Atari XEGS Field Service Manual?

[Staalwart]

Hello everyone! I recently purchased an XEGS with 4 carts, keyboard, light gun and original PSU, for about US$70 on eBay. Unfortunately, i was DOA (Red Screen of Death), but the seller promptly refunded me and allowed me to keep the item! So, I intend now to try to fix it, being that it is now literally free...

 

By doing some research, I understand that the first suspect for this RSoD is the RAM chip(s). I already ordered a couple of Samsung 64K x 4 RAM chips for a couple of bucks each (same class as the original ones, although I read that the Texas Instruments were more common), but I thought that maybe a field service manual for the XEGS would come very handy on this, and any future, repair. However I've found service manuals for almost any other 8-bit Atari computer but the XE Game System (I found just the owner's manual).

 

So, anyone have a PDF dump of those that could kindly share?

[gilsaluki]

Desoldering/soldering on an XEGS is fun. Challenging. Take your time. I recommend putting in sockets on the chips you do change out.

[Rybags]

If the screen doesn't go black, that means the OS isn't able to get going. Likely failure would be CPU, MMU or OS Rom. Chances are your Ram chips are fine, though if they're of the "MT" brand it's a good idea to replace them.

 

AFAIK there's no specific FSM for XEGS. There's FSM for 400/800 and Sam's Computer Facts for 800XL as well as the schematics produced by Jerzy Sobola.

 

Diagnosing such problems is usually a lot easier with a second good machine to test chips in. Though the MMU is specific to the XEGS and the OS Rom is as well.

[+Philsan]

http://atarionline.pl/v01/index.php?ct=utils&sub=A.%20Schematy&PHPSESSID=96dfb27be5357e5132482aec2a93c3f1

[TGB1718]

Had a look at the circuit diagram and chip datasheet and think this problem is caused by the data

being removed from the data lines too early, look at this timing sheet, you will notice the "VALID OUTPUT"

is still valid after the CE & OE go high and although this is a very short period, if the system is not reading

until the end of the valid period, then a faster chip may have removed the data before it has been fully read.

 

Why Cartridges are OK may just be down to the way the MMU handles access, without information on the

internal timing of the MMU it's a bit difficult to see why Cart's are ok.

 

This is my best guess why it doesn't seem to work with fast chips.

 

Edited September 22, 2021 by TGB1718
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