PDA

View Full Version : Atari 800XL erratic video



Neosodium
January 29th, 2011, 12:00 PM
I recently bought an atari 800XL from ebay, having been after an 8-bit atari computer for quite a while (still after a 400 but money does not permit)

There is an unusual problem with the video output, both on RF and composite (monitor port) where it goes into a bout of flickering for several minutes like the V.hold is bad on the TV, eventually settling with the screen shifted to the left and the colour monochrome or messed up.

I contacted the seller today and he will be sending a replacement machine, and has said not to bother sending this one back. (great guy!) Seeing as it's staying here, I wanted to see if I could work out what was causing the problem.

I did find a screw which can be accessed from the underside or the inside of the machine that adjusts the colour, but when one display mode is colour the others appear in monochrome or have garbled colour. I'm thinking a dodgy GTIA chip but I'm no expert on atari hardware.

tezza
January 29th, 2011, 12:19 PM
The GTIA might be the culprit. It certainly has a lot to do with the video and they can fail. I had to replace mine on my 400 recently.

Tez

Neosodium
January 29th, 2011, 12:22 PM
Which chip is the GTIA, one of the four large ones at the bottom of the board? (I know the POKEY is the top-right large socketed chip) are they easy to come by and cheap? or is this machine destined to be a spare parts provider?

tezza
January 29th, 2011, 01:24 PM
I don't know off the top of my head. I'd have to disassemble my Atari 800XL to tell you that. A search around the web should reveal the chip code numbers associated with GTIA so you can identify it.

Tez

carlsson
January 29th, 2011, 01:44 PM
Wikipedia has some chip numbers:
http://en.wikipedia.org/wiki/CTIA/GTIA

See also this post on AtariAge:
http://www.atariage.com/forums/topic/74377-master-chip-listing/

Assuming the articles are correct, you should look for C014889.

Neosodium
January 29th, 2011, 02:49 PM
Found the little beggar, it's the one on the left. I deseated and reseated all the chips earlier and that made no difference but deseating and reseating it this time seems to have put all the colours back where they belong and stopped the colour bleed, but the screen is still noticeably off centre. I'm going to give it some time and see if it goes all wacky again.

Incidentally, how necessary is the RF shielding? it's a real pain to remove when you have to do it more than once.

tezza
January 29th, 2011, 11:42 PM
Not sure, I've never operated my Atari 800XL without the shielding but it's there for a reason. I guess it depends whether the unsheilded RF plays havoc with nearby TVs.

Tez