I am upset because I wrote a long and detailed reply, and somehow Geekhack timed out and caused the whole thing to evaporate. It has happened before.
The Model F family (XT, AT, and F-122) are built with a metal plate with holes that the (individual) barrels fit through. The barrels seat into a foam pad and are held in alignment by tabs of various types. The PCB is also sandwiched with a metal plate and there are slots and tabs that slide fit to hold it all together.
The capacitive switches are connected to the flat rectangular copper traces built into the PCB itself, and the hammers just drop down to make the capacitive environment they need to fire. Clearly, I am no physicist, someone else can explain the actual working much better than I can, but suffice to say that the "switches" themselves do not have "moving parts" and the springs/hammers above are modifying the environment surrounding the switches, rather than conducting electricity themselves. That is pretty esoteric and I may not be understanding it perfectly, either!
My wiki is concerned with the foam mat (which deteriorates badly into a horrible sticky grit) replacement, and the bolt-mod for a 122. Other information and mods are detailed elsewhere.
http://geekhack.org/showwiki.php?title=Island:24615&highlight=ibm+model+conversionThere are 2 links at the top, the first to Soarer's and the second to Kishy's. Use Soarer's instructions for the Teensy converter, but read Kishy's for other valuable information.
To take these beasts apart, you bend up the metal tabs, if need be, and slide the plates apart. A hammer is often required. Barrels and springs fall out loose. These can be moved around, and you can use the cylindrical inserts in barrels without springs for the longer key stabilizers. Obviously, you need keys in the proper size and shape for your new layout. (It is very helpful to have a 1989 or later Model M as a donor about now.) Generally, there should be a barrel in each holes, but there are not always holes to correspond with switches, in my experience. Odd, that.
To re-assemble, you have to hold everything together, compressing the rubber mat, while making a very tight sliding fit. If you have small and/or weak hands, forget it. You may well be doing it multiple times to get it right. This is very difficult, and is part of the reason I did the bolt-mod.
I think the prevailing opinion (mine, anyway) is that the XT has the "lightest and tightest" key feel. Factors influencing this are probably the smaller footprint (and corresponding inner plates), tighter curvature, heavier plate (I don't have an XT anymore and I cannot remember this for sure), metal case back, and different alignment pegs for the barrels. The mile-long spacebar and its under-carriage is something else, but that is another story.
The AT is very similar but lacks some of these specifications (the all-plastic case is brittle and devilishly hard to take apart at the front - be extremely careful there!). But it still feels pretty darn good, and the layout is considerably more modern.
The 122-key terminal board is much larger, and therefore probably flexes a lot more internally. There is a third row of slots and tabs down the middle, longitudinally, which probably helps, but I did the bolt-mod to tighten it up as much as possible. I wish there were more places to put bolts all around, but the barrel bases all butt together and do not leave any space in the main body of keys. And unlike a Model M bolt-mod, where I tighten the bolts only very gingerly, I screwed these boys down tight.
I sold my last XT almost a year ago, before I did the F-122 bolt mod, so I never compared them side-by-side, but it compares very well with the AT.
The huge size and weight are not a problem for me, and to have a near-ANSI layout with bonus keys top and left is a great thing. It makes me very reluctant to use anything else. Even a beautifully bolt-modded early Model M seems hollow and dull in comparison, and my full-size Leopold blue seems like a dinky plastic toy.
Finally, with internal padding, a rubber desk mat, and flossed springs, even makes it tolerable at home with the wife and kids in the other room!
Thank you Soarer and Teensy!
PS - best find I have seen lately:
http://www.ebay.com/itm/ws/eBayISAPI.dll?ViewItem&item=390397297073&ssPageName=ADME:B:SS:US:1123
