Sunpak digiflash 3000 manual

I wonder if a little sand got into the threads...

If you simply cannot get it to loosen, have someone try CAREFULLY with a pair of ChanNelLock (brand name) or other brand "water pump pliers", Dorothy. Ordinary slipjoint pliers (the kind that look like two little opposing thumbs) are generally useless for things like this, but parallel-jaw pliers ("water pump pliers") might be able to get in to that little plastic thumbwheel to loosen it far enough to remove.

Really - GENTLY is the keyword here. If they grip the thumbwheel too tightly, it'll distort the plastic and the thumbwheel will never feel the same again.

Once the flash is off the camera, use some aerosol cleaner spray to clean the threads while you work that thumbwheel back & forth. Don't use anything with any lubricant in it, but Freon or plain-air "spray" works well. Whatever you use, it MUST evaporate COMPLETELY and not attack plastic.