the scout

Just acquired an old Raleigh Amazon hybrid about 25 years old I reckon. It is in exceptionably good condition however as to be expected the brake and derailleur cables are rusty. I have replaced the brake cables no problem but the derailleur cables are causing me some concern. It is a Shimano100GS and it is not obvious to me how to start. There is a crosshead screw on the top of the shifter and another on the bottom.
Do I undo these or is there a risk that the whole thing will explode with parts shot everywhere.
Anyone got an diagram or any advice of how it works?

JiveTurkey

Is the shifter a single thumb shifter on top of the bar or two paddles/levers under the bar?

The thumb shifters I've worked on don't need to be taken apart. With the shifter in the correct position, the cable can be slid out by pushing the cable toward the shifter; the head will pop out of a hole.

If you have two paddles, then I bet one of the screws just holds the shifter onto the handlebar clamp, while the other just holds the lid on, under which you'll be able to remove the cable. I doubt removing either of these screws will lead to the shifter falling apart as there are other screws inside.

MichaelW

Push the slack cable into the shifter and see where the other end moves. Grab the cable feral (cylinder plug) with needle-nosed pliers and pull it out, usually you can do it with your fingers. Ive never heard of a Shimano shifter needing to be unscrewed to change cable.

Mr IGH

Deore Deerhead shifters, there, now you've have

cyccommute 

Here's a link to a complete disassembly but you shouldn't need to do that level of teardown to change a cable.Thumb shifters just have the cable coming out of the back of the shifter. Look at this picture stolen from Fleabay to see how the cable goes in

the scout

It is the twin paddle under the handlebar type

cyccommute 

Those aren't old time shifters then. (Old time shifters are thumb shifters like the picture I posted). Go here and look under 'Shimano® STI Shift Levers'. Rapidfire systems all have a cover of some kind over the cable. In some it's a plastic cross that just screws out but in others it may be a snap in plate. It's hard to tell which one they used on which model. The only thing they all have in common is that the access plate should be above the shifter paddles. You don't have to remove the levers to access the cable.

LarDasse74

iirc, the rapidfire 'push-push' shifters required no disassembly to remove the cable... with the cable detached from the derailleur and the inner wire pulled free of the housing, shift into the lowest (cable all-the-way-out of the shifter) gear, and push the cable back into the shifter... on the side of the shifter opposite where the cable comes out should be a small circular hole and the cable head will be protruding there... sometimes there is a kink in the head-end of the cable and the head does not exit the hole perfectly, and it takes a few tries to get it out.

IF the calbe is totally corroded or jammed in the shifter and pushing on it does not free it, you may have to remove the bottom cover off the shifter to figure out a method to work it free. Don't worry about the shifter springing apart - the bottom cover is just a cover and the shift mechanism is (or should be) held securely together by another nut underneath the cover.

Good luck!

Eidt: I just went to the garage and got an old rapidfire push-push shifter (I think mine is a 100 or 200 GS)... Cyccommute was right - there is space above the 'paddles' where the head of the cable should come out no problem, not a little round hole.

Edit2: Here is a pic of the shifter. You can see a little spot of light above the top paddle... this is the hole through which the cable passes. The head of the cable should come out thought the space above the top paddle.
https://www.flickr.com/photos/67415609@N02/6136490733/