Tech question - picture distortion - Videokarma.org TV - Video

mashaffer · #1 06-14-2009, 05:08 PM

I have a freebie RC color TV model GGR649L Manuf. Oct 1982. We use it only for movie playback and don't even have an antenna for it. We feed DVD player into a modulator and then rf cable to the TV for video.

We often have distortion of text such as credits and subtitles (but not always) that make it unreadable. Often when the text starts to fade out it clears up for a second as it is going away.

I thought it might be too strong a signal so I tried an RF attenuator on the signal line with no change (I am not too savvy when it comes to TV). Any idea as to whether this is an adjustment or just a dying set (or other)?

Appreciate the help.

mike

Findm-Keepm · #2 06-14-2009, 08:50 PM

Your set should be a CTC-120 chassis by the year and model group. Sams 2125 Folder 1 is close, although Sams doesn't list your model. I haven't worked a CTC-120 in years, but here are some tips from a database I have. Hope it helps. C105 and C120 are probably "must" replace items if you do tear into it.

Cheers,

zenith2134 · #3 06-15-2009, 12:10 AM

I've encountered one ctc-120 since I started messing with TVs a few years back.

FindmKeepm certainly knows for sure though, and judging by the repair tips he posted, your problem could just be a weak or shorted transistor in the video circuit or a cap.

But by your description the first things I would check on a set of this age are the focus, and the rf/if section but be careful in the latter sections...

mashaffer · #4 06-15-2009, 09:58 AM

Thanks for the input guys. It looks like I could get in over my head pretty quickly here. I didn't think it could be focus as it is only the lines on the screen where the text is that become distorted. I think it is always white text also but I am not 100% sure.

Here are some pictures of the controls on back. Would any of these have an effect?

mike

Duane · #5 06-15-2009, 10:12 AM

The RGB bias and drive controls are used to set grey scale,so the answer is no.

andy · #6 06-15-2009, 10:33 AM

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richms · #7 06-15-2009, 11:49 AM

It will be high video level - most things seem to put out a signal thats a bit high, since there is an AGC on any composite input that takes care of it, and it means that there will be less noise in the pic.

Some modulators I have used have had a screw on them for the video level - with onscreen graphics and menus from a DVD player it was massivly overmodulating and causing problems in the white areas and turning it back solved it. That also stopped the problem with the modulator on ch36 having the bright parts show ontop of the content showing on ch 38 (have to double space them since they are cheap DSB ones)

mashaffer · #8 06-15-2009, 02:39 PM

OK, are we looking at too high of a signal from the DVD into the modulator or too high of a signal from the modulator to the TV? The attenuator I tried went on the cable from the modulator to the TV so I never tried reducing the signal to the modulator.

I searched the menus on the DVD player (philips) and found nothing relating to output levels and the book with the modulator was pretty useless. I will check both again carefully. Is there any kind of outboard attenuator available for the composite out on the DVD?

The only other source I have is an older VCR. I don't remember whether there was a problem with it but have not used it much. I will check that also.

mike

Trance88 · #9 06-15-2009, 04:11 PM

Is it possible the contrast or bright might be too high? From my experience if the bright is too high it might cause picture distortion especially on bright colors.

richms · #10 06-15-2009, 11:30 PM

Quote:

Originally Posted by mashaffer

OK, are we looking at too high of a signal from the DVD into the modulator or too high of a signal from the modulator to the TV? The attenuator I tried went on the cable from the modulator to the TV so I never tried reducing the signal to the modulator.

Could be either, but too high into the modulator is the usual cause of it.

I have only seen adjustments on decent quality modulators - the cheap ones that are basically the same as what you get for a games console with an old tv dont have them.

The ones I use for getting the cameras and the media player out to the other TVs here are all kingray ones which are proper ones used in multi unit installations with level controls for video, audio, and output level and on the dvd player I was able to take the video way down and not have any effect.

Try some resistance in the video line to the modulator if there isnt a control on it - its a 75 ohm signal so start very low like 5 ohms, 10 ohms and see what happens. Cheap RF modulators always provide minimal signal - most cant even survive a 4 way split and still have enough for a noise free signal. Its all to do with when people do dumb stuff and connect it to an antenna - a 95dBµV output will quite effectively transmit for a good 20m or so into rabbitears or a loop antenna, whereas a 60dBµV will only work if its virtually touching the receive antenna.