Atari 800 video problem

[jimerson]

Hi, I got an older Atari 800 beige s/n 946-2193 a couple weeks ago. It came with the ROM and 16K module. Also Centipede, Star Raiders and Basic cartridges. The cartridges all seem to work and I can play the games but there is always "garbage" scrolling down behind the text characters. I believe this has a GTIA because "GRAPHICS 9" (and POKE 623,64) command shows a black screen. Also because I saw it when I re-seated all the chips I would appreciate any help troubleshooting this, I don't really know where to start.

 

To make the photo below, I used GRAPHICS 2:PRINT#6;"ABCDEFG..." to enlarge the text to get a much better look at the problem. I hope the video works.

 

Thanks, Jim

[Sugarland]

Do you have another known good A8? Try swapping the antic and GTIA from the bad machine to the good one.

[xrbrevin]

ANTIC generates the characters for the GTIA to display on the screen so swap them out if you can

[ClausB]

Wow, crazy! Some parts of the characters are not affected: top of A, bottom of V and Y, middle of I and Z, and all of the left bracket. Only where there are pixels in the left side of the characters does the noise show up.

 

Also the downward scrolling is mostly on the left side of the screen. Elsewhere the noise looks more random. The left side is when ANTIC does refresh cycles, so there might be data timing issues. Timing is handled by the smaller chips on the CPU board and the ROM board, so check them if swapping ANTIC chips doesn't help.

 

It wouldn't hurt to reseat all the chips again and look for bent pins and pins with excess corrosion. I have a few early Ataris and they always need reseating. Their boards don't have gold edge contacts and they corrode more quickly. Try running a pencil eraser across the darker tin edge contacts until they start to shine brighter.

[jimerson]

Thanks for all the advice. When I reseated the chips I just pried them up a bit then pushed them back down. I will remove them and check for damage and corrosion. If that doesn’t work I will order an Antic and swap it (I don’t have any spare parts). Will report back in a few days.

[videofx]

also if you have 48k RAM pull 2 of the 16k modules and try it with just 16K and keep swapping 16k RAM modules to see if it finds the problem with RAM

[Rybags]

That'd be Antic for sure.  There's some sort of screwed up behaviour in a counter or something.  GTIA has next to no "buffering" so wouldn't generate such behaviour especially between adjacent scanlines.

 

Can you try some other stuff - like maybe GR. 7 : C. 1 : PL. 0,0 : DR. 100,100

 

[jimerson]

2 hours ago, Rybags said:

Can you try some other stuff - like maybe GR. 7 : C. 1 : PL. 0,0 : DR. 100,100

 

Ack, not anymore! I decided to pull the chips from the cpu board and look at the legs and sockets. I noticed some brown crud on pin 8 of the 6502B, and that leg broke off when I pulled the chip. Looks like pin 8 is Vcc so it was probably going to fail soon anyway. The board is very clean and all solder very shiny, looks almost brand new. I should have the replacement ANTIC and 6502B before Friday.

[Rybags]

Just solder a donor leg onto the CPU.  It should work fine.

 

In my earlier days of bad desoldering I butchered an Antic to the point where about 5 legs broke off, I soldered on some pulled from an old network card's DIP chip and it works fine.

 

[xrbrevin]

have you ever had to attach a donor leg when the break is very close to the chip body?

i have a DRAM chip with a broken leg that has next to nothing exposed. does anyone know if can i 'dremel' the body to expose more contact?

[TGB1718]

Should be ok to expose a tiny bit of whats left, obviosly very carefully

[+tf_hh]

On 1/5/2020 at 3:11 PM, xrbrevin said:

have you ever had to attach a donor leg when the break is very close to the chip body?

i have a DRAM chip with a broken leg that has next to nothing exposed. does anyone know if can i 'dremel' the body to expose more contact?

 

Take a machine-head precision socket, plug the defect chip in, fill the intact legs with solder, so that nothing can move even for a nano-meter 🙂 - Then take a little piece hard wire, plug in into the unused hole where the broken leg is. let flow some solder into the hole of the socket. Place this "new leg" against to piece from the former leg of the chip and solder again. This way I salvaged some ANTIC, GTIA and other from disposal.

 

[jimerson]

Thanks for all the excellent assistance, it works now. On the CPU I soldered a donor leg from a bad C=64 PLA. Replaced ANTIC chip and video is now good!

[xrbrevin]

great success!

[mimo]

11 hours ago, jimerson said:

Thanks for all the excellent assistance, it works now. On the CPU I soldered a donor leg from a bad C=64 PLA. Replaced ANTIC chip and video is now good!

Nice to see a C64 being of some use 😁

[Sugarland]

19 hours ago, jimerson said:

Thanks for all the excellent assistance, it works now. On the CPU I soldered a donor leg from a bad C=64 PLA. Replaced ANTIC chip and video is now good!

Awesome. Great to hear!! That donor leg may sometimes interfere with the boot process. If it refuses to boot try typing load "D:*.*",8,1 into notepad.