Many models of Ten-Tec transceivers suffer from hardening grease in their PTO. This can be remedied by removing the PTO and cleaning out the old, hardened grease. Then give it a good re-greasing and it should be ready to go for several more years.
That's just what I did for my Omni D (Model 546). The PTO knob was getting "tighter" when tuning. Strangely, it was more pronounced when rotating it left. I had recently had a similar tightening issue on my Omni A, so I knew it was time to do a cleaning.
Sorry I didn't record some of the smaller disassembly/reassembly. Couldn't really get the camera AND my hands in there at the same time. Hope this video helps someone out.