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Atari 400 shows random characters on startup?


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#1 tezza OFFLINE  

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Posted Sun Aug 15, 2010 6:27 AM

Hi,

While exercising my Atari 400 the other day the machine suddenly developed a fault. The bottom 1/3 of the screen is black and the top 2/3 is full of random characters all changing very quickly. You can see a photo here:

Posted Image

I've swapped out the RAM and almost all of the logic chips (except the 74LS158s on the RAM board...I didn't have any spares....however, I did transpose the two that were there and that didn't make a difference). I've also swapped out the CPU. No change.

Power voltages to the board are fine.

Has anyone come across this before on a 400 or 800? I suspect a fault in the ANTIC IC. Before I go and order one however, I'd be interested in what others think? I've been using the 400/800 field service manual as an aid to diagnosis but it doesn't specifically mention this symptom.

Tez

#2 mimo OFFLINE  

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Posted Sun Aug 15, 2010 6:38 AM

Have you tried booting with the keyboard detached? I had a 400 that displayed a similar fault years ago, and I am pretty sure it was the keyboard
what happens when you insert a game cart?

#3 Rybags OFFLINE  

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Posted Sun Aug 15, 2010 6:44 AM

Might be Antic... if it was bad RAM or select logic, then you'd not expect to get what looks like a normal display as the Display List should suffer the same corruption.

Can you try the Basic cart, just hold down RETURN. Observe whether it repeats at the same constant speed you'd expect, or slows down somewhat, like errored lines are being entered.

Probably worth trying a game cart or three as well.

Also, if Basic appears to function normally, try blind typing:

POKE 106,32:GR. 0

Try other values instead of 32 like 64, 96 etc... up to 160 (assuming you've got 48K in there)

#4 tezza OFFLINE  

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Posted Sun Aug 15, 2010 2:47 PM

Thanks for those replys.

Yes, I've detached the keyboard and no change. The symptom is the same whether a cart is inserted or not. I'll reconnect the keyboard and try pressing the RETURN with the BASIC cartridge inserted as suggested.

The unit has only 16k.

Tez

#5 Divya16 OFFLINE  

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Posted Sun Aug 15, 2010 4:20 PM

Thanks for those replys.

Yes, I've detached the keyboard and no change. The symptom is the same whether a cart is inserted or not. I'll reconnect the keyboard and try pressing the RETURN with the BASIC cartridge inserted as suggested.

The unit has only 16k.

Tez


Is it the same pattern everytime or different everytime you boot up?

Does it give a beep when you boot with START key pressed?

(This could be a smarter alien that can communicate without using the XL self-test.)

#6 tezza OFFLINE  

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Posted Sun Aug 15, 2010 4:43 PM

Is it the same pattern everytime or different everytime you boot up?

Does it give a beep when you boot with START key pressed?


The pattern is not static. As soon as the screen is visible the letters/characters all change rapidly (and seemingly randomly) many times a second. It's so fast you cannot tell what the initial pattern was. The colours don't change though. Wait, I better qualify that. The character and background colours do change, but only every 15-20 mins or so. However, I noticed the Atari 400 did this if you had it in a BASIC loop displaying something for a long period of time, even when it was going properly. I'd assumed this was normal behaviour (is it?).

Tonight I'll try reconnecting the keyboard and see if I get a beep with start, and also try pressing return with the BASIC cartridge in.

Thanks for the replys. I've got to order the ANTIC chip from the US and given shipping costs from there to New Zealand, I want to make sure that's the problem before committing myself to that course of action.

Tez

Edited by tezza, Sun Aug 15, 2010 4:46 PM.


#7 tezza OFFLINE  

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Posted Sun Aug 15, 2010 4:44 PM

Here is the unit when it was working.

http://www.classic-c...n/atari-400.htm

Tez

#8 kurtm OFFLINE  

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Posted Sun Aug 15, 2010 6:03 PM

The pattern is not static. As soon as the screen is visible the letters/characters all change rapidly (and seemingly randomly) many times a second. It's so fast you cannot tell what the initial pattern was. The colours don't change though. Wait, I better qualify that. The character and background colours do change, but only every 15-20 mins or so. However, I noticed the Atari 400 did this if you had it in a BASIC loop displaying something for a long period of time, even when it was going properly. I'd assumed this was normal behaviour (is it?).

Tez


The color changes is normal once it's been left on a while. That's the "attract" mode that also cuts the brightness to save your screen. So, that actually tells us a bit, I think. Because that is an OS function. So the CPU is running "normally" enough for that to take effect. And the ram works well enough for the timers keeping track of that to work.

A question on the picture, it looks like the very bottom of the random characters is two sets, almost like some double-scan. Is that just an artifact of how you took the picture?

(And I can't wait to eventually read your write-up of this on your blog :D )

--Kurt

#9 tezza OFFLINE  

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Posted Sun Aug 15, 2010 6:49 PM

Hi Kurt,

(And I can't wait to eventually read your write-up of this on your blog :D )


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.

A question on the picture, it looks like the very bottom of the random characters is two sets, almost like some double-scan. Is that just an artifact of how you took the picture?


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

Edited by tezza, Sun Aug 15, 2010 6:51 PM.


#10 kurtm OFFLINE  

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Posted Sun Aug 15, 2010 7:50 PM

Hi Kurt,

(And I can't wait to eventually read your write-up of this on your blog :D )


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.


Check out the 800 some time. That has 3 RAM slots, an OS slot, and then the same CPU/support board.

A question on the picture, it looks like the very bottom of the random characters is two sets, almost like some double-scan. Is that just an artifact of how you took the picture?


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


Since that line is just an artifact, I don't imagine it's the CTIA. I couldn't imagine the display would be so normal otherwise. If you really suspect the CTIA, you should be able to substitute the GTIA from your 800xl, since I don't believe the GTIA changed much (and it was an upgrade for the 400/800 early on).

--Kurt

#11 Rybags OFFLINE  

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Posted Sun Aug 15, 2010 8:59 PM

It won't be GTIA - that looks fine.

I'd put 70% on it being Antic.

Do you know if it responds to entered commands in Basic? Try something like POKE 712,8 and see if it changes the border colour.

#12 tezza OFFLINE  

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Posted Mon Aug 16, 2010 4:00 AM

Right, I reassembled the machine...

Do you know if it responds to entered commands in Basic? Try something like POKE 712,8 and see if it changes the border colour.


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

#13 Rybags OFFLINE  

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Posted Mon Aug 16, 2010 4:12 AM

I'd say so... unless somehow the RAM select logic is being selectively screwed only when Antic references it.

#14 atariksi OFFLINE  

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Posted Mon Aug 16, 2010 4:50 AM

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.


Where are you located? Perhaps, may be cheaper to pick-up a 800XL if you need multiple chips.

#15 tezza OFFLINE  

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Posted Mon Aug 16, 2010 5:32 AM


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.


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

#16 tep392 OFFLINE  

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Posted Mon Aug 16, 2010 8:22 AM

ANTIC is on the CPU board, right? You've probably thought of this already, but have you tried cleaning the contacts on the board edge? I bought a 400 on Ebay that was DOA until I cleaned the board connectors.

#17 Rybags OFFLINE  

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Posted Mon Aug 16, 2010 8:32 AM

You should be able to use the Antic from an 800XL. The difference among the Antic revisions is the range of addresses they perform memory refresh on... although that <might> cause problems of it's own.

#18 Shawn Jefferson OFFLINE  

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Posted Mon Aug 16, 2010 12:47 PM

That screenshot doesn't seem to have enough lines (at least it seems to me.) The display list might be getting corrupted?

#19 atariksi OFFLINE  

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Posted Mon Aug 16, 2010 3:07 PM

You should be able to use the Antic from an 800XL. The difference among the Antic revisions is the range of addresses they perform memory refresh on... although that <might> cause problems of it's own.


Is there a specific part number on that ANTIC that's doesn't work on XL/XE or the other way?

I have been swapping chips between A5200<->A800<->A800XL all the time except CPU on A800 is different.

I have seen ANTICs that state CO12296-31, CO12296D-01 and I think CO12297-xx.
I have GTIAs which always have CO14805 in the beginning, 6502C CO14806, POKEY CO12294, and PIA 6520.

#20 tezza OFFLINE  

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Posted Mon Aug 16, 2010 4:12 PM

ANTIC is on the CPU board, right? You've probably thought of this already, but have you tried cleaning the contacts on the board edge? I bought a 400 on Ebay that was DOA until I cleaned the board connectors.


Yes that was the first thing I did, along with reseating all the chips.

That screenshot doesn't seem to have enough lines (at least it seems to me.) The display list might be getting corrupted?


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

#21 tezza OFFLINE  

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Posted Mon Aug 16, 2010 4:18 PM

You should be able to use the Antic from an 800XL. The difference among the Antic revisions is the range of addresses they perform memory refresh on... although that <might> cause problems of it's own.


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

#22 tezza OFFLINE  

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Posted Fri Aug 27, 2010 9:39 PM

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

#23 tezza OFFLINE  

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Posted Sun Sep 5, 2010 2:23 PM

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-c...creen-issue.htm

Tez

#24 Stephen OFFLINE  

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Posted Fri Nov 25, 2011 4:02 PM

I just fixed a 400 with a bad ANTIC. I remembered the details of this thread, and it did not sounds like a memory issue, so I swapped another 400's ANTIC in, and all was good.

#25 Philsan OFFLINE  

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Posted Fri Nov 25, 2011 4:16 PM

Congrats, you have a very good memory!




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