Help: Atari 400 power up troubleshooting as per field service manual

[Arcanis-Will]

I am attempting to troubleshoot a dead Atari 400 using the flowchart on page 5-4 of the Atari 400/800 Field Service Manual. It's the sequence called "400 Power-Up Catastrophic Failures and Basic Operations Black / Grey Screen".

 

My symptom is exactly that - a blank / black screen on powerup.

 

I have followed the sequence and found the following:

-- Is there -5V... YES

-- Is there +12V... YES

-- Check the CPU Board for Clock Circuit - YES *

-- All testpoints listed in table 5-1 had signal patterns at the indicated frequencies - except for the first 2 entries in the table. I could not figure out where pins 21 and 22 of J101 were. I could not identify J101. Therefore skipped those two measurements. If anyone knows where those testpoints are please let me know.

-- Chip Reset Voltages:

Is there +5V on A302 pin 36 - YES

and on A303 pin 30 - NO - it's not 5V - it is a fluctuating value around 1.5 to 2V. Seeing this apparent failure, I retested it using another CPU card that was tested good, and have the same result - not +5V.

 

EDIT: Looking at the actual CPU board schematic, it appears the flowchart is wrong - the reset line for A303 is pin 40, not pin 30. Pin 30 is a data line which would explain the fluctuating values. It looks like I am unstuck - I will have to go back and confirm on the right pin but I already packed up all my stuff for the night.

 

The next step after that would be to replace the GTIA but I was told this board was tested and had a good GTIA so I will go on to the next step which is apparently to replace a bunch of chips on the motherboard (chips that I don't have). Will have to resume this tomorrow.

Also, just to confirm the reset back at the source, I probed the output of the reset circuit on the motherboard and it is outputting 5V (A111 pin 15).

Edited March 6, 2014 by Arcanis-Will

[Rybags]

I'd be concentrating on the CPU, Antic, OS Rom and the Ram.

 

If you have a game cart like Asteroids or Star Raiders, give it a go.

I suppose already you've tried for key presses and get no clicks.

 

If you have the means to burn/flash a diagnostic cartridge then it can be helpful also.

 

CPU boards - there are 2 versions at least. Earlier machines used mainstream 6502, later ones have the 6502C "Sally" version which requires less supporting logic chips.

Edited March 6, 2014 by Rybags

[Arcanis-Will]

I'd be concentrating on the CPU, Antic, OS Rom and the Ram.

 

If you have a game cart like Asteroids or Star Raiders, give it a go.

I suppose already you've tried for key presses and get no clicks.

 

If you have the means to burn/flash a diagnostic cartridge then it can be helpful also.

 

CPU boards - there are 2 versions at least. Earlier machines used mainstream 6502, later ones have the 6502C "Sally" version which requires less supporting logic chips.

 

Yes, no key click sounds either. I've tried 2 different CPU cards and 4 RAM cards with no change so I am looking more towards motherboard and ROMs. I do not have any means to burn a cartridge for diagnostics. However, I just realized that even though I did try the cartridge slot (which did not work), it was actually done while using the MS Basic hidden inside Star Raiders which is not at all the same. I will go back and reverify with a real Star Raiders cart, which I have recently acquired.

 

 

Thanks.

[Rybags]

I'd suspect the OS Roms then.

 

If you have a steady screen but otherwise no response then Antic + GTIA are probably OK.

[Arcanis-Will]

Solved!

 

After some 2 hours more troubleshooting I found that I had a bad Pokey (A101).

 

First I tried the Star Raiders cart - no improvement.

 

Then I remembered I had an extra OS Module from the pile of stuff I bought, so I swapped all three ROM from the module to the 400 motherboard, one at a time - still no improvement.

 

Then I decided to pull the only other large component on the board and try running the computer without it - so I pulled the Pokey off and voila! I got a working display just like that. So the computer will boot to Memo Pad (or Star Raiders) without Pokey.

 

After I found that out, I replaced the dead Pokey with another pulled off my dead 800XL board, and it powers up! Finally! All keys on the keyboard work too. What a relief!

 

I just finished putting it all back together.

 

As a side note, I took the time to lube up the 400's door mechanisms. Earlier this week I had ordered some Labelle 106 plastic-safe grease. It came in today and so I used it to grease up the 400's door mechanism, and the release latch where it rubs on the shield, and also the power interlock rod which had been sticking in the shield. The door and interlock work amazingly well now! It's really cool.