The Model M1/M2 (IBM Selectric Touch Keyboard)
The Model M2, part number 1395300, was an attempt at making a cheap and lightweight Buckling Spring keyboard for IBM's PS/1 - a home computer released in 1990. The keyboard was also used with the later PS Value Point. Compared with the contemporary Model M 1391401 it -
Doesn't have the inbuilt slope of the Model M - it's flat.
Has shorter, flatter single-piece keycaps.
Doesn't have a backplate.
Has a straight, non-removable cable.
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On paper, the compact and lightweight M2 looks like an excellent design. Unfortunately it is plagued with electronics problems. Specifically, the capacitors in the logic board are prone to failure. It is believed that the capacitors used immature surface mount technology and where therefore inherently prone to failure. The usual method of diagnosing capacitor failure is that the Caps and Scroll Lock stay on whilst the keyboard doesn't respond to keypresses. However, there is plenty of detailed advice on Geekhack on how to replace the capacitors, and this seems to take care of the problem. It really should be stressed that this is a prevalent problem - the owner of clickykeyboards.com claims that 4/5 of all M2s he deals with are duds. Potential buyers should not expect long term reliability from an M2 unless they are prepared to replace the capacitors.
Another unusual thing about the M2 is that instead of metal stabilizer bars, the large keys have plastic stabilizers permanently attached on to them. When disassembling, care should be taken to ensure that they don't snap. There are other signs of lower grade construction quality - instead of the membrane connecting into the controller PCB with a ribbon cable, the underside of the PCB has electrical contacts, and the contacts on the PCB 'sit' on top of contacts on the membrane, and are held in place with flimsy plastic tabs.
The Model M1, part number 60G3570, appears to be identical to the 1395300, except that it is sold under the "Easy Options by IBM" branding as a retail unit, as opposed to the M2 which was bundled with IBM computers.
One should also be on the watch for Rubber Dome versions of this keyboard. Fortunately, the most common rubber dome variant, part number 60G0817, can be spotted by it's large plastic border. However, there are some further rubber dome variants that may not have this border, such as part number 73G4614.
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There are also 3151 terminal variants that connect via a RJ connector, part number 42H0468 (and some others). Aside from the connector, the easiest way to spot these is by their keycaps which have non-standard markings and shapes.
In general, one should make sure to stick to ones with the 1395300 part number if they want to buy one for their PC.
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