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Mutual Capacitance of an Overwind...

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Alastair
(@alastair)
Posts: 294
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Having replaced/substituted the EHT Overwind on a Bush TV125, I had a lack of width issue. At full width setting on the 'width/stab; control, it wasnt quite making the edges of the tube.

I also noticed the LOPT Primary was hotter than I would have expected....

I got to thinking, Tuning of the LOPT stages is now wrong as it hasn't the added inductance/capacitance of the overwind anymore, only the load of the doubler circuit.

So, looking at the schematic, I decided to add a cap from pin 1 to deck. That coil drives the deflection-coils and is centre-tapped to deck, the other coil leading into the deflection circuit also.

My idea was, If the tuning was OK without a cap, then with a cap width would be even less, as the additional capacitance would upset the tuning....

I put a 1000pF cap to pin 1 and earth, fired it up and sure enough, had much more width. Not wanting to go too far, I dropped the value to 500pF and still had full width at about 3/4 width control setting.

After some hours of use, I checked the LOPT which appeard to be much cooler than it had been yesterday after a similar time in use....

What do you reckon...?

 
Posted : 12/02/2013 12:06 am
Anonymous
(@anonymous)
Posts: 16832
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Yes, seems logical, but then it's years and years since I dabbled with LOPT.

 
Posted : 12/02/2013 1:29 am
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