June 1940 Radio-Craft
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
Do you know what a 'gimmick' is in the RF circuit world? If you
have ever had the occasion to repair or recondition inductors
(coils, chokes, etc.), then you have
probably seen one and probably didn't know what it was. Read on
to learn about a gimmick capacitor (they
yield about 1 pF/inch, see Stackpole ad below and
twisted pair impedance calculator). Working with the
tiny wires on those old coils can be a real challenge. Words you
hadn't uttered in a long time
to move to the forefront of your memory in the process. Having struggled
with a few multi-layered RF coils from old radios, I am familiar
with the intricacies of trying to use fat fingers to wind and splice
40-gauge enameled wire in spaces 1/8-inch wide. Many of those older
coils are wound in thin, tall rings whose wires have a tendency
to slide off the sides during manipulation. A useful trick not covered
in this article is to make thin wood (I use balsa, but pine or thick
cardboard will work) forms to press against the sides and keep everything
in place. There are lots of great tips here.
Servicing R.F. Coils
This article, No.1 of a series, shows Servicemen how to repair
radio-frequency coils so as to save the time and money otherwise
involved in making exact replacements or in waiting for factory
repairs. Much valuable and practical information you cannot find
in books is published here for the first time.
Lawrence V. Sorensen
Typical replacement of primary windings.
The question of replacements for defective radio-frequency coils
has long been a nightmare to Servicemen. Many of these technicians,
realizing the amount of design-work embodied in such coils, have
insisted upon exact duplicate replacements or that the defective
coil be repaired by a coil company. In either case, considerable
delay often resulted.
Most Servicemen apparently have not realized that the part of
the coil that fails in normal service is the part which was manufactured
to rather broad tolerances of inductance, and that this part (the
primary) can easily be replaced since it has only a minor influence
on the "tracking" of the receiver circuits. Therefore, if the defective
primary is replaced, the original coil with its accurately-controlled
secondary inductances can be salvaged.
The Serviceman can give best service to his customers, when R.F.
coils fail, if he is familiar with replacement primary windings
and knows how to use them, and repairs the coil himself.
Repairs - In Detail
The advantages of being able to repair radio frequency coils
are obvious to every Serviceman. It remains only for him to convince
himself that he, personally, can do the job.
Many Servicemen, who are well able to correct trouble in any
make or type of receiver, hesitate to tackle the repair of an R.F.
coil because of inadequate knowledge about the design constants
of such coils, or because they believe that only trained feminine
hands can properly handle the fine wire used. The first objection
has been overcome by studies made by the Meissner Mfg. Co. which
has determined that for any Broadcast-band Antenna or R.F. coil,
one of 3 values of inductance will serve admirably, and as far as
replacements for Shortwave primaries are concerned, these primaries
usually consist of only a few turns of wire which can be replaced
with an equal number of turns of No. 36 S.S.E.
The question of ability to handle the fine wire of which broadcast
primaries are made can easily be settled after a few minutes' practice
with a piece of sandpaper and a piece of No. 36 S.S.E. or No. 38
S.S.E. wire, or the outside lead of one of the replacement primary
windings. (If the outside lead should break off too short, a few
turns can always be peeled off the coil to give the required lead
length without materially altering the performance of the coil.
This, of course, positively is not true in the case of secondaries,
which must be held to close tolerance of inductance.)
It has been found that if a piece of No. 00 sandpaper is folded
and cut in accordance with the sketch in Fig. 1 and is held between
the thumb and forefinger the insulation - first the fabric, then
the enamel - can easily be stripped off of the wire no matter how
fine the wire, if appropriate pressure is used on the sandpaper
while sanding the wire. Much too little pressure will require a
long time to strip the insulation, while too great a pressure will
break the wire. A few moments' experiment will quickly inform the
Serviceman of the proper pressure to use. There is one point that
should be stressed, and that is, that in his determination not to
break the lead, the Serviceman should not make the mistake of failing
to properly clean off the enamel. If the latter insulation is merely
scratched through in a few places, it is not possible to make a
good connection. The solder will not stick to the few bright scratches
so made. The wire must be thoroughly cleaned.
Sandpaper for cleaning wire.
A few trials at cleaning size 38 enameled wire will soon convince
the Serviceman that he can do just as good a job of cleaning the
wire without breaking it as can the trained fingers of feminine
coil operators, although undoubtedly he will be somewhat slower.
A trick that may help to avoid breaking the wire, is to rub the
2 surfaces of the sandpaper together before attempting to remove
any insulation. This action removes the high spots on the sandpaper
which tend to grab the conductor, and makes the action of the sandpaper
smoother and much easier to control.
Replacements - Step-by-Step
The following section is a step-by-step set of directions for
replacing a defective primary on a radio-frequency coil. A typical
group of replacement Primary Windings is shown in Fig. 2. Some sections
of the directions may seem obvious when read, but may easily be
overlooked until too late if the task of replacing a defective primary
is started without careful consideration of each step, in sequence,
because once a coil has been torn apart it may be too late to observe
certain details, that it may be necessary to know, in order to properly
complete the job.
(1) Make a clear diagram of all leads connecting to the coil terminals,
marking the color of each wire and the position that the coil occupied
in the receiver. This should be done carefully and rechecked before,
or as, the wires are removed. (See Fig. 3.)
Coil winding direction.
Measure separation distance of solenoid or transformer.
Replacing or reinstalling secondary winding cylinder
Reconnecting winding wires. Creating a "gimmick."
Observe direction of windings to ensure proper phasing.
(2) In removing the leads from the coil, take care to put no
unnecessary strain on the coil terminals lest the lugs move and
perhaps break off some lead from a good winding attached to the
lug. If the hook-up wires are hooked through and twisted around
the coil terminal so that it is difficult to get them loose, it
is best to cut the wires, close to the lug. After being cut, the
short pieces of wire are usually easy to remove or, if such is not
the case, the ends had best be left attached to the lugs and the
hook-up wire merely soldered to the lugs without going through or
around them when the coil is re-installed.
(3) Carefully examine the defective winding, which is to be replaced,
in order to determine the winding direction and the lugs to which
the ends of the winding connect. This information should be carefully
recorded. A convenient method of designating winding direction is
to use an arrow pointing as if its shaft were the outside end of
the coil, and the head of the arrow were the end of the wire. (See
(4) The exact location of the winding on the form in relation
to the other windings should be recorded, and the defective winding
removed carefully to avoid damage to other windings or connections.
(See Fig. 5.)
(5) If the defective winding consists of only a few turns of
wire wound adjacent to, over, or between the turns of a secondary,
this winding can be replaced with an equal number of turns of No.
36 S.S.E. or 36 D.S.C. wire.
(6) If the defective winding was of the "Universal" or honeycomb
type, a (Meissner) replacement primary should be chosen as near
the physical size of the original winding as possible, and yet be
able to slip into place. In some cases, unfortunately, lugs or other
windings interfere with slipping onto the coil form a new winding
close to the size of the defective winding. In such cases a new
primary just large enough to slip over the obstruction should be
selected and fastened in place by means of small hardwood wedges
held in place by wax, or "radio cement." (See Fig. 6.) (Make certain
that the winding direction is correct.)
The inductance of the replacement winding selected is determined
by the type of coil being repaired. A Broadcast R.F. coil takes
the highest inductance, approximately 7.5 millihenries; an antenna
coil for use with an outside antenna takes the lowest inductance,
approximately 1.7 millihenries; while an antenna coil for an inside
or "hank" antenna takes a value between the other 2, approximately
2.25 millihenries. Since these divisions have been so clearly drawn
after a study of the replacement problems by the Meissner Manufacturing
Company, there should be no doubt in the Serviceman's mind as to
which value to select.
(7) Connect the replacement winding leads in accordance with
the notes previously made concerning winding direction and connections.
(8) Check the coil for continuity on all windings and re-install
it in the receiver in accordance with the notes made in section
(9) Align the receiver, and adjust the coupling if necessary
as described in the following paragraphs.
When a Serviceman is called upon to repair a radio-frequency
coil from which the defective winding has already been removed,
or has been so badly damaged that the winding direction can not
be discovered, there are 4 questions that must be answered:
(1) Which lug was the antenna or plate connection?
(2) What was the probable inductance of the coil?
(3) Where was the winding located?
(4) Which way was the outside end of the winding pointing?
If there is no data to show which lug was connected to the antenna
or plate, the question must be answered from an inspection of the
coil, or an answer must be assumed and the coil repaired and rewired
If there is any kind of a coupling condenser used it will be
found connected from the grid end of the secondary to the plate
or antenna end of the primary, which immediately establishes a certain
lug as the plate or antenna connection. The coupling condenser may
take the form of 2 metal plates separated by a piece of mica and
attached to the coil form by means of rivets or lugs (Fig. 7A).
It may be a loop of heavy wire circling the secondary near the grid
end (Fig. 7B). (This form is used only with solenoid or bank-wound
coils.) It may be a few turns of insulated wire wrapped closely
round another insulated wire forming what is commonly known as a
"Gimmic" (Fig. 7C). In any case, the purpose of the capacity coupling
is to transfer energy from the primary to the secondary.
In the case of "choke coupling," used frequently in R.F. coils,
the choke is either at right-angles to the secondary, or at a considerable
distance from it, and the coupling condenser constitutes the sole
means of coupling between primary and secondary. Unless the coupling
condenser is properly wired to the coil and into the receiver, practically
no coupling would exist in the coil concerned. The winding direction
of the primary in this type of coupling has practically no effect
on the gain of the coil, and it may accordingly be connected either
In the case where both magnetic and capacity coupling are employed,
the purpose of the capacity is to hold up the gain at the high-frequency
end of the band. The capacity coupling aids the magnetic coupling
in such cases. Should a primary be connected reversed, the capacity
coupling would oppose the magnetic coupling and would produce inferior
performance at all frequencies and approximately zero amplification
at some one frequency resulting in decreased sensitivity at all
frequencies but especially poor sensitivity at the one frequency
where the magnetic and capacity coupling cancelled.
In some antenna coils, especially in sets with only 2 sections
in the tuning condenser, the stray capacity between the "hot" end
of the primary and the "hot" end of the secondary is used to buck
out the magnetic coupling at some frequency above the band (in frequency)
for the purpose of improving the rejection of interfering signals
in that frequency range. In superheterodyne receivers this improves
the "Image Ratio" of the set. With the exception of this case, which
is by no means universal, the rule for capacity coupling on radiofrequency
coils is that the wires leading away from the coupling condenser
must o around the coil form in opposite directions.
In the event that no physical coupling condenser exists, and
no data is available to tell which were the "hot" and "cold" ends
of the primary, the corresponding lugs should be chosen arbitrarily
and the primary connected "capacity aiding," that is with the wires
from the grid of the secondary and the "hot" end of the primary
going around the coil form in opposite directions. (See Fig. 8.)
After repairing a coil and re-installing it in the receiver,
the circuits should, of course, be aligned. The normal practice
should be followed, using some form of service oscillator for a
signal source, connected through a satisfactory dummy antenna to
the radio set. The usual values of dummy antenna are 200 mmf. for
band of sets intended for use with an outside antenna, 85 mmf.
for the broadcast band of sets using a "hank" antenna and 400 ohms
of resistance for shortwave bands.
If the repaired coil is used on the broadcast band, the circuits
should be aligned at 1,400 kc. and then "tracking" checked at 600
kc. If the set originally "tracked" well and the coil has been repaired
as directed above, it will "track" well after the repair. Of course,
there is usually no chance to find out how well the set "tracked"
before the repair, but in the case of multiband sets, it is reasonable
to assume that if all of the other bands track well, that the band
having the defective coil also tracked well.
When tracking is poor at the low frequency end of the repaired
coil and is good on all other bands, the coupling on the repaired
coil probably needs adjustment, but if tracking is poor in the
same direction on all bands it is probable that the gang condenser
is off its normal value by a small amount.
An experienced Serviceman sometimes bends the plates of the condenser
to improve tracking but this "emedy should not be attempted by
some one not thoroughly familiar with the work, If the plates are
bent to improve tracking, the adjustment should be made on a band
that has not had a primary replaced, and then the primary adjusted
on the broadcast band to obtain good tracking at 600 kc.
If the circuit appears to require more capacity at 600 kc. than
the gang condenser supplies, the coupling is too tight and should
be loosened by moving the primary farther away from the secondary;
while if the condenser seems to be supplying too much capacity,
the coupling is too loose and should be tightened by moving the
primary closer to the secondary or, in the case of solenoid windings,
closer to the center of the secondary. The latter case is likely
to occur when the replacement primary had to be larger than the
original in order to slip over some obstruction.
A convenient method of checking tuning capacity at 600 kc. is
to insert between the plates of the tuning condenser a thin piece
of celluloid while watching the output meter. This adds a little
capacity to 1 section of the tuning condenser without changing
the tuning of the other circuits. If the meter reading decreases
when the celluloid is inserted the capacity is too low or the coupling
is too tight. If the meter reading decreases, the capacity may
be correct or high. Coupling should then be tightened until the
celluloid slip test just shows too little capacity, and then loosened
If care is taken to see that the replacement winding is properly
placed (coupling adjusted if necessary) and proper attention is
given to the winding direction and connections, there should be
no difficulty whatsoever to prevent the Serviceman from giving
his customer a satisfactory job in much less time than would be
required to obtain an exact duplicate replacement coil
or to return the defective coil' to a coil manufacturer to be
repaired or duplicated.
Next month: "Servicing I.F. Coils."
Posted June 22, 2015