I have a customer who brought in a bike that has Shimano STX 7-speed shifter/brake levers.
It shifts up and down from the small ring to the middle ring fine, but it won't shift from the middle ring to the large ring. The thumb lever will move, but with little or no resistance, and no cable movement. The problem is all in the lever. There's no cable obstruction and the limit screws are not stopping the derailleur from it's full range of motion. I've spraying the inside with Triflo. This makes the small to middle ring shifts crisper, but does nothing to fix the missing shift. I'm hesitant to start taking out screws or the bottom nut. I know some of these shifters have a tendency to puke springs and gears once you open them up.
The rear shifter has a problem also. It will shift up the cog fine. But shifting from the big cog down has problems. While in the big 7th cog, activating the trigger shifter one click moves the derailleur direcly down to the 5th cog, skipping over the 6th. One single click from there moves the derailleur down to the 3rd cog, skipping over the 4th. One single click from there moves the derailleur down to the first cog, skipping over the second. Again, there's no cable obstruction.
Are both these units shot?
The only replacement I can order with a cantilever brake compatible lever is an Altus 7-speed. Other than the shifters, the bike is in excellent condition, smooth bb, headset, good condition hubs, cassette, chain, true and even-tensioned wheels.