CRT 'shimmer'

[Minwah]

I have an old (presume 90's) 14" TV which I would like to use for a Mame project. It is a budget brand but it has a really good, bright, clear picture...much better than the picture of many other similar TV's I have tested. Also it has a nice goldfish bowl tube as opposed to some of the later flatter ones.

The only problem with it is, after is has been on for a few minutes the picture 'shimmers' slightly. It is not noticeable if the image onscreen is moving a lot, but for images which remain still (eg the maze in Pacman), it is obious (to me). The average person probably wouldn't notice it TBH. I should add that I am using RGB scart, so this is not a crawling issue which can happen with RF, composite or S-video signals...the signal is good, and I don't think it is interference (otherwise I would think it would do it as soon as switched on cold).

I am in the process of replacing the electrolytic power capacitors in it (although they look ok) - hopefully this will fix it. Just wondering if there is anything else I should check or replace while I have it apart? The solder joints all look good to me.

Thanks!

[rCadeGaming]

Not sure what you mean by it "shimmers."  Does move around or just vary in brightness.  Is it interlace flicker?  Are you running 480i or native res?

[Gray_Area]

Pics or it doesn't happen.

[Minwah]

I don't think a camera picture would pick it up very well, and it's taken apart at the moment...

This is running at low resolutions (320x240 etc). The brightness is fine, constant. Hard to describe but the image wiggles around slightly. Imagine a vertical line on screen...this is what it would look like (4 frames):


|     |     |     |
|    |      |      |
|     |      |     |
|     |     |     |


Hope that makes sense. I'm still hoping new capacitors will fix it, just waiting for one to arrive in the post.

[azathoth]

I've had the shimmer when I first did my setp-up, as far as I can tell it's the Video Card that does it. It seems the worst at the lower resolutions, something to do with timings?
It was a brand new ATI 9200 card, but the problem has since gotten much better. It's been 2 years and now its barely noticeable it, if at all.
My setup is a 15khz chassis, tv tube, j-pac, 9200 ati video card.

Perhaps there is a break in period? Is your video card brand new? What kind is it?

[rCadeGaming]

Old consoles do this sometimes.  Does it happen more noticeably with thin bright lines, like white text on a dark background?  It could be electrical instability in the video signal.

How steady is your power supply?  Be sure you have a good steady power supply that has more than enough power capacity so it isn't straining (if your computer actually need 250W, use a 400W supply, etc.).  Try capacitors on the color lines and sync lines (experiment with 1uf to 220uf) to smooth things out.  If the signals are too strong, try bringing them down with resistors.  Use pots to experiment.

[Minwah]

Thanks for your replies, very helpful.

I am using an old P4 Dell Dimension PC...I am not sure what power the PSU is. The video card is a similar age GeForce (again, sorry not sure what model, not at home at the mo). Using Soft15khz.

I have finished replacing the capacitors...and basically it is the same. What made me think it is to do with the TV is that it is fine for a while, but once it has warmed up it starts to become noticeable, and eventually quite bad. I will try using it until I can see the problem, and then plug in a games console to see if I can still see it...

It is more noticeable with bright lines on a dark background. I will try using capacitors/resistors on the video signals too - I hadn't thought of that.

Thanks again!

[rCadeGaming]

Make sure your contrast and brightness aren't up too high.  On my Trinitron there is an edge enhancement feature, which being turned off makes a big improvement with this.  I doubt your set has this if it's from the 90's but it's worth mentioning in case.

Even if they don't help with this, pots on the color lines will allow you to calibrate your color levels.  Try 100 ohm pots.  Not sure about sync.  Try a 5k 25-turn pot, which will give you fine adjustment over a large range.  It will may give you adjustment of picture position as well.