Tech Tipp: Strange behavior of an Atari XEGS - and solution

[+tf_hh]

Hi,

 

today I found an absolute new and strange behavior reparing an Atari XEGS. I never have had such an error before neither I found something similar here. So maybe this could help other people with the same strange issue.

 

The XEGS I got comes with the fault description "System hangs if Missile Command is started or sometimes the game just resets" (reset = start from the beginning, same if you would press START at any time). I can reproduce this error easily.

 

So first, I watched the SYNC signal at the CPU - but even when the game hang, there´s activity on SYNC. So I decide first to change the ROM - I have a lot of ROM failures specially by the XEGS. After ROM the MMU was changed. But the fault remains. Ok, next step would be the CPU. CPU failures are from my experience the 2nd most related things after RAM memories. CPU exchanged, but you can imagine... that also didn´t help.

 

BTW: Before I change any part, of course I watched every single pin of the CPU, GTIA, ANTIC and the FREDDIE. All fine.

 

I connect a keyboard, start BASIC, enter some simple calculation endless loops with PRINT´ing them, this works for hours. I attached an SIO2SD, load Missile Command (file version) and... it works. No problems at all.

 

Ok, which components might cause the CPU to make a break... ANTIC? GTIA? All signals (RDY, HALT, IRQ) looks good. After all, I didn´t found other ideas, so I decide to remove all chips, solder sockets in and put well-known working parts into the XEGS mainboard.

 

Start the next test, no keyboard, Missile Command starts and... hang. Now - wished I had test this first, a lot of spent hours would have been saved - I connect the keyboard, start the XEGS with SELECT hold down and Missile Command starts. I didn´t know this before, that Missile Command could be started also with keyboard instead of Atari Basic. I don´t have an XEGS for my own, so I haven´t play around much with this.

 

WITH keyboard attached the Missile Command works fine. No problems.

 

Now I examines the POKEY more then before. Without keyboard I see on the scope, the IRQ line is sometimes low for over 10 times of a PHI2 phase! At every time IRQ goes low for such a long time, the game hangs, resets or makes other weird things. With keyboard attached this never happens, IRQ remains constantly high while the game is running - what I expect, then Missile Command didn´t read the keyboard nor SIO etc., it uses just the joystick ports and console keys, which are attached to the GTIA.

 

After some more examining the POKEY I saw, that pin 16 and pin 25 of the POKEY - KRx lines - are heavy floating from -0.5 volts up to 2 volts. Now it would be clear: POKEY "thinks", a key a pressed, generate an IRQ, but the OS (which is active during the game, ATTRACT will also work) can´t read the source. In case of the bad floating signal the POKEY will generate a long IRQ, longer than normal, which cause the CPU to halt - like some of the "CPU breaker" with R/C links from the 80s.

 

But... the XEGS is MADE for usage without the keyboard... so why this happens?

 

Checking with another XEGS boards, the culprit was found very fast: It was the parsimony of Tramiel´s area, removing some necessary parts to save some cents. The "good working" XEGS has line filters in the KRx (and other) lines, which will prevent these open inputs from floating around and beeing using like an antenna. At this special board, Atari replaces these line noise filters by simple 0R resistors. See this board:

 

 

The fix is absolute simple, after all I´m so silly not to find this earlier, but... now it works. And maybe it helps others from spending so much time to repair an XEGS

 

Just solder two 1nF capacitors between ground and the both KRx lines - that´s all. Of course you can use wired capacitors, too. Now there´s no floating inputs and the system works fine - with or without keyboard...

 

Jurgen

[_The Doctor__]

You have hit the nail on the head...so many XEGS end up in pile due to this situation... time for them all to be repaired and let back out into the world! I used to pull and replace the jumpers (zero ohm resistors) with the original xegs filters

Edited November 3, 2018 by _The Doctor__

[Paul Westphal]

Timing is everything, thanks. I have two XEGS on the bench that may benefit from your time consuming research. WORD

[Faicuai]

Sick!!!

 

Man, gotta LOVE they way you pulled that needle out of a large haystack!!!

 

Another reminder of the Tramiel legacy: el-cheapo design, manufacturing and LOWEST possible quality standards... It is what it is...

 

BRILLIANT dignostics work there!!!

[Mathy]

Hello Jürgen

 

Can you tell us which pins/locations are connected? I'm not sure what I am seeing in the picture.

 

Sincerely

 

Mathy

 

PS great job!

[_The Doctor__]

the caps on close up go from plane to 41 and from plane to 37

Edited November 4, 2018 by _The Doctor__

[+tf_hh]

Can you tell us which pins/locations are connected? I'm not sure what I am seeing in the picture.

 

As mentioned in my text: KRx lines (KR1, KR2), Pin 16 and 25 from POKEY.

 

Jurgen

[_The Doctor__]

I should clarify... the caps on close up go from plane to L41 and from plane to L37

[+mytek]

Stumbled across this while searching for something else.

 

Another fix, and one more inline with a non-XEGS, would be to put 4.7-10K pull-up resistors on the Pokey /KR inputs on the motherboard. That would insure the correct idle state (HIGH) on those inputs. Those resistors are present in the keyboard PCB, but not on the Pokey /KR inputs on the motherboard, thus the reason they float without the keyboard connected. For extra protection (including better ESD protection), the resistors could be combined with the capacitors that Jurgen proposed.

 

XEGS Keyboard PCB Schematic

 

XEGS Keyboard PCB

 

[t-poz]

Hi all,

 

I have a similar problem to what is in the thread.  I recently picked up an Atari XEGS and everything seems to work fine, except it will not load missile command.  I just get a blue screen with a solid cursor in the upper right corner.  I have several game cartridges which all seem to work fine.  I also have the keyboard which allows me to create/run a simple Basic program.  I get the same blue screen when I try to launch Missile Command with the keyboard attached (select + power).  Sometimes when I hit the Reset button while in the blue screen, it will change the screen color to a dark green with solid cursor.  I've opened up the console and did some cleaning though it was in pretty good condition.  While the case/shield was off, I plugged in and turned on and none of the chips seemed to get 'hot' while letting it sit/hang.  I am no expert with when it comes to understanding these boards and my soldering skill are very novice, so wondering if there's an easy fix or just leave things 'as-is' as I don't want to make the system any worse based on my skill set.

 

Any help or adivce is much appreciated.

 

Thanks!

[BIGHMW]

On 11/3/2018 at 9:12 AM, Faicuai said:

Sick!!!

 

Man, gotta LOVE they way you pulled that needle out of a large haystack!!!

 

Another reminder of the Tramiel legacy: el-cheapo design, manufacturing and LOWEST possible quality standards... It is what it is...

 

BRILLIANT dignostics work there!!!

....and THEY say its better than having a 5200, HA!

 

Even though she has thousands of titles during to her 8-bit computer bloodline at least Big Sexy never has THESE types of failures, even I owned an XEGS she had issues with some ports on my Ultimate cart, in some instances not reading it and reverting to the built-in Missile Command program, one of note was the Glenn conversion of 5200 Centipede, there were others too, but lo and behold the A8-to-5200 conversions work fine on Big Sexy. So even though there are only about 500 titles on her available I'll stick with my 5200, here since 1983,  and, forever, even after the smoke clears.

[mimo]

Ok, bye 

[+Stephen]

19 minutes ago, BIGHMW said:

....and THEY say its better than having a 5200, HA!

 

Even though she has thousands of titles during to her 8-bit computer bloodline at least Big Sexy never has THESE types of failures, even I owned an XEGS she had issues with some ports on my Ultimate cart, in some instances not reading it and reverting to the built-in Missile Command program, one of note was the Glenn conversion of 5200 Centipede, there were others too, but lo and behold the A8-to-5200 conversions work fine on Big Sexy. So even though there are only about 500 titles on her available I'll stick with my 5200, here since 1983,  and, forever, even after the smoke clears.

Let me know when you can run a Disk Drive, keyboard, printer, and 1MB of RAM in that old clunky 5200 and we'll talk:)  Bonus points for when you get SDX, and hard drives of 64GB running with full FAT32 read/write capabilities via the SIDE3 cart.  Stereo, Covox, and MIDI optional of course.

[invisible kid]

3 hours ago, t-poz said:

Hi all,

 

I have a similar problem to what is in the thread.  I recently picked up an Atari XEGS and everything seems to work fine, except it will not load missile command.  I just get a blue screen with a solid cursor in the upper right corner.  I have several game cartridges which all seem to work fine.  I also have the keyboard which allows me to create/run a simple Basic program.  I get the same blue screen when I try to launch Missile Command with the keyboard attached (select + power).  Sometimes when I hit the Reset button while in the blue screen, it will change the screen color to a dark green with solid cursor.  I've opened up the console and did some cleaning though it was in pretty good condition.  While the case/shield was off, I plugged in and turned on and none of the chips seemed to get 'hot' while letting it sit/hang.  I am no expert with when it comes to understanding these boards and my soldering skill are very novice, so wondering if there's an easy fix or just leave things 'as-is' as I don't want to make the system any worse based on my skill set.

 

Any help or adivce is much appreciated.

 

Thanks!

Folks will be able to help you troubleshoot. To me, it sounds interesting that it isn't loading missile command at all. It's not like it's loading and crashing. Does the xegs look modded at all? Could the previous owner have installed a custom OS chip? Basic works. Does Self test work? Option at startup or BYE from BASIC. I might try and load an external Missile Command just for kicks.

Edited July 8, 2022 by invisible kid

[BIGHMW]

9 hours ago, Stephen said:

Let me know when you can run a Disk Drive, keyboard, printer, and 1MB of RAM in that old clunky 5200 and we'll talk:)  Bonus points for when you get SDX, and hard drives of 64GB running with full FAT32 read/write capabilities via the SIDE3 cart.  Stereo, Covox, and MIDI optional of course.

@Stephen if I need to use a printer, a disk drive and a keyboard I already have two laptops for that a windows xp pro and a windows 7 pro one as well.  Besides even though I did enjoy some games that only the XEGS could run it came down to the heartbreaking choice between my first (the 5200 and the huge amount of money I put into her) for almost 40 years or the XEGS with her tremendous support and lineup of titles, but, the real gamebreaker that made me lean to the 5200 when the smoke all cleared was no official Tempest or the Mean Hamster titles for the 8-bit or XEGS so that swung the pendulum to the 5200's favor. Also I didn't want the efforts of @Ryan Witmer or Paul Lay @playsoft to go to waste or for them to re-port their 5200-improved versions of Super Breakout, Asteroids or Yar's Strike back to the XEGS just to please me or anything like that. If all of those titles i mentioned earlier on were available on both maybe it'd be a different story. I would've even gone out of my way to send her off to get both a VBXE upgrade and also an Ultimate 1MB job done on her as well. Who knows maybe good things will start to happen and maybe I can reacquire both another XEGS and a 7800 in the not so distant future.

[+x=usr(1536)]

Quote

Could the previous owner have installed a custom OS chip? Basic works. Does Self test work?

Possible re: a custom OS chip, but given that the XEGS only has a single ROM containing the OS, Missile Command, and BASIC, it's probably less likely than with a different machine.  See the following thread for details:

 

 

Of course, if it's opened up and there's an EPROM in the space where the mask ROM would usually be, well, there's your answer

 

Quote

Option at startup or BYE from BASIC. I might try and load an external Missile Command just for kicks.

 

Good point re: Missile Command from a cartridge.

 

I don't believe that the XEGS self-test checks the state of either the BASIC or Missile Command ROMs, so any issues with those wouldn't show up.  That said, it would at least give a rough idea of where things stand.

[t-poz]

Thanks for the replys....

 

To me, the board does not appear to be modded - need to look at attached threads more to confirm, but all chips appear to be original and soldering is clean (see pic).

 

With the keyboard attached:

• Basic runs fine
• Typing BYE brings me to the self test - all self tests run fine
• holding Option while turning power on brings up self test - every test runs fine
• holding Select while turning power on hangs with blue screen/solid cursor - Boo!

I do not have a missle command cartridge to test that game, but I do have pac-man, pitstop, pole position, joust and centipede which all work fine.  

 

Thanks!

[1050]

An inordinate number of XEGS have been found with OS ROM that are faulty. Since you've tried this one about every way except upside down, I'm leaning towards the opinion that it has a faulty OS ROM chip and replacement would be my next step. A 27256 Eprom with a speed of 200ns would be perfect and of course a $40 eBay programmer to burn the file onto it with. The TL866II plus seems popular but I see it's 60 dollar range now.

 

A standard eprom that would do nicely for $5.95

https://www.jameco.com/z/D27256-Intel-Corporation-IC-27256-25-EPROM-256K-Bit-250ns-Production-UV-Erasable-PROM_40061.html

 

And a One Time Program only (OTP) type for $3.49 that would also work fine. It won't have the quartz window for erasure, but instead just be like any other chip with dark plastic top on it. But you only get to program it once, and that's exactly what Atari bought in the first place.

https://www.jameco.com/z/AM27256-1D-Advanced-Micro-Devices-EPROM-OTP-256K-32Kx8-8-Bit-200ns-5V-DIP-28_2303055.html

 

So you can program PAL, GAL and many other types of eproms beside the 27256 with the programmer above. In addition you can place a logic chip that has the numbers sanded off of it in the device and it will tell you what the number was.

 

And you can change the game in there if missle command isn't your first choice. Perhaps lot's to think about.

 

 

[t-poz]

@1050Thank you for your detailed respponse and links.  I think at this point, I will probably end up getting a missle command cartridge for around $10 and call it good.  I would love to explore getting a TL866II along with a eprom and trying to program and install myself, but with my current skill set, I don't think I am quite ready for that (mostly the soldering).

 

Again, I really appreciate all the help.

 

Thanks!

 

side note - I have picked up a 1050 disk drive for my XEGS and was wondering if there were any recommended games or websites to pick up some 5.25 floppy disk games?  I grew up with a Commodore 64 and had a ton of games (mostly copied).  Unfortunately many many years ago, I lost my collection due to a flooded basement at my parents, so am curious what's available to play on the XE (loved Bard's Tale - which was an actual purchase).

 

[invisible kid]

I can relate to the uncertainty on working on this equipment! I got a TL866II and was able to do a dump, but have not tried to program with it, but have parts to do a custom OS. If you are in the NY/NJ/PA I can give ya a hand. As far as games, if you are looking for physical disks, there might be a handful of places, but since a lot of this is abandonware, If you could get an SIO2PC/SIO2USB, you could download a huge collection here:

https://atari-cellar.net/Atari 8-Bit Preservation Archives/

I got it from this thread:

Good luck! It's a ton of fun.

[invisible kid]

Oh yeah, there are many other ways to access software than the SIO2PC/SIO2USB(mem carts, fujinet, etc), but it is a decent cheap way that's been around a long time. But check into the others too.

Edited July 14, 2022 by invisible kid

[XEGS'r]

I kind of have a similar situation with my XEGS... System boots up normally when the keyboard is attached, and running BASIC programs works perfectly. However, when the keyboard is NOT attached, I usually get a black screen, but I do hear the usual "pop" from audio when turning on the system.

 

Admittedly, this one is my fault. I just upgraded the A/V output using TBA's UAV board and their IAB (Internal Audio Board) as well. Initially after installing these, the XEGS worked perfectly. I was also planning on installing a number of other upgrades which necessitate removing the major ICs. So, you know what that means... sockets! I socketed most of the major ICs and tested the XEGS after each successful socketing. Then, once again, I flew too close to the sun by deciding to socket ALL the ICs. I got to the NE555 timer. I did socket it and tested all the connections, all check out. However, when I power on the XEGS, the games behave very weirdly, not dissimilar to what was posted at the beginning of this topic. System works fine when the keyboard is attached, but when no keyboard is attached, various games behave erratically but are at least a bit playable. The built-in Missile Command starts, but all the timings are off. Donkey Kong is playable, but most of the sprites come off as inaccurate garbage, and usually, the colors are off. Q*Bert plays perfectly, but again, the sprites are distorted and garbage. The backgrounds and the sound do not seem to be affected. I've replaced the NE555 a few times and again tested all the connections, nothing wrong there. I tried the built-in diagnostics, and when I am able to get them to come up on screen, they all pass with flying colors. There is no problem with any of the other ICs I've socketed as none of these problems started happening until I got to the NE555. I even tried the solution posted by tf_hh with the two 1nF caps, and it actually made the situation worse as I couldn't get the display to come up at all. I even de-socketed the NE555 just to see if that made any difference, and nada. I'm kind of at my wits' end here... anybody got any ideas?