Atari made a few versions of the 800 keybaord during its five year production run. Each has its quirks, but all can be serviceable after 30+ years.
1) The original Hi Tek keyboards are very rare, I've never seen one, and they have a male ribbon cable connector like the 400 keyboard. These were only on the very first 1979 800's. Really can't comment on this one much.
2) The majority of 800 keyboards are the excellent mechanical Hi Teks that have white plungers. As stated, they are mechanical switch keyboards and many still work wonderfully and will continue to for many years. They do have one flaw. Due to repeated pounding of the spacebar, the load (force) is transmitted to the solder pad on the PCB. After many thousands of cycles, the solder pad at location 57 on the back of the PCB fails and the contact pin falls out leaving with a bad spacebar. You just need to reinsert a wire in the hole in the solder pad at location 57, experiment a little with a multimeter to find the position that restores continuity and re-solder the lead. This is probably the best made keyboard Atari ever made.
3) For a short while Atari used mylar membrane keyboards made by Mitsumi. They look feel and are constructed just like the 1200XL keyboard also made by Mitsumi. If you can find a mitsumi keyboard today that works, its a miracle. Everyone I've ever encountered had a few or no keys working. They can be easily identified as they are darker in appearance and have a distinctive glossy look. These, when they work, are awesome as they are not mechanical switched and feel smooth, but lack feedback. They can be fixed and will have to be, count on it, using the same procedure BOB1200XL has on RETROBITS.net.
4) The last run of keyboard was made by Stackpole, which have yellow plungers and are mechanically switch like the Hi Tek. These will last a long time and are reliable except, they too, have a flaw. The yellow plastic plungers all start to crack at the corners, even if not used. This causes the the keys to seat too deep and either the loose fit causes the key top to pop off or the key binds and sticks in the down position. The only way to fix these is to replace the yellow spacers. You just need to be careful you don't bend the fragile little gold plated contact fingers in the middle of the mechanical switch.
The Hi Tek and Stackpole will be the most reliable over time, but the Mitsumi has its rewards if you can revive them.
Edited by ACML, Wed Feb 12, 2014 10:44 AM.