[codefox1]

Could just be crud of some sort inside. My standard procedure is tighen fully and record precicely the turns and fractions because you will forget later. Next open completely up, and carefully remove mica. It is extremely fragile. Next blast out the contacts and screw with Deoxit 5, and very carefully clean the mica with same. Leave everything alone for a day, and reassemble. Back off the exact turns which should get you close to original. No guarantees if magic fingers have been there ahead of you, but usually less than two turns from shut will do. Alignment will confirm, which you should do anyway.

Yes, a piece of old fsahioned clear "document protecter" can and has been used by me successfully to restore a "barn fresh" trimmer.

Best of luck!