[R-390] Trouble Shooting R390A/Backlash

[Barry Hauser]

Hi Tim

Jon wrote:
>>  The lower and upper frequency ends seems to move about 1 to 2
>> clicks while the mid freq range, approx 300 to 500 appears to move
> >as much as 4 clicks. Can anyone point me in the right direction as
> >far as adjustment(s) to correct this? Hopefully nothing is warped.

Tim wrote:
>Check the Oldham coupler (between the kc knob and the PTO)
>and the split gear that drives the counter for missing springs.  From
>the bottom you can see where the springs are supposed to go.

Doubtful that the extreme backlash would be due to a missing Oldham coupler 
spring -- that mostly serves to reduce slop and rattle and maintain 
"reset-ability".  A rubber band will do in a pinch.

>If the gears are rusted or burred then it may not matter if they've
>got the springs or not.  Without taking apart the geartrain you
>ought to be able to wiggle the two halves of a split gear just
>a tiny fraction of a tooth and feel/see them snap back.

Possible - or insufficient or no offset on the split gears.  But more likely 
a broken or slipping gear clamp somewhere.

>If turning the kc knob causes neither a change in frequency or
>movement of the counter, then it's likely the Oldham coupler
>spring.  If turning the kc knob does cause a change in frequency
>but doesn't cause the counter to turn, then it's likely the gears
>that drive the counter.

I think Jon is telling us that the whole business is cranking back on him --  
won't stay put over some range of the KC setting.  More like recoil than 
backlash.  Sounded like everything -- counter etc. -- is turning, but spins 
back at certain points.  I suspect a clamp is not holding at maximum tension 
points.

>Also the two bevel gears that do the right-angle thing to drive
>the counter may be not close enough, that would cause some
>counter backlash too.

>Not sure exactly why the backlash would depend on position of the
>kc dial.  The springs pulling down the cams do exert some
>torque backwards through the geartrain.

The tension is not constant through the full range of the KC tuning. 
Sometimes more racks are being raised up through part of the cycle and 
stretching more of those slug rack springs, or stretching one further.  If a 
clamp is slipping, it may just slip on the most extreme upswing, then starts 
grabbing again on the downswing.  It's difficult to see when it happens - 
and maybe so hard to see that it just pays to inspect for cracked clamps, 
snug them all up and try again.

>Oh, wait, if one of the two springs on a split gear was missing,
>then you might see the effect you see, although it would depend
>not on kc display but on the angular position of the kc knob.

Usually even one spring is enough for the split gear to work sufficiently. 
Perhaps, if the deck had been reassembled without offsetting several of the 
split gears, I could see that happening.  Again, though, after re-thinking 
and recalling a similar extreme backlash/spinback situation, I'd strongly 
recomment checking the action through the full cylcle of the KC train and 
looking for a cracked or loose gear clamp.

Barry