I pulled mine off the shelf today to dust off the cobwebs. Much to my
chagrin I found the tuning capacitor almost frozen stuck. For those
that aren't familiar with the construction of these there's a lot of
nylon? bushings involved.

I suspect lubrication may be the culprit that causes the nylon to swell
up. I was finally able to get it loose enough to where the vernier
doesn't slip although it still feels very tight.

Too late for me to avoid lubrication but I thought I'd pass this along
for the next guy.

-Bill

Well that's sad. I doubt it's nylon which was invented in 1938,
maybe phenolic? Wood filled phenolic which swells... ugh!
Maybe you can rinse it with solvent then dry it out so it will
shrink back to nominal dimensions. The rest of it is constructed
of isolantite and brass which solvents won't hurt.


exray wrote in :

I pulled mine off the shelf today to dust off the cobwebs. Much to my
chagrin I found the tuning capacitor almost frozen stuck. For those
that aren't familiar with the construction of these there's a lot of
nylon? bushings involved.

I suspect lubrication may be the culprit that causes the nylon to swell
up. I was finally able to get it loose enough to where the vernier
doesn't slip although it still feels very tight.

Too late for me to avoid lubrication but I thought I'd pass this along
for the next guy.

-Bill

Grumpy The Mule wrote:
Well that's sad. I doubt it's nylon which was invented in 1938,
maybe phenolic? Wood filled phenolic which swells... ugh!
Maybe you can rinse it with solvent then dry it out so it will
shrink back to nominal dimensions. The rest of it is constructed
of isolantite and brass which solvents won't hurt.

Could be phenolic but it really seems like a plastic of sorts. Hard to
tell. The front bushing is where it got tight and there's no easy way
to get it out for a thorough cleaning.

-Bill

exray wrote:
Grumpy The Mule wrote:
Well that's sad. I doubt it's nylon which was invented in 1938,
maybe phenolic? Wood filled phenolic which swells... ugh!
Maybe you can rinse it with solvent then dry it out so it will
shrink back to nominal dimensions. The rest of it is constructed
of isolantite and brass which solvents won't hurt.

Could be phenolic but it really seems like a plastic of sorts. Hard to
tell. The front bushing is where it got tight and there's no easy way
to get it out for a thorough cleaning.

-Bill
That is a VERY weird capacitor. First of all it tunes through 270
degrees of rotation using a unique shape of rotor plate, stator plates
are triangular. Second weird thing is that the shaft bearings are
insulated from the frame, the rotor is grounded at a single point, a
flexible conductor running though the hollow shaft and grounded at the
rear of the rotor. I have a single gang version of that capacitor that
was 'featured' in the one tube regenerative receiver from the A. P.
Morgan 'Boy Electrician' book from the 50's. National did make scales
for their velvet vernier dials that went through 270 degrees of rotation
for use with these capacitors.

ken scharf wrote:

That is a VERY weird capacitor. First of all it tunes through 270
degrees of rotation using a unique shape of rotor plate, stator plates
are triangular. Second weird thing is that the shaft bearings are
insulated from the frame, the rotor is grounded at a single point, a
flexible conductor running though the hollow shaft and grounded at the
rear of the rotor. I have a single gang version of that capacitor that
was 'featured' in the one tube regenerative receiver from the A. P.
Morgan 'Boy Electrician' book from the 50's. National did make scales
for their velvet vernier dials that went through 270 degrees of rotation
for use with these capacitors.

Agree with WEIRD. After goofing around with it I had a heck of a time
getting everything realigned. Never did see a way to completely
disassemble it.
Whats your take on the type of insulating material?

-Bill

On Nov 9, 5:18*pm, Grumpy The Mule wrote:
Well that's sad. *I doubt it's nylon which was invented in 1938,
maybe phenolic? *Wood filled phenolic which swells... ugh!
Maybe you can rinse it with solvent then dry it out so it will
shrink back to nominal dimensions. *The rest of it is constructed
of isolantite and brass which solvents won't hurt.

I have witnessed 50 or 70 year old variable capacitors of
low-to-middle grade construction where I swear it's the metal
that's swelling. The spacers between plates seem to have
expanded or contracted so that while the rotor/stator mesh is centered
at the center of the capacitor, at the ends the rotor and stator are
nearly
touching.

Others tell me this is well-known but I still don't have any
understanding
of why metal would shrink or grow with time.

Tim N3QE

Tim Shoppa wrote:


Others tell me this is well-known but I still don't have any
understanding
of why metal would shrink or grow with time.

Tim N3QE

Where there's pot metal involved it certainly swells up. Thats a
scourge in 1920s sets that use pot metal pulleys, brackets and capacitor
frames. My understanding is that it is due to reaction/corrosion
between the various metals in the 'pot' mix.

-Bill

On Nov 10, 9:38*am, exray wrote:
Tim Shoppa wrote:

Others tell me this is well-known but I still don't have any
understanding
of why metal would shrink or grow with time.

Tim N3QE

Where there's pot metal involved it certainly swells up. *Thats a
scourge in 1920s sets that use pot metal pulleys, brackets and capacitor
frames. *My understanding is that it is due to reaction/corrosion
between the various metals in the 'pot' mix.

-Bill

This sounds like what may have happened to some old drives I had.
A guy gave me half a bushel of the things telling me he thought they
would work if I cleaned them up. I bet he had gone through the other
half a bushel trying. I was able to repair a few by drilling out the
holes
and inserting the brass ferrules removed from old pots. I hope this
proved
to be a good fix and the guy that bought them from me isnt cursing
me too loudly.


Jimmie

On Nov 10, 9:38*am, exray wrote:
Tim Shoppa wrote:

Others tell me this is well-known but I still don't have any
understanding
of why metal would shrink or grow with time.

Tim N3QE

Where there's pot metal involved it certainly swells up. *Thats a
scourge in 1920s sets that use pot metal pulleys, brackets and capacitor
frames. *My understanding is that it is due to reaction/corrosion
between the various metals in the 'pot' mix.

Well, my metallurgy knowledge has always been particularly poor.

With the problem variables I have on the shelf, the issue isn't
necessarily
the spacing between adjacent plates but the cumulative swelling
over a dozen or more spacers.

I have discovered that by disassembling the caps and grinding
every 3rd or 4th spacer down by a smidgen that the result seems
just fine.

This only helps on the capacitors that can be disassembled... but the
pot metal spacer problem seems to be worst in the kind that can be
disassembled.

Some later top-quality air variables I have use something that
is either welding or silver brazing to space/hold the elements on a
solid rod. These do not have the spacing problems. They're pieces
of beauty...!

Tim.