Heathkit EK 2A Book Part 1 100 Pages

Page 1

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SERiES

MODEL EK~2~UX

nEATH COMPANY BENTON HARBOR fmCHéGAN

Page 2

RESISTOR AND CAPACITOR COLOR CODES
' REBISTORS

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resent flu mu: In ohm. mu coloredbmdlxngmuped
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EXAMPLES

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CAPACITORS

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USING A PLASTIC NUT STARTER

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Page 21

_HEA'1HKIT

Page 11

Figure 2-14 is an enlarged view 01 modulated
radio signal in which you can see more clearly
the variations in amplitude caused by modula-
tion of the radio signal by an audio signal. This,
then, is a broadcast radio sigal, as it would
be graphed to show its make-up. Understanding
the nature of a broadcast radio signal is im-
portant because all of the functions of a re-
ceiver are designed to convert this signal
back into usable sound again, and this funda-
mental idea is essential in understanding the
functions of a receiver.

+

ZERO CURRENT

iilllilll

MODULATED RADIO FREQUENCY AC
CURRENT (BROADCAST RADIO SIGNAL).

Figure 2-14

HOW TO PUT UP A BROADCAST RECEIVING ANTENNA

PURPOSE

TO PLAN AND CONSTRUCT A LONG-WIRE
BROADCAST RECEIVING ANTENNA SUITED
TO YOUR PARTICULAR BROADCAST RADIO
SITUATION.

MATERIAL REQUIRED

1 75 ft. length of stranded bare antenna wire

1 30 ft. length of insulated wire for antenna
lead-in and ground wires

2 Porcelain insulators

1 Gromd clamp

STEP 1-CONSIDER ALL THE FACTORS

There are a number of things you should take
into consideration as you think about an ideal
location for your receiving antenna. Antenna
location is quite important since it will affect
the results you get with the various receiver
circuits built up later. You will want to plan
its location carefully by considering the following
antenna requirements:

1, A long-wire antenna, such as you will be
erecting, receives best off of its sides
(broadside) and poorest off of its ends (at
frequencies below 10 megacycles). Ideally,
your number-one broadcast station (from
the inventory chart in Lesson I) should be
located off one side of your antenna. if this
is not possible, the antenna should be ar-
ranged to favor the number-two broadcast
station from the chart.

2. The lead-in wire to your work areamustbe
connected to one end of the antenna, You
must also provide a ground lead from a
cold water pipe to your work area. This
means that the selection of a work area

in your home will be affected by the antenna
location you select, and the availability of
a cold water pipe or equivalent grounded
object from which to run a ground wire.

3. Two tall objects are required (buildings,
trees, poles, etc.) for hanging the antenna.

4. The antenna should be kept away from power
lines, rain gutters, metal buildings or
other large metal objects that might tendto
block, shield, or absorb signals (or cause
interference).

5. You have about 75 it. of stranded wire to use
tor the antenna and support wires, and 30
ft. of solid insulated wire for use as antenna
lead-in and ground wire.

All of the factors listed above will have an effect
on the selection of your antenna location, work
area, etc.

STEP 2 - PLANNING THE ANTENNA BEST
FOR YOU

From the number of factors listed in Step 1 it
is easy to see that you must compromise in
locating the antenna, depending on your par-
ticular situation, You may not have antenna
supports that are exactly in the right places,
or the work area that is most convenient for
your antenna lead-in may not be ideal for your
ground wire, etc. It is up to you to find the
compromise that best meets the five important
antenna requirements listed. This will require
some thought and planning, and a map of your
situation will prove quite helpful.

Page 25

HEA'IHK yr-

Page 15

LESSON Ill
WHAT THREE THINGS MUST A RADIO RECEIVER DO?

In your previous lesson the nature and charac-
teristics of a broadcast radio signal were dis-
cussed. (See Figure 3-1.) You found out that a
broadcast signal was an AC current alternating at
a very high frequency (1200 kc was used as an
example) and that the amplitude (amount) of
energy in this signal changed up and down in
time with the audio signal with which it was
modulated (1000 cps was used as an example).
The announcer's voice, or the sound of the
orchestra playing in the studio, was found to be
contained in the broadcast signal in the form of
these chance in amplitude.

ZERO CURRENT -

Tigure 3-1

Yet, if a loudspeaker were connected to a re-
ceiving antenna circuit, no sound wouldbe heard.
There are several interesting reasons why a
loudspeaker could not produce sound from a radio
signal as it is picked up from the antenna.

First, the form of modulated broadcast signal,
as sent out by the transmitter, is not such that
it will operate a loudspeaker or an earphone.
It needs to be "decoded," so to speak, to re-
cover the audio siflal that originated in the
studio. Remember, only an audio sigpal can
operate a loudspeaker or earphone.

Secondly, the signal on the antenna is so small
and weak that it could not operate a speaker,
and would be taint in an earphone, even if it
were in the proper condition to perform this
function.

))

And finally, since no means of selection is pro-
vided, there would not be just one signilinvolved,
but many signals being picked up by the antenna.
Even if the signals were strong enough, and
were of the proper form to operate a speaker, all
stations in the area would be heard at the same
time, and would interfere with each other.

This discussion of the deficiencies of a signal
when it reaches the antenna must suggest to you
the three main functions that a broadcast re-
ceiver must perform. it must first select the
desired signal, then it must amplify the signal
so that its strength is built up, and then it must
detect the signal, or in other words, transform
modulated radio frequency signal into its original
audio signal form. Knowing that these are the
three main functions a receiver must perform
before the signal is ready to operate a speaker,
will help you to understand how a broadcast
receiver operates.

RECEIVER CIRCUIT

A much-simplified block diagram of a broad-
cast receiver with speaker is shown in Figure
3-2. You will notice that the receiver divides
roughly into three sections: the radio frequency
section, the detector section, and the audio
frequency section.

The radio-frequency section (RF section) picks
up the signal from the antenna and performs
two functions. First it "tunes" the signal. In
other words, it selects one signal and rejects
all others, so the radio does not pick up more
than one station at a time. In addition, the
radio- frequency section also amplifies the
modulated radio-frequency signal in what are
called RF amplifiers. This builds up the signal
strength in preparation for sending it on to the
next receiver section.

BLOCK DIAGRAM 0F BROADCAST RADIO RECEIVER.

MODULATED
RADIO
SIGNALS Izoo Kc Iooo cPs
RADIO FREQUENCY DETECTOR AUDIO FREQUENCY
(RF) SECTION SECTION (AF) SECTION
ANTENNA SPEAKER

Figure 3-2

Page 42

Page 32

SCHEMATIC SYMBOL FOR FIXED-INDUCTANCE
COIL.

Figure 5-12
The schematic symbol for a fixed-inductance
coil is shown in Figure 5-12, and the physical
appearance of a typical radio-frequency coil
is shown in Figure 5-13.
COIL

COIL
FORM

SIMPLE RADIO FREQUENCY COIL.
Figure 5-13
A variable-mductance coil is shown schematic-
ally in Figure 5-14. The arrow drawn through
the coil shows that its inductance canbe changed.
This may be accomplished in a number of ways,
but a method frequently used in radio circuits
is to vary the core material by moving a
powdered iron core in and out of the coil wind-
ing. Such a coil, with a variable core, is shown
on Figure 5-15. Notice that a screwdriver ad-
justment is provided to move the core in and
out of the coil and thereby change its inductance.

SCHEMATIC SYMBOL FOR VARIABLE-
INDUCTANCE COIL.

C IL FORM

Wit

COIL

Figure 5-14

SCREW MOVES
POWDERED IRON
CURE IN AND OUT
OF COIL

VARIABLE-INDUCTANCE RADIO FREQUENCY
COIL.

Figure 5-15

The ability of both coils and capacitors to
"kick-back" a pulse, in reSponse to a pulse
applied to them, is an important point to re-
member lrom this discussion of coil and ca-
pacitor characteristics. You will find that this
kick-back action is what makes a tuned cir-
cuit work, when a coil and a capacitor are con-
nected together.

mm

COIL :2 CAPACITOR

PARALLEL TUNED CIRCUIT.
Figure 5-16

When the "kick-back" action of a capacitor
is combined properly with the "kick-back" of
a coil, a tuned circuit results. A parallel tuned
circuit (the type employed in the experiment
to follow), consists of a coil and a capacitor
connected in parallel (see Figure 5-16). The
size of the capacitor and coil (capacitance of
capacitor and inductance of coil), in a tuned
circuit must be such that, at a certain fre-
quency, the capacitor kick-back, the coil kick-
back, and the frequency being fed to the circuit,
are all three synchronized. The result is an
"oscillating" action within the tuned circuit,
stimulated by pulses from the incoming fre-
quency, which enables the circuit to "tune"
this one frequency and reduce its response to
all others. This synchronized action occurs only
at the resonant frequency of the tuned circuit.

The resonant frequency of a tuned circuit de-
pends on the value of the coil and capacitor
used. Changing the value of either part (the
capacity of the capacitor or the inductance of
the coil) changes the resonant frequency of the
circuit. Most circuits of this type are tuned
with a variable capacitor. In the circuit you
will build, the shaft of a variable capacitor is
rotated to change its value.

An example of physical resonance (involving a
tank of water), might help you to understand
even more clearly what electrical resonance
means in a timed circuit.

RE SONANCE

Water moving in a trough (Figure 5-17), is like
a tuned circuit in some ways, and you can
investigate its action to help visualize resonance
in a tuned circuit,

Page 43

Page 33

_HEATHK1T
FADDLE STARTS WAVE
31 ,. 'm LEFT ,.
r <7 7'
xFILE UP

ash-T

r PILE up; ,_

CYCLE COMPLETED

' R7-

nNIsH

WAVE ACTION IN TANK OF WATER HELPS
YOU UNDERSTAND TUNED CIRCUITS.

Figure 5-17

It a paddle gave the imaginary water in a tank
a push to the left as shown in Figure 5-17, and
was then removed, you would see a wave travel
to the left end of the tank, pile up for a moment
there, travel back to the other end of the tank,
pile up for a moment, and move back to the left
side of the tank again, etc. Each time the wave
travels back and torth, it decreases in size
because of the resistance it meets in trying to
flow back and forth. However, assistance from
the paddle, if it were synchronized, could keep
this wave action moving back and forth in-
definitely.

At the natural resonant frequency or the tank,
little effort would be required from the paddle
to keep the wave motion going continuously. A
paddle motion in time with tankresonance would
require slight exertion, whereas much greater
effort would be needed to push a wave motion
that was either faster or slower thanthe natural
resonant frequency of the tank. Shortening the
tank would increase its resonant frequency, or
lengthening the tank would decrease its reson-
ant trequencyI

An electrical tuned circuit is much like the
tank of water. The coil on one side and ca-
pacitor on the other side (like the ends of the
tank), act to answer each others pulses, and
cause circulating current at the resonant fre-
quency of the circuit. The resonant frequency
is determined by the electrical size of the coil
and capacitor. Only little stimulation from an
outside circuit (equivalent to the paddle) is
necessary to keep this circulating current going
at the resonant frequency.

If either the coil, or the capacitor, is made
larger, the resonant frequency of the circuit
decreases, while if either part is made smaller,
the resonant frequency increases. Tuning is
accomplished by changing the value 01 either
part, but the capacitor is usually the element
changed to tune the circuit to various resonant
frequencies.

CURRENT CIRCULATES 0R "OSCILLATES"
BACK AND FORTH FROM CAPACITOR TO
COIL AND COIL TO CAPACITOR, AT
RESONANCE.

Fig; re 5-18

Figure 5-18 shows how the circulating currents
move back and forth from capacitor to coil, and
coil to capacitor again. This circulating current
dies out if not stimulated by current from out-
side the parallel tuned circuit.

Page 44

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Page 34
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/ FROM ANYENNA.
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INCOMING SIGNAL STIMULATES CIRCULATING

CURRENT IN TUNED CIRCUIT WHEN SIGNAL

AND CIRCUIY ARE IN RESONANCE.

Figure 5- 19

Figure 5-19 shows a tuned circuit connected into
the antenna-ground circuit so that the incoming
signal stimulates circulating current when the
tuned circuit is exactly resonated to the fre-
quency of the incoming signal. Notice that
the current flowing up and down the antenna-
ground circuit, caused by an incoming signalY
acts on the tuned circuit to keep it oscillating.
Only a small amount of incoming signal, there-
fore, can stimulate a relatively strong circu-
lating current in the tuned circuit at its natural
frequency, thereby adding to signal strength,

and "favoring" the tuned signal over signals
at other frequencies not matching the resonant
frequency of the circuit. The idea of acapacitor
and a coil operating together as a tuned circuit
is a very fundamental one in electronics. There
are many types of tuned circuits, used in prac-
tically all equipment designed to handle radio
frequency signals. The fundamental principles
of tuned circuit operation remain the same even
though different frequencies may be involved,
or the physical appearance of the coils or ca-
pacitors may not be the same.

As the capacitor of your tuned circuit is changed,
the resonant frequency of the circuit changes
and sweeps across the broadcast band to select
first one, and then another of the stations being
picked up on the antenna. When the resonant
frequency of the tuned circuit is matched to the
frequency of the incoming signal, the signal will
be passed on to the rest of the circuit, and other
signals will be reduced in amplitude. This effect
will be demonstrated in the experiment that fol-
lows. You will add a tuned circuitto the detector
circuit built in the previous lesson, so that your
basic "radio" will have some degree of "selec-
tivity."

HOW TO BUILD A TUNED CIRCUIT FOR RADIO SIGNALS

w

TO ADD A TUNED CIRCUIT TO THE CRYSTAL
DETECTOR SET AND INCREASE ITS SELEC-
Tl'VlTY

PREPARING THE CHASSIS FOR THIS EXPERI-
MENT

Remove the 3.3 megohm resistor, the crystal
rectifier, and the hookup wire between T-2
and the antenna post. The chassis is now pre-
pared for construction of this experiment.

MATERIALS REQUIRED

47 mini ceramic capacitor
Regenerative detector coil
Two-gang tuning capacitor

Rubber grommets, 7/16" diameter
Rubber grommets, 5/16" diameter
#6 solder lug

#6 flat washers

#6-32 screws

Knob, (gray)

Spacers

Length insulating sleeving

Crystal diode rectifier, from previous
experiment

D-t-wHwaathHI-u-n

Page 57

-HEATHKTT

Page 47

Figure 6-18

swncu ON
I MEG. CONTROL

GREEN BLACK

SCHEMATIC DIAGRAM (SHOWING HEATER
CIRCUIT SEPARATELY).
When instructions call for wiring from one of the
tube socket terminals to the center shield at the
socket, use bare wire for this purpose. Bare wire
is obtained by merely stripping the insulation
from regular hookup wire,

The heater circuit, from the power transformer
to the heater terminals of each tube socket, is
wired first,

As mentioned earlier, all connections should be
soldered as the circuit is wired, unless otherwise
specified. Wires can be added to a terminal
rattler easily at any time if temporary-type
lap joints are used.

( ) Connect one green transformer lead to the
#8 ground lug next to the transformer,
and the other green lead to lug4of the first
t be socket closest to the transformer. See

igure 6-16.

Connect a length of hookup wire from lug
4 or! this same tube socket to lug 3 of the
second socket.

(9/ Connect another length of hookup wire
from. lug 3 fit the second tube socket to
lug 3\o(' the third tube socket (follow
Figure 6-16 for this wiring from socket to
socket).

( VL/onnect a length of hookup wire from lug 3
of the third socket, overto lug4ot the fourth

tube socket.

(bi/Connect a short length of bare wire from
the center post of the second tube socket
to lug 4 on this socket. (See Figure 6-16.)

( / Connect a short length of bare wire from
the center post to lug 3 on the first and
fourth tube sockets.

(V3 Connect a length of wire from the center
post of the fourth tube socket to ground
lug F.

( )The third tube socket (which is the [5%
socket at the moment) does not use a
ground jumper wire at this time. Connect
the long lead from the pilot lamp to lug 4
of the 6C4 socket by passing it through the
chassis hole next to this terminal. (See
Figure 6-16.)

(ii/)1 Connect a bare jumper wire between lugs
5 and 6 of the 6C4 (3rd) socket.

(V Connect a wire from the orange solder lugot
e regenerative detector coil over to lug
oi the 604 tube socket.

I
(A Connect a wire that is long enough to reach
T-Z, to lug 5 of the 6C4 socket. Do not
connect this wire to the "'1" terminal strip
at this time, since it must be disconnected
later for a test.

(La/Twist two 7" lengfl'isothookup wire together
and run them between the two switch
terminals on the back of the 1 megohm
control, and L-3 and L-4 o! the 4-lug
terminal strip near the power transformer.

(yonnect the two .001 mfdceramic capacitors
between L-l, L-Z and L-3 of the 4-1ug
strip as shown in Figure 6-16.

(LiCBnnect the black and red-black trans-
former leads to L-l and L-S of the 4-lug
terminal strip as shown.

(146131 the line cord leads and connect one
to L-l and the other to L-4 of the 4-lug
strip as shown.

( )vcfiiect the 150 Kohm (brown-green-yel-
low) resistor across terminals 1 and 2 of
the earphone socket as shown in Figure 6-1 6.

This completes the wiring of this experiment.
All connections should be soldered except the
free lead from lug 5 of the 6C4 socket and the
free crystal diode lead.

Page 63

Page 53

Egan-nun-
GRID CONTROL OF CATHODE~TO-PLATE
PLATE CIRCUIT
G RID
Mius (7 7
(mar) mg
VOLTAGE
CATH ODE - «1

SCHEMATIC SYMBOL or A TRIODE TUBE.
Figure 7-4
tube in Figure 7-4 also shows the grid posi-
tioned between the cathode and the plate, so
that electrons moving through the vacuum tube
must pass through the grid to reach the plate.

f... _

+
Pws

FULL VOLTAGE

~I |+
I|
POSITIVE GRID AIDS ELECTRON MOVEMENT
FROM CATHooE TO PLATE.
Figure 7-5

If a triode tube were substituted for the diode
tube in the simple circuit of Figure 7-2, the
circuit would appear as in Figure 7-5. If the
triode grid is then connected to a positive
voltage, it will act to assist the plate in pulling
electrons across the vacuum in the tube, and
the circuit will function as it did when a simple
diode was used. Current flows fromthe negative
terminal of the battery, through the tube from
cathode to plate, through the resistor, and back
to the positive terminal of the battery. In the
process of helping the plate attract the elec-
trons away from the cathode, the positive grid
also accumulates some of the electrons on its
spiral wires. However, this effect will be
ignored for the present since the grids limited
surface area accumulates a much smaller num-
ber of electrons than the plate, and because this
particular action is not essential to the basic
idea of grid control to be discussed in this les-
son.

,l .,
VERY NEGATIVE GRID STOPS ELECTRON MOVE~
WEN-T FROM CATHODE TO PLATE.

Figure 7-6

If the grid in a vacuum tube is connected to a
highly negative voltage, this voltage will ac-
tually block the flow of electrons from cathode
to plate. High negative grid voltage repels
electrons and forces them back to the cathode.
This keeps electrons from passing through the
grid to reach the plate (see Figure 7-6). Under
this condition, the grid, with a high negative
voltage, is actually causing the cathode-to-grid
circuit to become "open" so far as the battery
is concerned. No current can then flow from the
battery around the circuit, because it is blocked
at the vacuum tube. Neither is any voltage
developed across the resistor in the circuit
because no current flows through it.

It seems quite clear that the grid in the vacuum
tube can, in effect, turn the cathode-to-plate
circuit g or g, depending on the voltage
applied to the grid. When the grid is positive,
it aids and accelerates the movement of elec-
trons irom the cathode to plate (Figure 7-5),
while when it is highly negative, it can actually
block the movement of electrons from cathode
to plate (Figure 7-6). In a sense, the grid
acts as a "switch in these two extreme condi-
tions. The grid has been given the name control
flfor this reason.

Between the condition where the control grid is
highly negative and stops all current flow, and
the condition where it is positive and assists
current flow, there is a range of grid voltage
that cuts down on the movement of electrons
from cathode to plate but does not stop this