{Collins} exciter tuning and copper caps

I am new to the list, helping a friend get his KWM2-A operational. 
The exciter tuning knob does not move the slug rack up and down.  With a little finger pressure it will tune OK, but not on its own.  Seems to be a mechanical friction issue, but not apparent from just looking.  Anyone run into this?  Is it an easy fix, or does the plate with the vox pots have to be removed for access? 
Has anyone used the Copper Cap rectifiers to replace the 5U4 and 5R4 in the 516F-2?  Is it simply plug-and-play or are other mods required? 
I would appreciate any help I can get on these issues. 
BTW, I worked on these guys back in the 60's while in the USAF.  I was the only guy in the squadron who had ever seen a piece of Collins gear so they put me in charge of all the systems, 30L-1s, etc.   The rigs spent most of their time on 20M, "just checking out the radio, sir" says I. 
Barry - W1HFN 

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