Atari 800 Repair

[venom4728a]

I recently picked up another 800, the initial issue was no color or sound.

 

I found a Bad GTIA chip and a bad transistor array IC CA3086.

 

Now I have color and sound when I turn on the computer with a game cartridge installed.

 

With no Cartridge installed I expected to see the memo pad but I only see a blue screen with white cursor. Cannot type.

 

Also unknown if related, I have tried 3 BASIC carts and none of the three would run basic. Every game cart I tried did work, only other non game cartridge I tried was Atari Writer and it appeared to be working.

 

I have swapped out personality board, all memory boards, CPU board, and most of the chips on main board under keyboard area, but not yet under memory slots.

 

So did all 800's have the memo Pad? What would cause that and possibly BASIC to not load?

 

 

Thank You

 

Best Regards

Robert

[Kyle22]

You should have a memo pad. Some game carts are seen by the computer as "diagnostic carts", and they take control of the boot process earlier than a BASIC cart does. What games worked, and do you have any games that don't work?

 

Will it attempt to boot a disk?

[venom4728a]

I used Star Raiders, Asteroids, River Raid, Pacman, Breakout, Dig Dug, Frogger and Star Trek all of the games worked. Only Carts not working was 3 basic carts.

 

Robert

[venom4728a]

I am wondering if there is a relay/transistor IC that switches between cart present and no cart present? If it has failed and always thinks a cartridge is present even when not and is preventing Memopad from starting. I swapped out the 4051s on main under keyboard area, but I spotted a few more small chips on main board, under the plastic OS/Memory slots I have not gotten around to swapping out yet. I will try to change those out this weekend and plan to try to clean Cartridge slot contacts to see if that helps.

 

I am hoping the 3 BASIC cartridges not working is just a coincidence and not a symptom of something else.

 

Robert

[+Stephen]

Wondering if it a RAM issue. I assume your 800 has 3 16k modules in it. Try running it with a single one (and try all 3, but only using one at a time).

[Kyle22]

If the RAM checks out OK, look at (on the motherboard) Z101, a 74LS42 decoder, and Z102, a 74LS32 Quad OR gate. Both involved with the cartridge control lines.

[venom4728a]

I did swap all 3 ram modules with known good ones, as well as OS and CPU Cards. Z101/Z102 are probably the 2 or 3 chips I have not gotten to yet under the plastic os/Memory housing. Figures it would be the last 2 chips I can change because of how far I have to disassemble to reach them.

 

Thank You for you help!!!

 

Robert

Edited April 8, 2016 by venom4728a

[ClausB]

I had an 800 with the same symptom. I did not have the speaker plugged in so I did not hear it beep as a prompt to boot from cassette. It just sat there with blue screen and white cursor.

 

It turned out to be a bad CTIA with a stuck START switch bit. Replacing it with a GTIA fixed the problem.

 

[venom4728a]

I swapped out the 74LS32 but do not have a 74LS42 to try so I ordered a few.

 

I did have a bad GTIA, changed out to get the color back.

Edited April 11, 2016 by venom4728a

[+tf_hh]

Hi,

 

if you can´t wait to test... maybe you´ve one of the uncounted Happy 1050 clones... they have often a 74LS42 onboard

 

Jurgen

Edited April 11, 2016 by tf_hh