Atari 800 XL black screen

[alortegac]

I just got it from a garage sale. It is great physical condition but only shows a black screen.

Tested power supply working ok.

Check the board visually and it looks ok.

Tested 5v on ics.

Any suggestions? The service manual suggest Q8, Q9, C109 and crystal.

I suspect ROM chips. Any way to test them.

 

Any leads will be greatly appreciated. I want to bring it back to life!

 

Thanks all!

[+Nezgar]

Are your chips socketed? If so, pull the OS ROM out and see if you get a red/brownish screen. And remove/reinsert each of the other socketed IC's to help with any potential contact issues.

 

Otherwise, a 2nd known working machine is useful for swapping IC's into to test each in 1 at a time.....

 

Could also be RAM.

 

the OS ROM is pin compatible with a 27C128 EPROM. If you have a programmer, you can try dumping it and comparing it with known OS revisions. It should usually be one of 2 for XL's.

[alortegac]

 

All IC's are socketed, even the smaller ones. The board looks pretty clean.

I do not have another XL machine, do not have replacement OEM IC's, RAMS, etc.. I sprayed it with electronic cleaner to help with any contact issues. I do not want to begin removing ICs before I have a plan on what to do next. I do not want to create more problems, they feel pretty well attached. Where can I find directions to check the correct voltages on each IC?

 

My apologies,. I said ROM in my original posting, I meant RAM. Is there any way to test them with multimeter?

 

I removed the OS, I do get a greenish/brownish/red screen. Color on display can be adjusted with the variable resistor. is this a good sign? The original OS p/n is CO14806-03, and I tried another 14806-12. I also tried a CO14806C. These tests yielded no change, only black screen. I also replaced the 4050B just in case, but no change.

 

I do not want it to go to recycle bin, looks like a nice machine to be preserved. Any help will be greatly appreciated.

 

Al

Edited August 23, 2018 by alortegac

[+Nezgar]

OK the fact you get the Red screen with no OS would suggest the main IC's (GTIA, ANTIC, CPU, POKEY) are probably OK, and RAM is where to concentrate now... I'm not sure you can test much with a multimeter for RAM, maybe with a logic probe. Others would have to pipe in about that.

 

Does it have Mt RAM chips? Those are notoriously bad. When the system has been on for a few minutes, feel if any of the 8 get hotter than others, if so that's likely your bad one. One trick mentioned on these forums is to lightly spray isopropyl alcohol on the chips while on and look for ones that evaporate faster than others.

[alortegac]

Thank you so much. Fortunately, all IC's are already socketed on this board. Including RAM. It will be a very careful swapping exercise as they seem to be pretty tight.

 

Would you let me know the IC' part number for the RAM? You mentioned 4164. I just want to be sure I buy the correct ones.

 

The ones in the unit are MOSTEK 8325, MK4564N-12. ATARI P/N CO60612.

 

Are they the same as 4164's?

 

Picture attached. thanks a lot!!

Al

[+Nezgar]

Thanks for the pic, yours are MOSTEK MK4564N-12. Supposedly 4164-15, 2164-15, 4564-15, 6665-15, or 3764-15 are all interchangeable, they're all 64Kx1Bit.

 

So... some example sources online:

From China:
https://www.ebay.com/itm/251666089003- $5.95USD for 8 from China
https://www.ebay.com/itm/392103363888- US $8.75 for 10 from China

From US, probably faster assuming that's where you are:
https://www.ebay.com/itm/382531975129- US $2.25 each, 16 available from Texas
https://www.ebay.com/itm/253811317142- US $3.85 each, from Florida

[+Nezgar]

You can also try rotating all your existing RAM chips into the next slots while you're reseating them, repeat 7 (so every chip is rotated to the next 7 positions) just in case you get any different powerup behaviour.

[+Nezgar]

Just checking against this previous post: http://atariage.com/forums/topic/281328-faulty-ram-on-800xl-what-ram-to-buy/?p=4081775

Check which ANTIC chip variant you have, C012296 or C021697.

If I have this right:
C012296 (older) has a 7 bit refresh counter and works only with 64Kx1 DRAM with 128 cycle refresh. (2ms)
C021697 (newer) has an 8 bit refresh counter and works with both 64Kx1 DRAM with 128 & 256 cycle refresh. (4ms)

so if you have ANTIC C012296, confirm part # with seller, and double check the datasheet before ordering that it's 128 cycle and not 256 cycle refresh.

[alortegac]

Thank you so much! I really appreciate it.

I have this version.

[MrMartian]

I just want to say that I have an 800XL here showing the same symptoms.. Changing RAM made no difference, changing MMU did - for about 10 minutes and now it's back to the same. I've swapped out all ICs, they work in another board but not in this one. I've replaced some sockets and still nothing..

 

May be similar problem, I'm not sure where else to check..

[_The Doctor__]

Sometimes you need to find out what killed the chip before running with the swap... check voltage and look for AC hum/ripple the chips can take abuse but I always meter everything out after something is fixed to make sure about such things.

[shoestring]

Check the two RAM muxes at u26 & u27 ?

 

Those will also give you a black screen if they're toast.

[alortegac]

Hi, so far replaced GTIA, All 8 Ram and 14806. No change. I am waiting to receive u26 and u27. But I am thinking this machine is getting closer to the recycle bin. Running out of options.

Thanks for all your help.

Edited August 28, 2018 by alortegac

[+Nezgar]

Definitely not for the recycle bin. Worst case, there are probably people out there for instance with working system boards without cases... And most of the chips on your board may be good, useful in other systems, but determining that requires testing each in a known working machine... They don't make these parts anymore!

 

Hang in there a bit more here. Whereabouts in the world are you? There may be someone near you who could look at it in person.

 

I'm also curious about MrMartian's suggestion about trying to replace the MMU.

[+DrVenkman]

I never would have been able to save one of my 1200XL’s without a logic probe ($18 on Amazon). With it I was able to determine that several sockets for the large ICs (specifically POKEY, SALLY and PIA) were not making good contact with the chip legs on one or more pins each. Replaced the sockets and saved the machine.

 

So please don’t recycle the machine! Someone can save it even if that someone isn’t you right now.

[alortegac]

Ok, not to worry....I am just a bit frustrated after all my attempts have failed. I do not have spare parts, that is a problem. I do not have another functional board to test or compare against. I do not know what I can test next in this situation. It can be anything. It can be the mmu or rom ic or even little capacitor......

 

I am located in Massachusetts. I can send the board out to anyone interested in looking at it. I will pay For shipping postage both ways, of course. Also, for any spares,etc. I just cannot pay for prof. electronics service.

I will investigate on the logic probe. Thanks for your suggestion.

 

My very last option is to buy a populated board from Best (if still available). It is $65 without shipping. (?)

 

Any other ideas are welcome!

Thanks!

Al

[alortegac]

Does anyone know the basic testing protocol for 800XL with the logic probe? Thanks in advance. Al

Edited August 28, 2018 by alortegac

[+DrVenkman]

Does anyone know the basic testing protocol for 800XL with the logic probe? Thanks in advance. Al

First, watch a couple YouTube videos on the basics of logic probe use. It’s pretty simple and a pretty simple device but once you know how to use it and what you’re seeing with the results, it makes a lot of sense.

 

Then go to AtariMania and download a copy of SAM’S COMPUTERFACTS for the 800XL. It has complete accurate schematics for the board plus - very helpfully! - a full set of logic charts for each IC and a set of troubleshooting steps for pretty much every problem.

 

Using the logic probe and comparing the results with the logic charts in the book, you should be able to significantly narrow down what’s causing the issue and probably identify it definitively.

[alortegac]

Thank you! Will do and report back!

[shoestring]

This is pretty good video on a vic20 repair which includes some trouble shooting with logic probe.

 

 

And onecircuit's videos are great too

 

 

[+DrVenkman]

Now that I think about it, here's a short video of me uncovering one (of several!) problems in the 1200XL I restored last summer ...

 

[alortegac]

Thanks a lot guys! Very helpful material. I watched all videos, read SAM’S COMPUTERFACTS and got my logic probe.

 

Problem Recap: I know GTIA and CPU are good, tested both of them on my Atari 5200. I also know all RAM is new and good. I have replaced both u26 and u27 too. An the 4050.

 

With the probe, I tested most IC's following the SAMS charts, and I have bigger problems! . Most IC's appear to have issues, even the known good ones (?) when tested with the probe. Some Pins in high when they should be in low or not pulsing, or only in solid high. The problem appears more frequent in the pins D (data), PB and pins with functions like R/W. "A" pins appear to be ok for the most part. Also, pins with VCC, not used and ground, all ok.

 

So we know some or 1 may be bad? but, which one?. is there an order for replacement? ROM? MEMORY MANAGEMENT? ANTIC? This chips are not easy to find or cheap. I really need a way to know which has more probability to be the culprit.

 

Another thing, is I checked voltages following the schematics, some voltages are marked 4.3V, my readings are a bit higher like 4.7V or 4.8V, not sure if this is significant.

 

Thank you again for all your help.!

Al

Edited September 2, 2018 by alortegac

[+DrVenkman]

Okay, before you jump to conclusions, a couple things:

 

First, voltages generally in the range of 4.5 - 5.1V are generally not going to be a big deal. So don't sweat minor differences from what a reference manual says they "should" be.

 

Second, your known-good chips are still almost certainly good. If they work in another system, they're not the problem so check them off your list.

 

Okay, deep breath ...

 

Now with that out of the way, good chips can still give different results with a logic probe when either the socket isn't making proper contact or another chip or component in the socket is bad. My guess from what you've posted is that some other chip is bad and it's causing the "good" chips to give odd signals. As for possible replacements for the ROM and MMU (and the rest of the chips), Best Electronics has new-old-stock Atari chips in stock. Check their website and then send an email.

[+Nezgar]

If you want to rule out the OS ROM, and don't have another machine or an EPROM programmer to verify it in as I mentioned in an earlier post, I have the chips and a programmer, so I'd be happy to drop one in the mail for you for a few dollars + postage. PM me if interested.

 

Otherwise, here's the parts from BEST according to this page: http://www.best-electronics-ca.com/custom-i.htm

CO61598   	(XL / XE OS)   	$12.00
CO62024   	(600XL OS)   	$8.00
CO61618   	(XL / XE Memory Management.)   	$14.00

Hmm, Wonder if the 600XL OS is the Rev 1 XL/XE OS, hence cheaper? (Due to some minor bugs) Maybe easier to just get OS ROM from BEST with an MMU since I see the price isn't that bad...

 

EDIT: yes CO62024 is Rev 1 XL/XE OS according to the FAQ. Should be just fine for testing if you want to save a few $...

[alortegac]

Thank you so much for your support! Greatly appreciated.