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Excess flicker (snow) on a TRS-80 Model 1 clone

T

tezza

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Hi guys,

One of my Dick Smith System 80 machines (TRS-80 Model 1 clone) shows a lot of flicker on the screen, especially when the screen is refreshed a lot such as in this example of Meteor Mission.

snow.jpg

Any idea of what might cause this? I was thinking video RAM but diagnostic software shows no fault in the video RAM? Maybe something in the video RAM circuitry though?

The Model 1 circuitry always did have SOME flicker but never to this exent.

Apart from a few cracks in the case this is the only thing wrong with it. I have two other good System 80s which I've been playing with lately, but I'd like to get this one at 100% too, just for the challenge.

Tez
 
Just an update: I swapped out the 2114 video RAM ICs with replacements. I get the same symptoms so they are not the culprits here.

Tez
 
Might be a timing issue (50 vs 60 Hz?) as flicker is usually caused by the CPU accessing the video RAM at the same time as the display logic. Maybe the board has been overclocked?
 
conflict access should not be "white" and the lines are usually wider. The graphic blocks are generated by gates I would suspect them if it is not some kind of ghosting. Try poking in basic FORX=0TO1023:pOKE15360+X,128:NEXTX then FORX=0TO10000:pOKE15360,128:NEXTX etc. to find out if it is access that makes the noise or if it is the graphic characters that are bad.
 
retrohimpi said:
Might be a timing issue (50 vs 60 Hz?) as flicker is usually caused by the CPU accessing the video RAM at the same time as the display logic. Maybe the board has been overclocked?
Click to expand...

Hmm.. A good thought but I don't think the speed has been upped. The machine HAS been modded in some aspects. For example, it's been altered to use the external cassette as the primary cassette storage unit rather than the built-in one. But the games aren't any faster. If it had had the 4MHz speedup mod I would expect them to zoom along. I can't see any evidence of any other tampering except around the cassette circuitry.

All the System 80s and TRS-80 model 1s I've ever seen has some flicker just like this. Its a normal phenomenon. In this machine it's a lot worse than normal though.?

Tez
 
knutrl said:
conflict access should not be "white" and the lines are usually wider. The graphic blocks are generated by gates I would suspect them if it is not some kind of ghosting. Try poking in basic FORX=0TO1023:pOKE15360+X,128:NEXTX then FORX=0TO10000:pOKE15360,128:NEXTX etc. to find out if it is access that makes the noise or if it is the graphic characters that are bad.
Click to expand...

Thanks Knut, I'll try that program. I'm sure the graphic blocks aren't bad. They look normal in a character dump. It's when the screen is being refreshed that these lines appear.

Tez
 
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