Despite the R-390A receiver having been extensively documented over the past 40-50 years, I couldn't find a procedure for overhauling the Zero Adj. clutch assembly.   Based on my sample of 4 units,  the clutch failure rate is 100% and without a working clutch, it's impossible to calibrate the receiver so the PTO and Veeder Root counter display coincide.     The calibration procedure is well documented, but I had to learn to disassemble, clean, and reassemble the clutch from scratch, so this is my small payback for those who have shared their knowledge of these great receiver with the rest of us.

The clutch can be easily removed by only removing the front panel and the locking ring on the KHZ tuning dial to gain access.   Loosening the clamp on the clutch shaft will allow it to be pulled straight out the front but it will be necessary to loosen (don't need to remove) the screws holding the Veeder Root counter so the clutch gear can pass by its bevel gear.

All the R-390A docs show removing the front panel by folding it forward with the chassis propped up on blocks.  That doesn't work for clutch removal,  but I found it's actually very easy to turn the receiver 90 degrees,  resting it's left side with the panel hanging over the edge of the bench,  then simply pulling the panel out and rotating it 180 degrees.  The wiring harnesses provide more than enough flexibility to do this and some kind of simple clamp can be rigged up to support the panel while it's in this position.    In this position you have easy access to the clutch and related area to remove the riveted dial lock ring and the clutch assembly.

The photos describe the procedure pretty clearly.  After disassembly,  clean all parts in solvent and polish any rough spots.   See the attached Army assembly drawing as well.   The most difficult part is keeping the four Bellville spring washers precisely centered so that when the anti-backlash gear is pressed in place they do not cause interference.   A steady hand is required to lower the gear into position and engage its two alignment pins with the holes in the clutch disk without moving the washers out of position.  

Two simple tools make the job much easier.   One is just a piece of hard stryofoam or something similar that has clearance holes for the three protrusions so that pressure can be applied to the brass gear.  The other is a piece of tubing that has an ID that is a snug fit over the shaft and an OD that is smaller than the recess in the gear bushing.  This makes it easy to start the snap ring around the top of the shaft and then just press it down into position.  A "click" will be heard when it snaps into the groove in the shaft.

Removing the snap ring is a challenge because the usual tool for this job cannot be used as the "ears" of the snap ring sit in the recess and cannot be spread apart.  I broke the first snap ring trying to get it out, but found that with practice,  two knive tips (like Exacto) can be used to pry the ears up and out of the groove and the snap ring removed intact.

A working clutch is a nice addition to an otherwise well-performing R-390A!