Repairing an Atari 600 XL upgraded to 64 KB RAM

[masteries]

 

Greetings mates,

 

Currently I am trying to fix this rare behaviour in an Atari 600 XL (NTSC),

upgraded to 64 KB RAM,

 

These are not my screenshots, but the problem is the same;

the machine crashes after few seconds after being power up,

showing corruption at screen.

 

Initially it appears a RAM problem, but I exchanged the two chips,

with another working ones, same result.

 

Look at the example screens:

 

<This is the RAM upgrade method>

 

 

< Games are crashed with random garbage pixels and colour changes, in few seconds. 

You are lucky if you manage to load a game entirely >

 

 

< This usually happens at self-test screen after 10 - 50 seconds >

 

 

 

Researching over the web, I found this topic, talking about Freddie chip; but

this 600 Xl doesn´t have Freddie chip at all:

 

 

 

 

The motherboard is exactly as this, without the RF video output, replaced with the composite video mod

(as curiosity, this mod looks awesome in screen, better than my 65XE):

 

 

 

Are there any troubleshooting walkthrough in order to fix and check step by step for this machine model?

 

Thanks in advance,

 

 

[_The Doctor__]

clean and reseat all chips, use a memory tester after the machine has been running a while, make sure option is held down during power up when loading most games etc.

 

if no drive/850 etc or cartridge is attached it should go to a self test. if self test memory test or independent memory tester shows something bad, it's time to go a thread that has covered the failure it found.

if you keep getting basic your option key path is borked... then it's time to jump to a thread that covers fixing that

[Beeblebrox]

@masteries  Definitely do what Doc suggests and reseat all chips, etc. Ram is a likely contender.

 

Also this morning I was working on an 800 which was throwing up similar garbage on screen when I had my Star Raider cart plugged in whilst doing diagnositcs. (If you have a Star raiders cart pop it in and see what you get on screen after a while incidentally).

 

I ended up replacing the GTIA borrowed from another A8 and problem when away.

 

Not saying the same is occuring with yours as could be other issues, but if you are able to take the GTIA from this, (and also the Antic - might as well), pop em in another A8 with sockets, and see if they work, it'll elimninate them as causes if nothing else.

 

 

[Vyvyan B.]

Might want to check your power supply too, or test it out with another one just in case there's something going on there.

[_The Doctor__]

again checking the supply at initial power on under load, it may read fine at first, or it may read crap after heating up... like all things, checking it twice is advised.

If unsure report every detail and if need be post pics as you go.

Edited January 20, 2023 by _The Doctor__

[Beeblebrox]

@masteries  Definitely upload a high res image of your 600XL's pcb. It's been mentioned recently but someone might see something that will save you a lot of time and effort. 

[+Stephen]

Does the 600XL use the same delay line as in other machines?  They are seemingly becoming more of a common failure point too.

[Paul Westphal]

The ram mod looks good to me.. I have seen a lot of bad CPU's in the 600xl's, so you may want to check it.

 

Mine is the first Mod, and the second I obtained from Nir Dary

[Paul Westphal]

While we are at it, here is Nir's sharpness mod :

[+scorpio_ny]

1 hour ago, Stephen said:

Does the 600XL use the same delay line as in other machines?  They are seemingly becoming more of a common failure point too.

Yes it does. I believe it is U13 on the 600XL

[+scorpio_ny]

By the way, which power supply is being used? There is a model PSU that can lead to possible damage to the computer. The infamous “ingot” power supply. If you are using it, I suggest that you stop using it and get a replacement asap. I cannot search for the image at the moment, but someone else here can.

[_The Doctor__]

looks like spain/eu/uk ingot is not so big and issue there, the epoxy one they use doesn't fail nearly as often or badly as the USA 60hz boat anchor making and boat anchoring block that we have here.

[Geister]

I would check carefully the two spots where the wires go through the VIAs to ensure that they are actually soldered...especially if the problem happens after bumping the computer.  In my 600XL one of the VIAs was not fully soldered and the computer glitched out unexpectedly.

[masteries]

Thanks a lot mates,

 

I´ll post here the answers and actions performed, asked in Exxos forum:

 

Some of the actions are related to topics covered in your previous posts,

while the advice to exchange CPU will be done tomorrow; I have another 65XE (PAL) and 130 XE (PAL) good working computers

 

 

 

OK, so first off, the 64K upgrade looks correctly wired, so that's good, but I'd also use a multimeter to ensure there is continuity from point to point just to be 100% sure all the soldering is good.

There are continuity between the airwires and the corresponding points,

Next step, is the PSU good? Is it known good and working with another machine, if not then you need to verify with a known good machine and be sure the PSU is good and 5v is getting to where it should on the board. 74LS ICs tend to have a standard layout when it comes to power, with the highest numbered pin usually being 5v. So something like a 74LS04 which has 14 pins, pin 14 is VCC and should be 5v, for 16 pin ICs it would be pin 16 etc. You can pick off GND anywhere, but 74LS ICs usually have the last pin on the left hand side as GND when viewing from the top and pin 1 at the top left.

Yes, I have two modern replacement for Atari 8 bits PSU, these are 5 volts DC output, that provides 5.23 Volts and 5.26 volts;
observed in the osciloscope, the voltage is very stable; if you zoomed it, the DC signal is not completely straight, there is a random
white noise (frequency is not always the same) of less than 0.1 volts amplitude, aorund 0.08 and 0.05 volts amplitude.

I have replaced all electrolitic capacitors with the same value as originals,

What happens if you boot with option held down? Do you get into the self test menu, and can you run the RAM test? If so what are the results as red squares mean bad RAM.

The self test is opened, I can select Memory of Audio video and others...
audio video performs ok, as well as memory test... but after 20 or 30 minutes of use,
the machine started to perform very bad, very unstable, such the screenshots.
If you powered it off, and restarted again, the machien remains unstable.

You need to power off, and wait at least 5 to 10 minutes,
in order to operate the machine another dozens of minutes
with no apparent errors.

Curiosly, when the machine will be turned unstable, there
are some sync problems in the video output. These sync problems
can appears (few ones) when the machine is turned unstable.


Can I ask, do you have a Star Raiders cartridge? If you do, then I'd try plugging in and seeing if it runs at all, as it cuts out the O/S IIRC. My own 600XL (It had bad RAM, a stuck bit) just gave a reddy brown screen, but would run Star Raiders. It might help with diagnosing if the Antic/GTIA/CPU are running ok if it will actually run.

I don´t have any cartridge, only a Fujinet device.

When Fujinet is connected, the machine is being turned unstable
quicker than without it, you can use the Fujinet, load and play games during 5 minutes
prior to turn unstable.


If it won't run with Star Raiders, do you have a spare machine you could test the GTIA/ANTIC/CPU in? It could even be an 800XL or XE, it doesn't have to be a 600XL as they share the ICs.

I have a 65XE and a 130XE, but these ones are PAL ones instead NTSC such the 600 XL;
is this important for IC exchange?

If you try the above and get nothing and have a scope I'd suggest check pin 5 of the RAM for the RAS signal and pin 14 for CAS. Also check all the address lines to check they are are not stuck at an odd value. On mine one was stuck at at a level that meant it wasn't high, nor was it low and never changed. I pulled out one chip and saw it start to behave normally and then realised it was a single RAM IC that was bad.

I have 4 64x4 bits RAM chips to test; the machine uses two of them;
as well these four chips were tested in a ZX Spectrum +2A, loading
128 K based games such Batman the movie and La Abadía del Crimen
and played them without problems.

Another problem I believe happens is that the delay line chip, U13 fails. If you can test in another machine you can verify one way or the other. I expect you could scope the delay line and compare to a working machine, but you'd need to look at the schematics to figure out the inputs/outputs in order to measure effectively.

I need to check this, due to I have no experience or knowledge regarding this,

Lastly there is an MMU IC at U2 which could fail, another one to test in another machine if you can. I know I've said test in another machine a lot, but it really is helpful to have a 2nd known good machine for testing. Doing it without really does make life a lot harder.

I have the european 65XE and 130 XE ones

One last thing. Have you tried spraying the sockets with DeOxit or electrolube at all? The factory sockets are single wipe and IMHO terrible, and I replaced all of my 600XL's sockets with new double wipe types to ensure good contact with the ICs. A bit drastic I know but the original sockets aren't great.

I used a product called Lubri-Limp that is manufactured and intended as electronics contact cleaner.
Clean process performed when all ICs were extracted and reseated,
 

[+tf_hh]

I would suggest to change the CPU. I don´t have a real good explanation for this behaviour, but specially when talking about the 600XL I have a signficantly higher defect rate of Signetics CPUs with 83xx datecode in the 600XL compared to all other systems where also Signetics CPUs were built in. Maybe it´s the closer architecture of the 600XL with less space and air ventilation. Maybe it´s only bad luck because mostly the Signetics CPUs are used in the 600XL, later productions of 800XL and above uses Rockwell, NCR and others, too.

 

Typically during my repairs this behaviour you mentioned looks first like a RAM problem, if RAM isn´t the culprit, change the CPU. Use any other Sally (6502C) from any XL/XE (or some CPU boards in the 400/800 have also the Sally CPU, mostly PALmachines and later NTSC ones)

[flashjazzcat]

The advice to reseat chips is also sound; I mention this since I was reminded yesterday that problems can exist owing to the fact 600XLs are entirely socketed. Someone sent me a 600XL which booted to screen corruption and all that was wrong with it was that pin 20 of ANTIC was bent and hanging over the edge of the socket.

 

Edited January 22, 2023 by flashjazzcat
Can't type!