"My wife has a "Smith Corona" Model SD800 electronic typewriter that will not advance the ribbon. Everything else seems to work fine. We have been unable to find a local repair for this unit. I think I can fix it with some advice from someone familiar with these machines."
Can you determine if it is a mechanical or electronic problem? For example, with the ribbon removed, does the gear or post that drives it appear to try to turn or not at all?
Modern electronic typewriters are a combination of keyboard, microprocessor, and printer. Therefore, the same sort of troubleshooting approachs can be used as for computer printers.
Common electronic problems include bad connections to the motor that advances the ribbon (cold solder joints, cracks in traces on flex cable to carriage), bad motor driver chip, or bad motor.
Mechanical problems include stripped or broken gears, misalignment preventing advance mechanism from engaging, and defective ribbon cartridge.
(From: Roger D. Waddell (email@example.com).)
This problem is usually caused by a broken 'E' clip on the bottom of the print hammer solenoid. The clip holds on a lever that works in conjunction with the ribbon/correction feed solenoid near the right front of the print carrier.
When the clip breaks, the lever falls out of position and never trips the lever that assists in feeding the ribbon.
I have seen this problem many times!!