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RCA "Big Can
Servicing and Repair Page
Click on the subject line below
Breakdown
of parts
Removal
of elements from headsets and "Cans"
Opening
the element up
Re-
Magnetizing the element
Adjustment
of armature ("Centering")
Tools
| Before we start, Never open a balanced armature
element up
unless
you have a real problem. It is ill advised to do so "just for a look
inside".
If it is not broken it might be by the time you are finished looking
around.
It is very important that if you have to open it up, it is done
in an area that is as clean as you can get. Small pieces of dirt will
find
it's way in if you are not careful. Another good piece of advice is get
help. Sometimes a picture is worth a thousand words", but it is
also
totally worthless without advice and help. |
Breakdown
of parts
Close up of "can" mounted in lower headband
Although this is a "Mic" element, it
is
virtually the
same as a headset element
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Removal
of elements from headsets
RCA MI-2045-E "Big Cans"
Remove the "Cans" from the lower head band. Pry apart
the
the lower
headband (with short pins) that hold the "Cans" in place. Take care not
to scratch the bakelite "Cans" with the pins, as they are being removed.
Removed "Can" from headset frame
Unscrew the "cap" from "can". "Cap" is on left side
in
photo above
Keep as equal pressure all the way around the cap while
trying
to
unscrew it.
A small strap wrench works well if available.
You will see the "face" or "diaphragm" side of the
element,
inside
the "can" (on right of photo above,yellowish item).
"Shuck" the
element
out.
You can "shuck" or pry the element out of the rear
housing
with
a sharp knife.
Be very careful and go slow.
Check out all
the photos
and figures below and read the
following section
prior
to trying
this.
| I'm assuming you're
talking
about RCA "Big
Cans" here.
After unscrewing the ear
cup,
the element will
be just sitting there in the can. Take a sharp knife and pry on the
element
by twisting the knife (see photo below). Work it gently all the way
around
the element, between the element and the housing, twisting the knife
slightly
as you go. There is a rubber gasket between the housing and the
element.
That is what is sticking the two together.
It's sort of like
shucking an
oyster. Slow
and very easy is the key here.
Check out the photos
below. A
sharp knife placed
at a tilt and slowly twist the knife. All of a sudden it will pop
out.
Tip: If
the
caps just won't unscrew
off by hand, heat them lightly with a hair dryer then give it another
go.
Just take your time and
you
won't have too
much problems.
The "section of knife" shown below is a "cross
section" of the knife.
We are not using the "tip" or "end" of the knife,
but
the "cutting
edge".
See photos below this one.
Note:
Not my
hands!!!
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Once the element is removed from
the
can,
you can solder new wires on to the "cans" contact arms fairly easy.
Element removed.
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Opening The Element Up
The three main units of the "can"
Cap (left), element Unit (middle), rear housing (right).
Another view.
Remove the screw in the center and carefully remove the
rear
cover
off of the element.
There is a rubber seal that sometimes sticks, so go
slow
and do not force it.
Tip: If it is sticky, heat
it
slightly
with a hair dryer. This soften the rubber gasket.
Now you can see the internal working of the element.
Remember, cleanliness is next to godliness at this point.
Any metal on your work place can find a way into the
element.
Now the rubber gasket is removed also.
Sorry for the bad photo!
Now what you have is the "workings" of the element.
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Re-magnetizing
Element
The magnets used to recharge the magnets on the
elements are
NdFeB.
Also known as neodymium or rare earth magnets.
As Gil Stacey says "They are so powerful that Steve
stores
them on
the roof of his
mobile magnet laboratory, which also serves as a pick-up
truck"(photos
below).
The round metal piece with a smaller pedestal is a pole
director
from a wrecked speaker which
concentrates the magnetism into the center of the
element’s
back.
Polarity is established with a compass and opposites are
placed
together.
A “controlled slam” is necessary to fully impart the
charge
and also
to prevent breakage of the magnets and bakelite housing. The
“controlled
slam” is nothing more than one hand holding the element to prevent the
element from striking the charging magnet’s pole director with
unrestrained
force.
Want to see a video of Steve Bringhurst charging an
RCA
element?
Filmed by that famous cinematographer, Gil Stacy
307 KB rm file (Real Movie)
Real Player
Click Here
Get Real Player Free at
2638 KB AVI file
Real Player, Quick Time (Apple)
Click Here
1.443KB WMV file
Windows Media Player
Click
Here
Cut! and Print!!
I'm hoping this works out as it is my first time
offering
videos
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Adjustment
of armature ("Centering")
Note: The label "perm. magnet unit" is not pointing to
the
magnet
itself, but instead
to the laminated plates that bring the magnetic field into play with
the
balanced armature leaf.
It is part of the "perm magnet unit".
Adjustments
Written by Gil Stacy
Technical skill by Steve
Bringhurst
One cannot determine the proper alignment of the
armature by
simply
viewing its relationship in the gap in which it sits. In other
words,
if the armature appears properly centered, that does not by itself
insure
that the element is in best alignment. Note in the attached photo
(below)
that the element is connected to the crystal set. It is tuned to
a station. While cupping the element in his hand to assist
amplification
so that the signal can be heard, Steve gently pushes the armature up
and
down to find peak volume. The sound is heard without the element
being placed to the ear; holding in the hand while working is
sufficient
to hear the sound, provided the hand is cupped. Manipulation of the
armature
is accomplished with a toothpick resting on the respective armature
upper
or lower armature nuts, alternately pushing up and down. The
direction
of best volume is noted and only then does he adjust the nuts in the
direction
of best volume.
With hemostats adjust the nuts in the direction
determined to
give
greater sound.
After retightening the nuts, the armature is
manipulated
again, and
if the sound change is the same in both directions, it is properly
aligned.
After alignment is completed, often the armature is perfectly centered,
sometimes it is not. This alignment technique while listening to sound
is as important, maybe more important, than re-charging the
elements.
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Tools
A good set of
hemostats or
maybe a
pair of them is the only thing needed for adjustment. If you can get
jewelers
wrenches that would be nice. Below is a spanner wrench that can
be
made for taking off the face of the element. I would not recommend
doing
this unless you have a major problem. The other reason for removal is
if
you are using the "mic" elements of a RCA handset and wanted to open up
the small holes in the center to 1/2" like the "ear" elements. It is
recommended
that you remove the face to do this. A nice description is below to
help
you make this spanner wrench.
 
Spanner
Wrench Construction
The spanner wrench was constructed from a
1/4" by
1" by 8"
piece of steel. It could be any size stock you may have on hand, but it
needs to be rigid enough so that it does not flex. The holes for the
pins
are 1 and 7/16" apart. The pins are 3/32" in diameter.
After
being pressed into the holes, they should be 1/8" long. If they are too
long they will hit the diaphragm, so it is best to keep them at
1/8".
Also, if the pins fit tight in the cover, file them where needed until
they fit good. A loose fit is better to avoid cracking the
fragile
bakelite. It's ok to use some light penetrating oil on the threads. The
hole in the center of the wrench is not required, I had a mark there to
find the center and just drilled a hole there when I was done. It can
be
used to watch the diaphragm while removing the covers with the big
(1/2")
opening.
Special thanks to Gordon McCall for the photo of
the
spanner wrench
and directions to build it.
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Crystal Radio
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