[Boatanchors] Repairing a Royal 7000 tuning capacitor (the AM/SW one), LONG POST

[Phil KO6BB]

Hi All,

The following is a re-post of one that I posted to the Zenith 
Transoceanic group.  I feel that others here may find it useful and the 
same repair procedure may be useful for other tuning capacitors. . .
  ************************************************************

Awhile back I mentioned that I was going to have to work on my Zenith 
Transoceanic Royal 7000 "daily driver" receiver.  The tuning capacitor 
had become erratic, killing reception completely , at the low end of the 
BCB and shortwave bands (around 580 KCs) as well as a couple other spots 
on the dial.

I'd hoped that it might be a fairly simple fix, perhaps blowing dust out 
of the receiver, which, as I expected, didn't help any.

My second hope was that the tuning capacitor might have the bearing
adjustment screw and locknut on the backside, typical of many old tuning 
capacitors.  Nope, no joy there either.

NOTE:  The tuning capacitor is in such a location that dust, or 
accidental damage to it is highly unlikely!

The third step was to put the receiver on the FM band to remove any
coils from across the tuning capacitor so I could use an ohmmeter to
determine which section was shorting out.

Here I digress a little.  It turns out that the tuning capacitor is
surrounded on all sides by sheet-metal which would be nearly impossible
to remove.  I could JUST BARELY see the capacitor section on each end of 
the deck of three.  Not enough room to "knife edge" individual
plates IF I could see a bad one. . .

Anyway, in checking the capacitor for shorts it turned out to be the
middle section that was bad.  Wouldn't you know it, absolutely no way to 
get to it without removing the capacitor completely.  What follows is a 
step by step procedure that I developed to make it as easy as possible 
(after removing the chassis from the cabinet which I won't cover here as 
most folks know how to do that).
************************************************************

Removing the Capacitor.
1.  Tune the receiver to the very bottom end of the dial.  With a
"sharpie" pen, mark the exact location of the holes in the tuning drum
to facilitate replacing it in the correct location.

2.  NOTE:  If you're careful, IT IS POSSIBLE to remove the tuning drum
from the capacitor WITHOUT messing up or un-stringing the dial cord.
Here's how.
      2A.  Have a small plastic tie wrap ready.
      2B.  Loosen the two setscrews, and keeping the dial cord taut,
carefully pry the tuning drum off it's shaft.
      2C.  Still keeping the dial cord taut, tie-wrap it to the screw
terminal strip where the whip antenna connects. Make sure the tie-wrap
is snug and the drum secure.  Now you're ready to remove the capacitor.

3.  With a large soldering iron, remove one end of each of the ground
braids from the chassis (easier to unsolder them from the chassis than
from the capacitor).

4.  Remove the three 1/4" hex head mounting screws that hold the
capacitor and set them aside.  This will let you move the capacitor
around in it's enclosure.

5.  Make note of the three different colored wires that are still
connected to the capacitor, then with a skinny soldering iron remove
each one.  NOTE:  These were NOT wrapped around their terminals, just
inserted in the solder holes and soldered.

6.  Now you can remove the capacitor and examine it to determine the
cause of the short.
************************************************************

FIXING THE CAPACITOR:

Examination of my unit showed that it appeared to have had a
manufacturing defect, and a very slight shift of it's plates over time
caused the problem.  I'll explain.

The two outer sections had almost ideal plate spacing, except that one
end of the stator (fixed) section was slightly closer than the other in
the rearmost section.

The center section appeared to have the rotor (rotating) section
completely off center in relation to the stator plates, at no time were
they centered, and it was worse as the plates were closed.

For this capacitor, "Knife edging" (nudging the plates with a thin knife 
blade) would NOT be a suitable fix.

************************************************************
Before beginning any further work I decided to clean out the old grease, 
oil etc with a soft paint brush and some gasoline (would be a no-no in 
the Leaded gasoline days). That way I could see the unit better and 
would re-lube it when finished.
************************************************************

The first order of the day was to repair the faulty middle section.
After careful examination under a magnifier lamp I determined that the
rotor was actually a little crooked, and all plates needed adjustment.
Here's how I fixed it.
1.  I opened the capacitor plates fully.
2.  With a flat blade screwdriver having a fairly broad tip I inserted
it between the wall of the capacitor and the Bakelite piece holding the
plates in position, and very gently twisted the screwdriver so that it
pried all the plates over.  You MUST be gentle here or you'll break the
Bakelite and/or destroy the capacitor.  I was so gentle it actually took 
me four tries to get it moved enough, but when I was finished, there 
were no shorts and the rotor was properly positioned between the stator 
plates.
************************************************************

The next order of the day was to correct the stator sections of two of
the units so that they were mounted straight.  This was actually easier
than I expected.

The stator plates are held in place by little rectangular pieces of
Bakelite that are crimped into the capacitor housing.  Using the same
screwdriver and method I used to position the rotor of the middle
section, I very carefully pried the offending sections over until they
were square with the rotors.  One I took a little too far and had to
move it back slightly.

These stator sections were actually easier to move than the rotor.
Perhaps the Bakelite had shrunk a little over time, causing the plates
to shift.

After all this I carefully examined everything, checking and rechecking
to make sure there were no shorts.  I left the ohmmeter connected and
pulled the capacitor shaft in and out while rotating it to make sure
that there would be NO shorts under any conditions.  There weren't. . .
   ************************************************************

SECURING YOUR WORK TO STAY FIXED:

As it was obvious that things could shift a little over time it was
necessary to secure it all (mostly just the Bakelite pieces holding the
stator plates).

To do this I used some 5 minute JB Weld (careful it has metallic
content).  I first secured each of the six Bakelite pieces to the
HOUSING of the capacitor.  I did this by JB Welding the Bakelite
sections on the OUTSIDE of the capacitor, no worry about the JB
conducting anything it shouldn't there.

After that had set (about 5 minutes), I did the other side of the
Bakelite mounts, securing the metal plates that hold the stators to the
Bakelite.  NOTE:  DO NOT get JB on the area that goes to the housing as
you don't want to risk shorting the stator to the housing.  While you
might not see it with an Ohmmeter, it could very well affect the
operation of the circuit.
   ************************************************************

Using a quality grease (Lithium is OK) re-grease the ball bearings and
any other sliding surfaces that you can.  DO NOT get it between the
plates of the capacitor.
************************************************************

Carefully re-install the capacitor and wire it into the circuit, doing
the reverse of the removal procedure, above.

NOTE:  BE VERY CAREFUL when you cut the tie wrap holding the tuning drum 
and keep the dial cord taut while reinstalling the drum (make sure the 
tuning capacitor plates are fully meshed). Position the drum using the 
index marks you made prior to removal.
   ************************************************************

Install the chassis in the cabinet, connect speaker and power supply etc 
and check the dial calibration.

If you're lucky the receiver dial calibration may have remained the
same.  In my case it shifted 'very' slightly.

Oddly enough, I had to tweak the MW and longwave calibration a tad bit,
BUT shortwave calibration at 5, 10 and 15 MCs against WWV checks was
better than it was before.  This may have been due to sloppy work on my
part when I aligned the receiver  a couple years or so ago!


TOTAL TIME SPENT ON THE JOB BREAKS DOWN LIKE THIS. . .
1.  Approximately one hour to remove/reinstall the chassis and tuning
capacitor proper.

2.  About two to two and a half hours actually spent on repairing the
capacitor itself.  This mostly because I worked VERY slowly and checked
my work time and time again.  One slip there and I could have damaged it 
beyond repair, turning the radio into a large "paperweight"!

That plus the fact that most of the work on the capacitor had to be done 
under a lit magnifier slowed me down too (these 68 year old eyes aren't 
what they once were).

-- 
73 de Phil, KO6BB
http://ko6bb1.multiply.com/ (OTR Blog) 
http://cbsmysterytheater.multiply.com/ (CBSRMT Blog)
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