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Repair help


leachicm

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I have 2 colecovisons with the same issue. The video will freeze randomly then slowly refresh then I get normal video for a few seconds. The atari module works fine so its not RF. I have a modern power supply and the issue also occurred with original PS. One coleco I've worked on a bit, new ram, recap, new video chip, Bios. Nothing has improved the situation. Oh, the audio works perfectly and is unaffected by the video glitches

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On 6/20/2021 at 5:30 PM, leachicm said:

I have 2 colecovisons with the same issue. The video will freeze randomly then slowly refresh then I get normal video for a few seconds. The atari module works fine so its not RF. I have a modern power supply and the issue also occurred with original PS. One coleco I've worked on a bit, new ram, recap, new video chip, Bios. Nothing has improved the situation. Oh, the audio works perfectly and is unaffected by the video glitches

"slowly refresh"?  You might need to describe that better.  Do you mean that the screen goes black and then slowly fills with the picture again?  Does it look like the game paused when the screen froze, or is it more like video calling on a crappy Internet connection, and you miss some of the action?

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So only the image freezes then.  That certainly sounds like a problem with the TV or the converter if you're using one.  The video chip on the CV runs autonomously showing an image on the screen.  It deals with 8x8 tiles rather than pixel by pixel, so this isn't something that you'd find in a malfunctioning CV (or TI99 or MSX or other systems that use this chip).  Instead, it could be a TV struggling to make sense of the signal.

 

The TMS9928/9929/9918 chips do not create a fully compliant signal.  It's good enough for naive CRTs of old, and many modern digital devices will put up with it.  But not all will.  For instance, my oscilloscope will not trigger properly on the signal that these chips output because it wants to be able to identify the even/odd frames as well as line counts and such, in order to always show you the line you want to inspect.  Instead it just runs free.

 

Your hardware may have similar issues.  Obviously it works with some less-than-complete signals such as the Atari module, but perhaps the TMS chips break its brain.

 

All of this is a long-winded way of asking, "did you try other TVs?"

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I considered the TV being the issue. I don't have another to test on but might have access to a CRT next month. All my other retro systems run fine on RF with this LCD. I got an AV mod and things looked promising then it died don't know if there being 13.2V on the 12V line had anything to do with it. Thanks for the explanation, its good to know that there is a quirk in the design

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