Just to clarify Michael's explanation a little, factory fixed tabs are - as the name implies - permanently fixed in position and can't be adjusted by the user.

Do the keys stick up by the paper or down at their resting place? If the former, it's an easy fix (usually) involving just cleaning. Work a solvent into the slots of the segment for those keys and work them back and forth for a while. I successfully use alcohol or naptha (aka cigarette-lighter fluid). If that doesn't do it, check for gunk at all the other places in the linkages and check for bent links that might be getting hung up on other near-by parts. Once you get it all cleaned and working well, you do not have to oil any of these parts. Light oil on the carriage rails is OK but not so much that it holds dust.

But if they stick while down, that's a bigger deal and I'm not going to worry about it until you confirm that's what the problem is.

How do they work? Smith-Corona electrics have a motor in the back that, through a rubber belt, spins a three-bladed, <1 inch diameter rotor near the middle of the machine. When you press a key, that action releases the corresponding link which drops down to be hit by the spinning rotor and then be kicked up to the platen. All the typebars are kicked with the same force, independent of how hard you hit the key, so the type impressions are all uniform. Do NOT let that rotor catch your finger while you're investigating!

You also have a powered carriage return and that is handled by the same motor through a whole different mechanism: a clutch, triggered by the CR key, engages a pulley that pulls a cord attached to the left end of the carriage in order to, well, return the carriage to the left margin. Early S-C electrics had a manual CR until the engineers perfected the powered one. Both manual and powered carriages are advanced by the same spring-and-cord arrangement as most other typewriters.

Similarly, you probably have a manual Backspace because of the difficulty of designing a powered one and the fact that the Backspace is much less used and so less of a selling point. Some machines also have that, though.

HTH

OK. Now I have to ask: are the Z and the 4 really sticking or just failing to move at all when you press a key? Do you hear any noise at all when you press the Z or 4, as if the linkage is moving a bit and maybe hitting the rotor but then not getting kicked up?

S-Cs have a point in their links that consists of a hole in one end of one link and the mating link has a knob that fits through that hole and a flat spring that rotates on the other side of the hole to prevent the knob from slipping out. Fairly often, I think through abuse, that joint gets disconnected anyway; maybe it's designed to disconnect rather than bend the rods. Anyway, it's a little fiddly but you should be able to move the spring out of the way and slip the knob in place and then turn the spring back to cover the hole. You'll see how it works when you're looking at it. I have always been able to maneuver the parts without having to take anything apart. A long tweezer and a dental pick or other thin stick help.

Then, too, it might be some other problem...Onward.
HTH

Last edited by M. Höhne (27-1-2015 18:14:11)

First of all, I agree with your choice between these two.

As for the repair, tell whoever buys it that she or he should set aside some time to carefully watch the mechanisms in motion to determine where the problem lies and get an idea of how to fix it. That's the point we're at now anyway; we can't do it remotely without a lot more info.

Have Fun!