My Collins 51S-1 developed a bad case of excessive main tuning knob slippage which essentially made the receiver difficult to use. Finally I was able to find some time to pull the front panel and repair it if I could. Here's how I did it which hopefully should help you in the event that you ever run across one of these babies with a similar problem. It's actually pretty easy.
The manual will outline the steps to drop the front panel but if you don't have one handy here's how to proceed:
Remove cabinet if applicable. Using a Bristol splined wrench remove the MAIN TUNING, MEGACYCLE and ZERO SET knobs. If you don't have one of these or know what they are you probably shouldn't be working on Collins gear. Remove the 4 Phillips screws on the front right and left side. Working on the underside remove the screw securing the black spacer to the inside of the panel and the set screw on the EMISSION switch shaft. You may find it easier to remove the knob and the nut holding the switch assembly to the panel. There is also one Phillips screw on the bottom near the AF GAIN but it is secured by a nut and you'll have fun getting that back on when you reassemble the radio. Remove the two screws on either side of the tuning shaft. Pull the front panel forward slowly making sure not to damage any of the wiring. Pull the tuning shaft assembly off the inside gears in the dial. Set radio aside.
Look at the gear on the back of the tuning shaft. It is fastened to the shaft with a 3/16" nut. Two wave washers and a flat washer provide a certain amount of friction that allows the tuning shaft to slip which was designed to prevent damage to the PTO at the end of its' range. Herein likely lies the problem. Remove the 3/16" nut, flat washer and wave washers. Clean in 93% isopropyl alcohol. You can carefully oil the shaft with the gear and wave washers removed but make sure to wipe off any excess oil. Reassemble the wave washers, flat washer and 3/16" nut. Tighten nut until the gear ships with some effort. Reassemble dial assembly. Note that the split gears inside the main dial are spring loaded so make sure to move the outer gear clockwise a few teeth to reset the anti-backlash gears when reengaging the drive gear.
Reassemble the front panel, replace the screws in reverse order and the knobs. The 51S-1 should be good to go for another 50 years.
73,
MrMike, W1RC
