Typical XEGS issues?

[AtariPhreak]

Greetings…

 

I have 2 XEGS systems - 1 of which works perfectly, and the other one the power light turns on, and I see a brief glitch on the screen, but I get no output at all. 

 

As mentioned elsewhere, I am not someone good at soldering - so self repair isn’t much of an option, but I thought I would at least ask if there are common problems that might result in the behavior I am seeing (and yes, I realize I haven’t given you much to go on..) .. I hate to sell it as broken as having a spare XEGS would be a wonderful thing … 

 

I looked for obvious things such as broken pins on chips, broken capacitors, etc.. In comparison to my working XEGS, everything board wise looks fine to me.. 

 

Thanks for any thoughts/advice on how to troubleshoot this thing and if there are any places that might repair an XEGS for ess than it would cost to replace it, I would love a referral .. 

 

.lp

[Rybags]

Build quality went down with XE in general so don't be surprised to see faulty solder joints.

If you can't DIY then you could inspect, looking for severed traces and capacitors leaking or bulging.

 

XEGS allows seperate audio out so you could attach to something that can play it and see if the computer is actually starting - attach a keyboard before power up and see if you can get keyclick - hold Start on powerup and see if you get the audio prompt.

[xrbrevin]

see if any chips get hot - the 2x RAM chips at bottom-left

try powering it up without the keyboard (or by holding select), this should make missile command kick in

[Beeblebrox]

Most of the issues I've had with the xegs are either bad ram chips or bad Rom. Both require desoldering, socketing and replacing. I've done this on several over the last couple of years.

 

Of course it might not be these things at all, but they would be my first port of call. The Xegs boards, much like the XE range, are not great on terms of build quality as mentioned. You can easily do trace damage.

 

So IMHO either send it to someone who can troubleshoot and fix it, or sell as not working, suspected bad chips. 

 

Edit: BTW the os Rom chip, as well as the mmu chip, for the xegs aren't the same as those on the XE line. 

Edited March 13 by Beeblebrox

[Paul Westphal]

What Beeblebrox said.

I'll add some other info's :

Yurgen :

Hi All,

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.

Next, I examine 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:

XEGS_without_LF.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...

Attached_1nF_Caps.jpg

Jurgen

 

[_The Doctor__]

Yep, that's it in a nutshell.