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Old 11-28-2019, 09:48 AM   #1
Hitto
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Default Improving CRT geometry by upgrading capacitors

I'm trying to improve the geometry of a CRT monitor by upgrading capacitors.

Compared to the original ones, I've upgraded by 15-20% the capacitance of each capacitor around the flyback trafo and used components with higher current capabilities.

Howewer, the scan capacitor (the big 3.3uF bipolar capacitor in the horizontal section) is what is giving me trouble.

Do you guys know if going higher with the capacitance will make possible to obtain a better geometry or will it only affect the horizontal width of the picture?

Is a polyester capacitor limited in current capabilities, giving that this capacitor will have to handle more than 8A of current? A parallel will be very hard to perform since the space is restricted by nearby components and polyester capacitors already have quite large lead spacing.

I've seen CRTs using 3.3uF, 4.7uF and even 10uF. This CRT has a 3.3uF capacitor and I'm currently thinking of using a 6.8uF capacitor.

Once the board will be assembled it will be quite troublesome to resolder a new one due to lack of space, so I would like to know more informations about the choice of this (critical) component.

Last edited by Hitto; 11-28-2019 at 09:49 AM..
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Old 11-28-2019, 10:28 AM   #2
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Default Re: Improving CRT geometry by upgrading capacitors

that cap is selected for a reason,
the only caps worth upgrading are the smoothing caps on the outputs of the transformer.
even then, most people wont need it - a full white screen can sometimes shrink the image because it draws more current - but a white field is unusual except in some games.
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Old 11-28-2019, 10:44 AM   #3
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Default Re: Improving CRT geometry by upgrading capacitors

This web page is pretty much a full white screen?
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Old 11-28-2019, 11:30 AM   #4
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Default Re: Improving CRT geometry by upgrading capacitors

Quote:
Originally Posted by Hitto View Post
I'm trying to improve the geometry of a CRT monitor by upgrading capacitors.

This CRT has a 3.3uF capacitor and I'm currently thinking of using a 6.8uF capacitor.
If you change this capacitor value the horizontal linearity will be effected and the geometry will look like shit. Like it was stated, the values are picked for a reason.
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Old 11-29-2019, 10:14 AM   #5
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Default Re: Improving CRT geometry by upgrading capacitors

Thank you all.

Do you think a polyester capacitor, let's say a 3.3uF 100V one, is alone able to withstand 8-9A of current?

Quote:
a full white screen can sometimes shrink the image because it draws more current
That is true. It also caused whining on that CRT. Would this effect be toned down by upgrading of capacitor filters on 160V line? The board has by default design 22, 33 and 10uF of decoupling near the pins of the trafo.

I assume that the 160V line is quite crude in terms of ripple.

Last edited by Hitto; 11-29-2019 at 10:16 AM..
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Old 11-29-2019, 10:40 AM   #6
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Default Re: Improving CRT geometry by upgrading capacitors

if B+ is 160v which sounds high, you can use a 100v polyester - a 250v would be o.k.
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Old 12-01-2019, 08:23 AM   #7
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Default Re: Improving CRT geometry by upgrading capacitors

I meant the scan capacitor, which will have to sustain 8-10A of current. Is a standard MKP capacitor able to sustain that much current or do you think that a parallel (2.2+1uF or 2.2uF+2.2uF) will be necessary?
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Old 12-01-2019, 12:06 PM   #8
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Default Re: Improving CRT geometry by upgrading capacitors

pulse current.
some polyesters and polypropylene caps wont be happy so read the datasheets.
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Old 12-03-2019, 06:02 AM   #9
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Default Re: Improving CRT geometry by upgrading capacitors

Thank you.

Found some capacitors which should be able to sustain the required current, which will be not less than 8A. They are the B32774P TDK film capacitors.

In case anyone will find it useful, these are the full specifications:
https://www.tdk-electronics.tdk.com/...2774P_778P.pdf

The 6,8uF 630V capacitor is able to sustain 10.4A of current while the the 3,0uF 630V capacitor is able to sustain 6.0A. I'm going to try the 6,8uF 630V and see if this difference (3,3uF>6,8uF) is going to alter the geometry or not (I guess yes, but the 3,0uF 630V may be not enough to carry the required current and, if I'm not mistaken, that scan capacitor is a critic component that cannot fail for the sake of nearby components).


Another serie with high ripple capability is the MKP1848H manufactured by Vishay.
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Old 12-03-2019, 06:25 AM   #10
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Default Re: Improving CRT geometry by upgrading capacitors

Other informations found while searching that I wanted to share.

In case you want an exact-fit replacement the electrolytic capacitor is the Nichicon UHA1H3R3KHD (3,3uF 50VDC rated at 6A).

The most capable component in this path, if we do not want to use a film capacitor, seems to be the UHA1H4R7KHD (10uF 50VDC rated at 10A).

I am going to test, in the following order:
6,8uF 630V (MKP)
10uF 50V (Electrolytic)
3,3uF 50V (Electrolytic)

Other stuff:
http://www.gradllc.com/images/HORIZONTAL.pdf

Last edited by Hitto; 12-03-2019 at 06:35 AM..
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Old 12-03-2019, 08:31 AM   #11
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Default Re: Improving CRT geometry by upgrading capacitors

you may want to think about how low the current handling of the old electrolytics was.
use the 3uf
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