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Atari 400 Green/Yellow Screen


vipah
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Hi,

 

I've got an Atari 400 (NTSC) which I purchased as non-working.

 

The symptom is the fairly common Green/Yellow screen on startup.

 

Things i've tried:

 

Swapped CPU board with known working board

Swapped RAM card with known working board

Purchased a 48k/ROM board from TF_HH on these forums to replace the RAM and ROMS (incase the roms are the issue)

Changed out all Capacitors on the power board along with the 10UF cap on the motherboard.

12V and 5V (A&B) all tested as good.

Oscillator on the board reads correctly 3.579MHZ

Tested with a Donkey Kong cart

Cleaned all connectors, re-seated all chips

 

None of these have changed the behaviour of the issue in any way.

 

Here is one wierd thing:

 

Tested clocks around the board as per Figure 5-1 in the Service Manual .. All clocks are right on the money EXCEPT pin 37 on the 6502 .. it reads 1.64MHZ .. both CPU cards read the same on this pin. I'm not sure if this is an issue, seems odd though .. the Service Manual says that it should read 1.140MHZ.

 

Anybody with any ideas? I've run out of em ;)

 

Thanks!

Edited by vipah
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I've got an Atari 400 (NTSC) which I purchased as non-working.

The symptom is the fairly common Green/Yellow screen on startup.

Purchased a 48k/ROM board from TF_HH on these forums to replace the RAM and ROMS (incase the roms are the issue)

 

Here is one wierd thing:

Tested clocks around the board as per Figure 5-1 in the Service Manual .. All clocks are right on the money EXCEPT pin 37 on the 6502 .. it reads 1.64MHZ .. both CPU cards read the same on this pin. I'm not sure if this is an issue, seems odd though .. the Service Manual says that it should read 1.140MHZ.

 

Just to be sure... did you remove the three ROM chips from the main PCB when you use my 48/52 KB RAM cart? I got several defect systems with a failure at the MATH ROM. This will result in short circuits, in that case without removing these ROMs the system won´t work.

 

Of course you should change the POKEY and PIA with also tested and working ones to be sure.

 

The clock speed read at pin 37 is not accurate when using a 6502B CPU (most all NTSC CPU cards found in the 400 and 800 series). Pin 37 is the main clock input for the CPU (PHI0) and to enable DMA access by ANTIC, this clock is paused when ANTIC wants to access the memory (level will be hold at high).

 

When power is okay, all major chips are changed, then at least one of the three 74xx chips can be defect. Also examine the mainboard carefully, maybe someone has scratch or cut a trace.

 

Good luck!

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Hey!

 

Well this morning I decided to pull the ROM chips as a last attempt (before seeing this reply) and lo and behold? Blue memo pad screen. It looks like it was the CO14599A (R3279-11) chip all along. Appreciate the response. Your card is working perfectly and helped me resurrect this system.

 

Thanks again!!

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  • 4 years later...
On 10/9/2017 at 5:02 PM, vipah said:

Hey!

 

Well this morning I decided to pull the ROM chips as a last attempt (before seeing this reply) and lo and behold? Blue memo pad screen. It looks like it was the CO14599A (R3279-11) chip all along. Appreciate the response. Your card is working perfectly and helped me resurrect this system.

 

Thanks again!!

I am having this problem with a stock 400 when you pulled the C014599A chip did you just leave it out or did you have a replacement?  This has been very frustrating for me.

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