I found the file "STABLXL.TXT" by Robert Puff on CSS-BBS.  It explains about modifications to the XL/XE which in some cases are needed when you're using them with large memory upgrades or the BlackBox.
 
I have the schematics to the 600XL, the 800XL and the 130XE (there must be better copies, but they'll do.).  After carefull examination, I have found out that Bob Puff sometimes mentions the WRONG numbers (that is, if the schematics are right).
 
In mod #1, for the 600XL, you should solder a wire from either pin 1 or 2, to pin 4 (not from 4 to 13, like on the 800XL) of the 74LS08.
 
In mod #2, Bob mentions that an unused gate of the inverter (74LS14) is used in mod #2 of both the 600XL and the 800XL.  You have to solder a wire from pin 6 of the 74LS08, to pin 5 of the 74LS14.
 
Well, on the 800XL, that would mean you connect the phase ZERO clock (pin 6) to the LS14 instead of the phase TWO clock.  On the 600XL, pin 6 (of the LS08) is the phase two clock, on th 800XL it's the phase zero clock.
 
On the 600XL, pins 5 and 6 of the LS14 are USED (for the inverted RESET signal (or is that "NOT RESET")).  I can not find pins 8 and 9 in the schematics, so I guess THEY are unused.  On the 800XL schematics, I can't find the pins 5 and 6, so I guess using them on the 800XL is OK.  All other pins on the LS14 in both XL's are used.
 
I've looked at the XEGS.  since it's totally different in it's set up (BASIC, OS and Missile Command all in ONE chip, only two RAM chips, stuffed in a corner, and NO hardware PBI.) modifiing it will be quite different.  But if somebody is gonna do a large memory upgrade on the XEGS and/or bring out the PBI, (s)he probably won't mind figuring out how to do these stabliser mods either.
 
CU          Mathy Van Nisselroy