Atari 400 troubleshooting - black screen

[ziggaboogi]

Hello, 

 

I'm hoping someone may have an idea on what I can check next on an Atari 400 that was recently gifted to me from a friend. 

 

Here is the current rundown after some testing 

 

-booting up the first time it was successfully able to get to memopad with no catridge inserted and was able to type a few characters, but then it blanked out and now only shows a black screen when powered on 

-I have tried reseating every single chip, and tried cleaning every leg on the chips and the daughter board connectors 

-I've verified that the voltages coming from the powerboard are fine and are nice and steady 

 

Currently the only things I have for this are a cartridge of Defender and a 410 with no tapes. I'm hoping that someone could help me figure out what I should check next. 

 

Edited August 20, 2020 by ziggaboogi

[Paul Westphal]

Replace the CPU card which is hidden inside the case.

Best Electronics and B & C Computervisions ( myatari on eBay ) have them. Look for a 48k ram board , The Brewing Academy might be selling them. Little known fact : A 400 upgraded to 48k will run the UNO cart, a 48k 800 will not.

Eightbitfix.com

[Sugarland]

User @tfhh sells a great 48k RAM card exclusively for the 400.  As far as your black screen goes Paul knows his stuff afaik but replacing the whole CPU card may not be necessary if the issue can be narrowed down to the specific cause. Perhaps it's a bad GTIA. Do you have a spare machine you can move the 400's GTIA to the working machine? Have you tried searching "black screen" in this forum?

[xrbrevin]

an OS ROM fault can be diagnosed by booting a star raiders cart - if the game works then the OS ROM is moody

[ziggaboogi]

Unfortunately I don't have another 400 to swap parts for. I did see that best electronic was selling replacement daughter boards but just wanted some way to confirm if either the CPU board or the ram board is faulty before ordering replacements. I know they also sell specific chips for the 400 so narrowing it down even more would be ideal, although more difficult.   

 

 

I recall that the 5200 should have a compatible pokey chip, so are there any compatible gtia chips from the 5200 or 7800? As I do have working versions of those consoles.

 

 It also sounds like finding a copy of star raiders would also be a way to narrow down the issue a little.

[Sugarland]

The 5200 has a GTIA and pokey, yes. A Star Raiders can be acquired for cheap and is a great idea. If the OS ROM is the cause then swapping the CPU card would not fix the issue.

[ziggaboogi]

So I swapped around both the pokey and the gtia between the two and found no issues when booting up the 5200. But the symptoms didn't change on the 400.

 

I've put in an order for a copy of star raiders so I can update in a few days once it shows up.

[ziggaboogi]

So update, I received a copy of Star Raiders but it's still only showing a black screen. As I've rearranged the ram chips, does that mean I should just bite the bullet and order a new CPU board from Best Electronics? 

 

[Sugarland]

First have you tried swapping GTIA and ANTIC to your 5200 to see if they work there?

[+DrVenkman]

If your 400 is late production, it's also likely to have a SALLY cpu versus the standard 6502B from earlier models. If so, you can try the SALLY, ANTIC and GTIA in your 5200 to verify that the chips are good.

[ziggaboogi]

So final update. I was able to get a new CPU and ram card. Turns out only the ram card was bad but now I finally have a working 400! Thanks again for all the help.

[eegad]

Same thing happened to me.  Several years ago I hooked up my 400 for the first time in 30 years.  Blank screen. Took apart and reseated chips and wiggled connections.  Nothing.  Put it aside. Decided to fiddle with it again during covid lockdown last winter.  Replaced RAM card with one ordered off ebay for about $15, swapped it out,  and it booted up fine.  ...not that I have any real USE for a 400, but it's  kind of nice to have my first computer from 40 years ago functional again.