Improving CRT geometry by upgrading capacitors

[Hitto]

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.

[stj]

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.

[eccerr0r]

Re: Improving CRT geometry by upgrading capacitors

This web page is pretty much a full white screen?

[R_J]

Re: Improving CRT geometry by upgrading capacitors

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.

[Hitto]

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?

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.

[stj]

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.

[Hitto]

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?

[stj]

Re: Improving CRT geometry by upgrading capacitors

pulse current.
some polyesters and polypropylene caps wont be happy so read the datasheets.

[Hitto]

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://cdn.badcaps-static.com/pdfs/...61730aa80a.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.

[Hitto]

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:
https://cdn.badcaps-static.com/pdfs/...96564c4d75.pdf

[stj]

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

[Hitto]

Re: Improving CRT geometry by upgrading capacitors

I've used a 3.3uF/50V electrolytic capacitor (made for horizontal deflection) and paralleled it with a small 0.1uF/50V MKP/Film capacitor (it should help with very high frequencies which the electrolytic is fisically unable to smooth).

It works okay.

Do you guys think that any MKP capacitor will be good, considering that it will only filter a small percentage of the total current, or should I replace it with something beefer?

I ask this because a lot of datasheets warn about using Film/MKP/MKT capacitors "with other types of waveforms".. and horizontal deflection on CRTs is a sawtooth.

Should I derate the MKP/Film capacitor in order to avoid problems?

[Hitto]

Re: Improving CRT geometry by upgrading capacitors

I've used a 0.1uF/250V MKP capacitor and called it a day.

[Hitto]

Re: Improving CRT geometry by upgrading capacitors

Other useful informations for the members of the forum and whoever arrives here with the same question:
1) Higher capacitance made no differences to geometry. Tried 6.8uF, 10uF, 12uF, 22uF, 22uF+12uF with zero changes to geometry. No overheating of the transistor by using higher capacitance but I cannot say if it causes problems on the long run. Not advised going higher than 15uF has I've seen monitors with such capacitance but never more.
2) Any electro capacitor, even a beefy crossover capacitor rated at 100V and able to sutain 900mA of alternate current, which is quite high, failed in a matter of minutes. The test was made while keeping a 3.3uF MKT capacitor in circuit to prevent failures of the horizontal transistor so we can assume that the electro capacitor was even not sustaing the whole current (!).
3) Installing 6.8uF capacitor (MKP/MKT) made the monitor more silent at 100Hz. Same obviously happened with higher capacitances.
4) If the original capacitor in this area measure much more than its rating (when I opened the monitor for the first time checking for faults the capacitor was a 3.3uF measuring 3.8uF with other parameters perfect but the screen was somewhat noisy) then you have to replace it. Producers back in the day used very cheap capacitors despite knowing that this component is vital for the working of the entire monitor.

Even if the replacement is a MKP capacitor, use at least two identical capacitors in parallel. Better if you use three in case of cheap/no-name MKP capacitors.

[Hitto]

Re: Improving CRT geometry by upgrading capacitors

Other informations for the readers and the members of the forum.

Found a very old CRT monitor which effectively uses MKP capacitors for this exact section. It parallels four MKPs: 820nF+820nF+1uF+1uF. They are 250V parts despite the voltage being no more than 60V (quickly measured with a cheap multi-meter in DC).

[R_J]

Re: Improving CRT geometry by upgrading capacitors

Other informations for the readers and the members of the forum.

Found a very old CRT monitor which effectively uses MKP capacitors for this exact section. It parallels four MKPs: 820nF+820nF+1uF+1uF. They are 250V parts despite the voltage being no more than 60V (quickly measured with a cheap multi-meter in DC).

You are forgetting about the high frequency pulse that is on that cap, I suspect it is greater than 60v peak

[Hitto]

Re: Improving CRT geometry by upgrading capacitors

I suspect it as well. My multi-meter is quite cheap though so I take its reading with a grain of salt and derate. Not that I would not derate nevertheless when it comes to delicate circuitry with almost-irreplaceable transistors.

[R_J]

Re: Improving CRT geometry by upgrading capacitors

The B+ that supplies the horizontal output transistor is usually around 130vdc, but the transistor is usually rated for 1500v or more and disc capacitors in the horizontal circuit can be rated 2~3KV it has is due to the high frequency ac peak voltage not the dc voltage.

[clearchris]

Re: Improving CRT geometry by upgrading capacitors

So what's the use case for keeping a CRT running? I'm sure anyone who hangs here can find a broken LCD and make it work.

Are you running old school nintendo and want to play "duck hunt" with the orignal zapper?

[eccerr0r]

Re: Improving CRT geometry by upgrading capacitors

It runs... save the environment: use till it breaks, then fix it, and use it some more until the cathode is contaminated and the heater burns out...