Video flicker in the System 80/Video Genie I

[Terry Stewart]

Hi guys,

As you can tell I've been playing with my System 80s lately. All
three are working. One of them (the 16k machine) has a minor issue
which I'd like to fix though.

The problem is with screen flicker. Small random horizontal lines or
"snow" appears when the screen is rapidly updated such as playing a
machine language game or even listing a program. Even just pressing
return can cause some of this flicker. When the screen is not
updating there is no sign of it.

I know these machines (like the model 1s) always have a bit of this
flicker, but this machine is far worse than the other two. At first I
thought maybe the 2114 video RAM but I've swapped this out with
replacement ICs and I get exactly the same symptom.

I've put a post on the vintage computer forums about this, where you
can also see a picture. See...
http://www.vintage-computer.com/vcforum/showthread.php?21104-Excess-flicker-(snow)-on-a-TRS-80-Model-1-clone

Has anyone come across this before or can suggest when the problem
might be?

Cheers

Terry (Tez)

[Knut Roll-lund]

Hi

I have never seen "white" lines like that, the video RAM access should be black lines that are about 4 or 5 characters wide which will only be there one frame.

Has this machine been repaired with modern parts? The original 74 and 74LS parts perceive undriven lines (tristate) as high, modern parts perceive them as low. I would actually think that if that is the case the lines should be wider.

Is it especially games that have this problem? The graphic characters are generated with some gates and a mux into the shift register. The mux switches between the character generator and these gates. If these gates are damaged it should affect character codes 128 and up.

Try filling the screen with 128 - FORX=0TO1023:POKE15360+X,128:NEXTX

If you get the noise without access I would expect the gates to be the problem. It could be some kind of ghosting but then I would expect also text screens to be affected also. It would require that some of the screen is filled with something white.

I would use pokes like above to try different things so I could see when the noise is there and not, and when it is worst. FOR is quite fast as you can see if you poke 191's on 191's and get the black lines. The fastest access you can do with basic is probably similar but without +X in the poke.

BTW I had to register to see the picture... so I'm now knutrl there.

BTW2 I have been trying to repair a Genie EI... It is really strange I have both a logic analyzer and an oscilloscope and everything seems to work, all signals are acting as they should but the Genie won't boot a floppy. There is also the hires on that machine and I wonder if there is some kind of conflict. :-(

Knut

[terry stewart]

Hi Knut,

Thanks for the reply. Good to hear from you.

The problem manifest itself whever the screen is refreshed be it graphics or
text. Even pressing new line several time will generate 2 or three of these
lines. They only last a split second. Where the screen in being refreshed
quickly and continuously, that's when the problem is worse. Such as in
machine language games as shown in the photo. The lines don't stay there or
in one place. They flicker on and off quickly and appear in different
places at evey screen refresh.

Some flicker like this occurs on every System 80 and TRS-80 M1 I've ever
used. This is far more than is usual though. I might put a note on the
yahoo 80s discussion group and see if anyone has ideas there.

The character codes above 128 look just fine, but I'll try that little BASIC
routine tomorrow.

Sorry, I didn't realise you needed to register to see that photo. Here it
is for those that haven't seen it.
http://www.classic-computers.org.nz/blog/images/2010-07-01-s80-snow.jpg

Re: your problem. It could be a conflict. Have you tried removing the
hi-res mod?

Terry

Hi

Knut

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[terry stewart]

Hi,

I've managed to fix the fault. See
http://www.classic-computers.org.nz/blog/2010-06-29-s80-screen-flicker-fix.htm

Thanks for the suggestions. They were helpful.

Terry Stewart (Tez)

----- Original Message -----
From: "Knut Roll-lund" <kr-...@online.no>
To: "Eaca discussion" <eaca-di...@googlegroups.com>
Sent: Thursday, July 01, 2010 9:12 PM
Subject: Re: Video flicker in the System 80/Video Genie I

Hi

Knut

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