tezza

Hi, I followed Bryan's instructions and successfully added the UAV to my NTSC Atari 800. Even here in PAL-land I now get a great picture on composite video! I'm not sure why as the UAV is not an NTSC-->PAL converter. Perhaps my TV/monitors are simply "modern" enough to allow it? Anyway, I wrote about the experience for those who might be interested. Tez

Found the cause. Atariksi was right. It was the GTIA chip. I had a spare working scruffy ATARI 800XL that I'd kept in case I need parts. I swapped out the GTIA chip in the 400 with the one in the 800XL. The problem now no longer exists on the 400 but the junk 800XL now exibits the symptom. The latter being a parts machine, this is of no concern. My 400 is fuly working again! Thanks for the assistance. Tez

If it is stuck on, it is probably the mechanical key spring. Try tapping it several times and seeing if it seems to rebound. No, its a 400. Membrane keypad. Also it is a replacement keyboard as I thought the problem was the original keyboard. It would seem not, as this new keyboard does exactly the same thing. Tez

Hmmm. I swapped out the keyboard with a replacement one but the problem with SELECT still exists. However I have discovered my initial assessment was wrong. The SELECT key is not non-working. It appears to be stuck ON !! Oh well, a disassembly and diagnosis awaits after Xmas. Maybe it is GTIA after all? Tez

I tend to write these fixes up after they are done and this one is no exception. It's at the URL below for anyone whose interested. http://www.classic-computers.org.nz/blog/2010-09-04-fixing-an-atari-400-screen-issue.htm Tez

Yes, I would tend to agree. The SELECT and OPTION keys would have been thrashed in the day and I wonder if the membrane has just had it as far as those two keys are concerned. If it was a ribbon connection issue, I would have expected a few more keys to be down. They are the ONLY keys which don't respond, even if you clasp the membrane keyboard between thumb and forefinger and squeeze hard. RESET and START are fine. Tez

I hear (and understand) what you're saying. It might be physically difficult to do though. I always worry that brittle ribbion which is the keyboard cable is going to snap if I play around with the innards with it still connected. It's hard to position the naked keyboard without putting strain on the ribbon. I seem to recall in my last thread regarding the ANTIC chip someone mentioned an a 800XL GTIA might be backwards compatible with a 400 one? Anyone know for sure? I could then do a swap over and check that way as I have a 800XL (in fact I have two..one is a parts machine which could be a donor). Tez

Hi, I just repaired a screen issue mentioned in another thread. The problem? A faulty ANTIC IC. Anyway, on testing the unit after reassembling I noticed another unrelated issue. The SELECT and OPTION keys don't work. I'm not sure it they were working before I disassembled the machine, as I've never really given this micro a good once-over until now. They many have never worked in the time I owned the machine! All other keys work including Shifted keys plus RESET and START. Would a good assumption be a problem with the keyboard membrane or maybe that brittle (and bastard to get in and out) keyboard connector? I'm assuming that if all other keys work apart from those two the keyboard driver chips are likely to be ok...but then I could be wrong? Anyone else had this problem or have any thoughts on the issue? Tez

Just closing this up. The problem was indeed the ANTIC chip. A new one arrived today. I replaced, reassembled and the Atari 400 is working as it should! Well...almost. I've just noticed the OPTION and SELECT keys don't work. Every other key does including RESET and START! These were probably existing problems as I never really had a good look at this computer until the screen issue. Most likely suspects are the membrane or maybe keyboard cable I guess. These two keys would have been hammered in the day. I might post a separate note about this and get some opinions? Thanks to everyone who contributed to this screen fix. Tez

Interesting. Had I known this I would have tried it when I had my Atari 800XL open to swap out the CPU. It may have given me a definitive answer as to whether ANTIC was the problem. Anyway, both the XL and the 400 are closed up now and I've ordered the part. Hopefully that's the cause. I can't think of what else it might be. Vintage computers never fail to surprise me though. I've had problems with a couple of my machines where the failed parts were static components namely a greencap capacitor and a resistor pack! Tez

Yes that was the first thing I did, along with reseating all the chips. No, it doesn't have enough lines. The bottom border is much larger than it should be. I know it is part of the border as it can be changed with POKE command. Tez

Where are you located? Perhaps, may be cheaper to pick-up a 800XL if you need multiple chips. New Zealand. Problem is the ANTIC chip is different in the Atari 800XL. I do have a spare Atari 800XL. Many of the other chips are the same but not the ANTIC unfortunately. I sent a paypal order off about 30 mins ago. I'll let the forum know if a replacement solves the problem. Many thanks to everyone who assisted. Tez

Right, I reassembled the machine... yes, it does. It changes the black areas (the top and side border and huge (1/3 screen) border at the bottom) a grey colour. There is a beep with every keypress. Holding START down when switching on causes beeping. I tried a few game carts. The screen is differet from No cart and BASIC, which are virtually the same. The game carts all show different symptoms but the result is always random coloured bars or squares on the screen with a dark background. Given that BASIC commands seem to work, the keyboard beeps when it should, and the fact the attract mode kicks in, I guess this is further evidence of faulty ANTIC, rather than, say, ROM? Tez

Hi Kurt, Hehehe. Yes, I've got a few projects which need writing up so there will be a few appearing soon. This one among them once I get the machine fixed. One of the entries which may be of interest to this community will contain a restoration project on an Atari 800XL which includes some retr0Brighting and the hazards of a dodgy power supply. Anyway..I digress... It was fun taking the Atari 400 apart. The design is like no other I've seen with two plug-in boards, one containing the RAM and the other the CPU and other propriety chips. For someone like me, used to single board computers like Apple IIs, more modern Atari's, PETs etc., this was an interesting design. It must have been expensive to make. No wonder prices for the 400 stayed so high for so long in the early 1980s. It's an artifact. The camera just picked up a bit of the scan line. Well, I just got a quote for a ANTIC chip. The email came through while I was writing this. $10 plus $15 shipping priority mail (or $6 "slow boat" mail). US dollars of course. One other question from those who might know? Could this symptom be possibly be caused by the CTIA chip? My feeling is that if this chip had failed I would be unlikely to get a clear picture at all. However, I'm thinking that if there is doubt, it might be wise to order both, given shipping costs. Tez

Here is the unit when it was working. http://www.classic-computers.org.nz/collection/atari-400.htm Tez