[R-390] Trouble Shooting R390A/Backlash

[Barry Hauser]

Jon wrote:
> Hi Folks,
>
> Just wanted to update the group on my restoration of the R390A. The 1/8amp 
> fuse was replaced and the radio came up singing. It was like an old Toyoto 
> commercial "Oh, what a feeling". Apparently it was a weak fuse, most 
> likely from the mid Sixies. Thank you for all your comments.

Fuses can be funny things.  They aren't fully sealed and subject to wear and 
tear as well as corrosion.  Sometimes they fail where the fuse wire is 
soldered to the end caps and it will look like the wire is intact. 
Sometimes the glue fails and the end caps will twist as you open the fuse 
holder to check the fuse -- or weakened enough to fail shortly after 
power-up.  (On the principle that no good dead goes unpunished.)  They also 
vary in quality.

> I'm focusing my attention now to the RF Stage and have a question 
> concerning backlash. I noticed a certain amount of backlash while turning 
> the KC knob counter clockwise. 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.

Seems familiar.  If you look at the cams and slug racks as you're turning 
the KC knob, you'll probably see that the backlash occurs as one or more 
racks are raising up and it's the pull of the rack springs pulling the cams 
backward.  It was mentioned recently that the exact position of the cams was 
not critical -- i.e. when lining up the holes in the cams with the scribe 
marks on the deck.  Not sure about that.  I seem to remember doing a 
mechanical allignment where I had one cam off a bit and correcting it fixed 
the backlash. Also possible you may need a bit more tension on one or more 
of the split gears.

Oh... one more thing -- a slipping gear clamp.  Hmmmm .. it's coming back to 
me now.  I had correctly aligned the cams, but one clamp was not holding, so 
in the alignment position (-7.000KC?) all the cams were OK, but didn't stay 
that way through the full cycle.  Carefully observe the works as you turn 
the KC knob from lock to lock -- several times -- and if that's the case, 
you'll eventually spot it.  Look for hairline cracks in the clamps.  Even if 
slipping or split, the clamps can hold for most of the rotation -- except 
where the reverse tension is at its strongest -- i.e. more of the slugracks 
are being lifted.  And the devilish thing -- they act like automatic 
clutches -- when the KC knob is turned back, they start grabbing again and 
"re-synch".  The loose/split clamp also caused the backlash action.  Tighten 
up any suspect clamps -- but don't overdo it - that's how they split and 
these problems often surface after a "tweaking" -- on that good deed 
principle.

Yup, a slipping clamp can turn this mechanical marvel into a Rube Goldberg 
contraption.

Barry








Barry