Understand issue with 28" crt and Intervideo

[Marcoqwerty]

Hello!!!

Lately i discover this on my 28" crt, looks like the image is normal or squeezed in the center and slightly big on the side, with a small bow on the top that push down slight under the top border line.

I tried to represent the issue with this image below and some screenshot.

Its very hard to show the issue but its clearly visible on the Horizontal Scrolling games, and more while the resolution of the game is low; i didnt find any tool to check this Horizontal Scrolling issue with mame, this is an arcade cab with windows installed.

This issue depends from the crt tube or comes from the crt board? ( here there is the pdf of the manual with the dedicated knob http://files.arcadeinfo.de/Monitore/Intervideo%20VP%20Series.pdf ) , and if yes how i can fix? please consider that my crt dont have the Yoke rings around the neck.

Eventually i still have another new Intervideo board, to make few test.

[princess prin prin]

There's nothing on the chassis (this one and ALL the others) to adjust north-south pincushion issues. It's due to a deflection yoke doing a less than perfect job. East-west pincushion issues on the other hand can be corrected by the trimmer named "pillow" (located just above the yoke connectors). The uneven horizontal linearity (sides wider than center, as per your pic) is related to capacitors C38 and C76 in series with the horizontal yoke. Decreasing their capacitance will reverse the effect. Do your tests and choose the optimal values for your tube.

I've checked another manual for the VP chassis and for all sizes and CRT brands (including 25" Philips like yours) the value of C38 and C76 is always 680nF. Only some of the bigger tubes require a different value. Now I don't know what's on your chassis but I bet they're 680 and since these caps lose (not gain) capacitance when they degrade which makes the picture more compressed towards the side, I don't think they're out of specs (i.e. they kept the nominal value). Then again nothing forbids you to try different values as per my previous suggestion.

[Marcoqwerty]

Thank you soo much!

The bad part is that i have to desolder the capacitors ( before find them on the board  ); i think i have to go too deep on my tech crt knowledge so for the moment i will try with the new crt board and if im not able to solve the issue i will try to follow your useful tips.

I will keep you updated!

Again thank you!!!! 

[Marcoqwerty]

I've checked another manual for the VP chassis and for all sizes and CRT brands (including 25" Philips like yours) the value of C38 and C76 is always 680nF. Only some of the bigger tubes require a different value. Now I don't know what's on your chassis but I bet they're 680 and since these caps lose (not gain) capacitance when they degrade which makes the picture more compressed towards the side, I don't think they're out of specs (i.e. they kept the nominal value). Then again nothing forbids you to try different values as per my previous suggestion.

Thank you!

This make me think... that i have to change these capacitors with new ones, the monitor have his age but i discovered this issue only lately; so probably these components become exausted or decrease their value during last years.

Eventually there is some way to measure them with some basic multimeter?

[princess prin prin]

No, you need a capacitance meter.