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

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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:

 

post-15670-0-11065900-1541260764_thumb.jpg

 

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...

 

post-15670-0-66089600-1541260770_thumb.jpg

Jurgen

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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 by _The Doctor__
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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!!!

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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!

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

Edited by _The Doctor__

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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

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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_schema.thumb.png.b82716a6d4d95ecb58a30a5eb4572ba1.png

 

XEGS Keyboard PCB

Atari-XE-Keyboard_pcb.jpg.74c8d2011cf348a0d9ff075c0f32617e.jpg

 

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