[AMRadio] SP600 expert needed

[CL in NC]

Current project, one SP600 for me and 2 SP600's for the Appalachian Radio Museum in Asheville, NC.  I need to know the details about the tuning drive.  It uses two brass wheels, one brass wheel about 3/8" wide rides in the groove cut in the drive wheel, which is 3/4" thick.  That one being turned by the shaft attached to the main tune knob.  The edges on the  inside of the main wheel has chamfered edges that engage the chamfered edges of the wheel riding between its edges.  Contact is maintained by the driven disk whose shaft is attached to  plate that is spring loaded, holding tension to all.  The smaller driven wheel, has a slot around its circumference that engages the edge of the metal freq dial on the right side, that dial then drives a series of gears for the entire tune mechanism, and is operated solely by friction of these two parts.  The main tune operates, turning everything as it should, until it doesn't.

Turning in one direction, and it never misses a beat, but go back the other way, you get several turns then it slips.  If you backlight the disks, you can see it is actually losing contact, not slipping, and nothing down the line is binding up, feeding back to make it slip.  Everything has been carefully decreased, the tension plate seems to be fine, not at its limit of range of motion, and its tension spring appears to be applying a healthy amount of pressure.  Anybody have any suggestions?  One person asked if there was supposed to be a rubber drive ring on that main drive wheel, but the machine work looks like it was intended to be brass on brass, and not have a rubber part that would eventually deteriorate.

Perhaps it is just worn out?

Charlie, W4MEC in NC