Glitchy 400

[mindlord]

I'm not an Atari expert at all, although I do have a fair amount of retro experience.

 

I have a 400 that works most of the time. It'll play a large number of cartridges, and some disks, but it seems some common video modes or rom instructions cause it to go off the rails. I've tried two different ram cards so I'm pretty sure ram isn't issue. Heat also doesn't seem to be a factor.

 

I've cleaned the CPU edge connector and re-seated any socketed chips I could find on the boards. I'm almost ashamed to admit that I tried the 1 inch drop test and things actually improved slightly.

 

Any better suggestions?

 

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[+DrVenkman]

3 minutes ago, mindlord said:

but it seems some common video modes or rom instructions cause it to go off the rails.

Some examples of programs or games that you can point to, along with pics of what you see on-screen would be helpful. 

[+Nezgar]

3 minutes ago, mindlord said:

some common video modes or rom instructions cause it to go off the rails

Can you describe the symptoms you're actually seeing for us in any further detail?

 

Maybe check the PSU is outputting 9VAC under load, and you're getting +5DC and +12DC internally. Both of those voltages can be measured from the SIO port, or +5V from the Joystick port.

 

http://www.mixinc.net/atari/pinouts/sio.htm

http://www.mixinc.net/atari/pinouts/joystick.htm

[griff3125]

It's a pretty easy machine to disassemble, I'll bet if you look at the power supply board on the printed circuit side, you'll find some cold solder joints, it's easy to clean and re-solder them.

Possibly on the main board as well, maybe around the SIO port.

If it's behaving erratically with the old drop test, this seems like a likely culprit.

 

[Rybags]

Probably worth the slight lift + reseat of socketed ICs.  Just one intermittent contact somewhere could be enough to cause the problem.

[BillC]

6 hours ago, mindlord said:

but it seems some common video modes or rom instructions cause it to go off the rails.

Maybe this 400 has a CTIA chip, and the software that is giving problems requires GTIA.

 

I upgraded my early 400 to GTIA and 64K Mosaic RAM board in late 1982 in order to be able to run more software.

[Rybags]

We need clarification of "off the rails".  The assumption there is crash or lockup.

CTIA instead of GTIA should never directly cause either of those.

[+DrVenkman]

2 hours ago, Rybags said:

We need clarification of "off the rails".  The assumption there is crash or lockup.

CTIA instead of GTIA should never directly cause either of those.

Exactly. There’s also plenty of software (carts included) that either require more than 16K or are not compatible with OS A/B.

 

As I said above, we really need details of what software is going “off the rails” and some photos at least of the screen to help.

[mindlord]

Thanks for the input so far everyone. I'll get some tests together and take some photos/video of the behaviors.

By "off the rails" the result could be any thing:
blue/black screen
static garbage
random fluctuating garbage
flickering garbage
partially rendered text with graphical garbage intermingled
Games that play, but the sprites and/or background are mangled


Are there any specific titles that will provide clues that I should try?

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[Rybags]

Not really.  Better to just post the machine specs (mainly, amount of Ram installed), which games fail, and I suppose if it's GTIA or not.

 

To find if it's GTIA - insert Basic cart and start.  Then POKE 623,64

If the screen goes black and text becomes near unreadable then it is GTIA.

[mindlord]

Pictures are in order of my comments: Looks like she's gtia, and 32k.
All the grim reaper mag disks do this.
Berserk.com and berserk.exe give a blue screen with garbage at the bottom.
Sysinfo.atr flickers garbage like this, but does things when I press keys.

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[Sugarland]

If after checking voltages and for cold solder joints everything else seems okay then consider swapping the primary chips into a known-working machine to see if the symptom moves with the chips, starting with GTIA, then CPU then Antic.  Before that a full physical examination is prudent. Are all the sockets making proper contact with the chips' legs?

Edited May 6, 2020 by Sugarland

[+Nezgar]

So you say it has 32K RAM.... Some of those programs might only work with a full 48K. if they assume 48K and load code segments into that missing upper 16K of the memory map, they'll crash, and if the screen is set to use that region, you'll just see random data...

 

Berzerk loading garbage at the bottom of the screen is a sign that the loader starts loading data just below MEMTOP (into screen ram) that's lower than on a 48K machine, and assuming it's free. The rest loads into nonexistant RAM, and will crash when it tries to run at the end of the load. Most 'cracked' games that originally came on cardridge load the code into that uppermost 32-48K area to replicate where it was originally compiled to run from.

 

With 32K, you should find that most 8K & 16K cartridge games will work just fine, as they will map their ROM into that 16K area where you have no RAM anyway, and use the lower RAM area for anything dynamic... BASIC programs that don't use every ounce of memory should also mostly work OK, you'd only be missing 8K of usable RAM with BASIC inserted.

 

So I think your first order of business for maximum compatibility is to get the machine upgraded to 48K. The simplest method is the 48/52K RAM Card from tf_hh, and optionally his SuperColorCPUCard to get a nice S-Video output.

[mindlord]

Thanks for the analysis. Makes sense. I can verify that by specifically targeting applications that should work and applications that shouldn't.

The ram card looks intriguing too. My retro stuff coffers are empty at the moment, but it's moved pretty high on the list.

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[mindlord]

Once again, thanks to everyone for the advice and analysis. I've done some pretty extensive testing. Voltages and solder joints all check out okay. I've lifted and re-seated the chips on the main board and CPU card. Nothing really changed, so I set out to try some variety of xex, atr, and cas files and pretty much nailed it down to just having 32K of RAM. I'll have to put in an order for that upgrade once I get some scratch. At least now I'm 99% certain there's nothing wrong with the board or CPU.