5.) IIRC plate current and HV low. Risk of burning vertical line into screen. Never let it or heard of it going long enough to cause failure.

6.) Cathode current will vary with brightness in a fairly predictable manner...I think I've ran brightness from dark screen to blooming with a meter on it before....if no regulator in the HV average brightness will modulate the HV some.

7.) Predictas have a width control that adjusts screen. Too low and there is no sweep/HV It obviously modulates width and HV, and too high can overdrive things and cause HV breakdown.
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