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John-Del[_2_] John-Del[_2_] is offline
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Default Reactive Overscanning

On Friday, February 9, 2018 at 9:37:41 AM UTC-5, wrote:
On Friday, 9 February 2018 12:56:15 UTC, John-Del wrote:
On Friday, February 9, 2018 at 7:23:21 AM UTC-5, wrote:


I have a small Sony in the bedroom. When I watch, I usually do it with the closed captions on. i have no need for music on TV, plus you can hardly understand the dialog, and some **** is just plain old annoying.

It overscans. Part of the reason is a bad cap. If the circuit is what I thik it is, the cap filtering the B+ input to the flyback will increase horizontal scan if the ESR goes high, and it also has intermittent immediate shutdown at startup. And it needs the 220 volt capo as well because it has jailbars, but that doesn't affect the scanning.

I think I got the caps laying around somewhere and intend to fix it. But I am sure the little **** will still overscan. It cuts off part of the captions, which is the main problem. So I have a choice of two ways of reducing the horizontal size. I am just wondering which one is better.

One way is to reduce the regulated B+. This might be easier ssaid than done if it is not adjustable and uses one of those hybrid DM type modules to control the regulator. ButI would still do it with a staclk of Zeners and just let the DM module be the current source. The regulation wouldn't be as tight but it would be good enough. the drawbacks are that the drive to the horixzontal poutput will be decreased which may result in higher dissipation. Also the B+ to the vertical IS being scan derived will be reduced and the on time of the bottom transistor may be longer and increase the dissipation of the IC. Adjusting the vertical height will not fix this. It would have to be adjusted anyway though because the ramp amp is probably run off a sub-regulated supply. Otherwise I get that foldover at the top.

The other alternative is a bucking winding on the flyback. Usually on these I can get in between the core and coil of the flyback so
I just need to wind some wire in there which would be in the same polarity as the yoke feed, but inserted in series with the yoke return line. The problem here is that it will decrease the high voltage and increase the deflection sensitivity which would be working against itself. Also the switching optimization might be compromised with a longer duration pulse there, plus that lengthens retrace time which also works against itself.

Just for ****s and giggles which way you think would be better ? I'll get it done no matter, but I am just trolling for opinions. I got too much time on my hands I guess. I need some work.

So, if you got too much time on your hands and know how these ^%&*##$s work, type it up.



I wouldn't play with the B+, but maybe add a bit more capacitance across the HOT. You'd lose a little HV admittedly, but on smallish CRTs that doesn't seem to be a problem IMO.


I'm not clear on the details of what you're suggesting, but anything that reduces horiz scan power also reduces EHT, and vertical scan too if that runs on a LOPT derived rail.


NT


I'm assuming he's trying to reduce horizontal overscan as vertical overscan is always adjustable and he knows that. Any modification to horiz width that causes vertical height changes can be adjusted out. By adding capacitance to the Horizontal Output Transistor (I believe you call it Line Output), the horizontal width will be affected. The HV (EHT?) will change a bit but it shouldn't be enough to cause issues.