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 Post subject: Re: philco predicta holiday not making high voltage
PostPosted: Jun Thu 23, 2022 1:46 pm 
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I thought everyone did it that way? Sometimes it was just a metal shorting strap on the CRT base.. I've never seen an electrostatic focus set with a pot.


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 Post subject: Re: philco predicta holiday not making high voltage
PostPosted: Jun Thu 23, 2022 6:06 pm 
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Joined: Sep Fri 03, 2021 9:33 pm
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I'm not done looking at all the parts and I have not measured any voltages yet but the only capaciter that is not what it is supposed to be is C5. It is supposed to be a .15 but it has been replaced with a .1. Do you think it's close enough to leave it in or should I replace it?


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File comment: C5 IS BY THE SWITCH.
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 Post subject: Re: philco predicta holiday not making high voltage
PostPosted: Jun Thu 23, 2022 8:05 pm 
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Location: Middletown NJ
Replace it with the correct value, .1 or .10 would have a difference of .05 uf from .15


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 Post subject: Re: philco predicta holiday not making high voltage
PostPosted: Jun Fri 24, 2022 12:56 am 
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bandersen wrote:
I thought everyone did it that way? Sometimes it was just a metal shorting strap on the CRT base.. I've never seen an electrostatic focus set with a pot.

Most color TVs (except very cheap brands/models) had pots for their electrostatic focus, some monochrome sets did too but mostly SS era stuff.
Many monochrome CRTs basically self focus and the different taps or pot settings don't change things much.


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 Post subject: Re: philco predicta holiday not making high voltage
PostPosted: Jun Sat 25, 2022 10:34 am 
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Electronic Memory wrote:
Many monochrome CRTs basically self focus and the different taps or pot settings don't change things much.



Well, that is not really true, maybe for the taps, but not for the pot.

Many electrostatic TV sets did have CRT's that would be close to a good focus with the focus electrode set at either gnd, or some higher voltage on a tap. But the reality, even for these CRT's, there is no substitute for a focus potentiometer, to put the actual focus through a knee where there is a good compromise between the focus at the central area of the screen and the corners.

Of course in domestic TV sets you can "get away with murder" with the focus a lot of the time. It becomes much more obvious though, this problem, if more is expected from the CRT, say to produce crisp graphics or text for a computer display.

A classic example of this issue was with the early Commodore PET computers where the VDU's CRT focus electrode could only be set for +85V or GND. Most of these CRT's focus with the better compromise at around 300V on the focus electrode. So it is better to add the appropriate supply and the focus potentiometer:

http://www.worldphaco.com/uploads/RESTO ... ER%209.pdf

One reason manufacturers avoided the focus pot is that it was generally an expensive part, needed to be about 2Meg (typically) and rated for around 600 to 700V, when the pot was at its mechanical center and there was around 300V presented to the focus electrode. So many TV designs saved money, by avoiding the pot, but it was not ideal.


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