Heathkit AR-14 Schematic

CIRCUIT DESCRIPTION

Each major section of the Receiver will be
described separately in the following Circuit quency control) voltage to lock-in the local

Description. For ease of explanation, the Source oscillator frequency with the station being

switch will be described in the FM position.

Follow the circuit on the Block Diagram (fold­out from Page 66) and on the Schematic (fold­out from Page 79) while reading the Circuit

Description. The letter-number designations

(R4,

C115, R212) for all resistors, capacitors,
and diodes have been placed into the following
groups to make them easier to locate on the
chassis and Schematic.

1

-

99

100 - 199

200 - 299

FM TUNING UNIT

The FM signal from the antennaisappliedto the

primary of balanced input transformer

FM tuning unit. The secondary of transformer

forms a tuned circuit with trimmer capacitor Cl

and capacitors C2 and ClA (antenna section of
tuning capacitor). The signal selected by this
tuned circuit is coupled through capacitor C3 to
RF amplifier transistor

The signal is amplified by transistor
tuned circuit of
and couples it through capacitor C8 to the base
of mixer transistor Q2. The RF tuned circuit con-

sists of coil L2, trimmer capacitor C6, and ca­pacitors C7 and
itor).

The local oscillator transistor, Q3, operates at a
frequency that is 10.7 mc higher than the received
FM signal. The oscillator frequency is de­termined by a tuned circuit composed of coil L4,
trimmer capacitor C14, and capacitors Cl3 and
C14A

(oscillator portion of tuning capacitor). The
output signal from this oscillator is coupled
through capacitor
transistor Q2.

A small DC voltage is coupled from the ratio de­tector circuit through resistor R5 to diode
in the collector circuit of the oscillator. This

DC voltage,

which changes as the tuning is

FM tuner section.
Amplifier section.
Power supply section.

Ql.

Ql.

Ql

selects the desired signal

C6A

(RF portion of tuning capac-

Cl0

to the base of mixer

Tl

in the

The RF

Tl

Dl

changed,

tuned in.
The oscillator frequency is locked-in by the AFC

voltage in the following manner: The capacitance
between the elements of diode Dl changes when
the AFC voltage that is applied to it changes.
This capacitance is connected in series with

capacitor

connected in parallel with part of coil L4. Thus,

when the capacitance of diode Dl is changed by
the AFC voltage, the total capacitance across
coil L4 is changed. This change in the tuned
circuit capacitance changes the frequency of the
oscillator in such a way as to maintain proper
tuning.

The oscillator and the received FM signals are
mixed in transistor Q2 to produce a 10.7 mc IF
(intermediate frequency) signal, that is coupled

through transformer T2 and capacitor C21 to

the first IF amplifier transistor, Q4. The
amplified IF signal from the collector of tran-

sistor Q4 is coupled through transformer T3
to the base of second IF amplifier transistor
Q5. This IF signal is again amplified by tran-

sistor Q5, coupled through transformer T4,
and amplified by transistor
signal is coupled through transformer T5 and
amplified by the fourth IF amplifier transistor

Q7.

A portion of the signal voltage is taken from the

collector of transistor

D2 to produce an AGC (automatic gain control)

voltage, This AGC voltage, which increases and
decreases with the strength of the received FM
signal,
R12 to the base of transistor Q4, where it
automatically controls the gain of the IF signal.

The AGC voltage is then coupledfrom the emitter
of transistor Q4, through resistor
of RF amplifier transistor
ically controls the gain of the FM signal in

the tuner section.

is used as an AFC (automatic fre-

C19.

and these two capacitances are

Q6.

From Q6 the

Q6

and rectified by diode

is coupled through resistors R14 and

RlO,

to the base

Ql.

This automat-

Figure 11

All, or only one of the IF amplifier stages may

operate as limiters. For a very weak signal,

only the fourth IF amplifier

and transistors Q4, Q5, and

Q7

may be limiting,

Q6

would be
amplifying the IF signal. For a very strong
signal, all four IF amplifiers may be acting as

limiters. This limiting action removes

ampli­tude modulation from the FM signal. Limiting
action is also provided by the self-limiting
characteristics of the ratio detector circuit.

RATIO DETECTOR CIRCUIT

From the collector of transistor Q7, the IF signal
is coupled through resistor R26 and ratio de-

tector transformer T6 to the ratio detector

cir-

cuit. This circuit, which separates the audio sig-

nal from the 10.7 mc IF signal. is shown re-

drawn for greater clarity and simplified in Fig­ure 11. Transformer T6 is represented in this

Ll,

figure by primary coil
ondary composed of coils L2 and L3.

a center tapped sec-

and a

third
or tertiary winding. L4. L4 is just a few turns of
wire

tightly

mary

wrapped around the bottom of pri-

Ll.

NOTE: In the actual circuit, choke L5
and coil L6 are also connected in series with
coil L4, resistor R31, capacitor C39, and re­sistor R39,

Notice that each diode has its own separate loop
through which its current flows (indicated by the
arrows). Current flowing in diode D3 is con­trolled by the voltage induced in L2 and L4 which

charges capacitor C33. The current flowing in
diode D4 is controlled by the voltage induced
in coils L3 and L4 which charges capacitor C34.
Current flows through L4 in both directions,
since this coil is common to both current loops,

The two currents flow through capacitors C33
and C34 in the same direction. Electrolytic
capacitor C35 is connected across both of these
capacitors through resistors R27 and R30.
This large capacitor keeps the total voltage
across these two capacitors from changing, thus,
any amplitude changes on the IF signal are
damped out by this capacitor.

The audio output signal from the ratio detector
circuit is applied to the base of Q8. Note that
the two loop currents are flowing in opposite
directions through coil L4, resistor R31, capac­itor C39, and the input resistance of Q8. At the
FM IF center frequency of 10.7 mc, the diode
currents are equal, thus they cancel each other
out and no voltage appears across the input re­sistance of Q8.

Consider a separate voltage to be induced by the

primary into each of the windings, L2, L3, and

L4. L4, which is closely coupled to the primary
introduces a voltage that is in series with both
L2 and L3. This voltage across L4 is relatively
constant in amplitude as long as the voltage

Ll

across
voltage across

does not change. (Remember, the

Ll

will stay relatively constant

due to the limiting action of transistor Q7.)

When the IF frequency deviates from 10.7 mc
due to FM modulation (audio signal), the current

in one diode loop increases while the current in
the other loop decreases. These changes are
caused by a change in phase relationship in the
signal current across coils L2 and L4. and L3
and L4. Now current flows through the input
resistance of Q8 in the direction of the larger
signal, and an output voltage is developed

RIC>HT signal

R

Figure 12

across the input resistance of Q8, The ampli­tude of this output voltage is determined by
how far the IF frequency deviates from the
center frequency of 10.7 mc. The frequency
of this audio output voltage is determined by
how often the frequency deviates from 10.7 mc.

.-

The slug in the secondary of coil T6 is used to
balance the ratio detector circuit. Capacitor
C36 and L5 removes any remaining 10.7 mc IF
signal from the audio signal. Resistors R28 and
R29 are load resistors for diodes D3 and D4.

FM STEREO MULTIPLEX CIRCUIT

Figures 12A and 12B show two sample signals

that might appear from the left (L) and right

(R) channel microphones of a radio station that
is broadcasting a stereo FM signal, The trans­mitting circuits then combine these signals to

produce the

L+R

signal shown in Figure 13Aand

the L-R subcarrier signal shown in Figure 13B.

The L-R subcarrier signal is a suppressed
carrier amplitude modulated signal on a 38 kc
subcarrier, and is called the subcarrier channel.

Figure 13

These two signals,

L+R

and L-R, arc then com-

bined with the 19 kc pilot signal shown in Figure

13C. This whole complex signal modulates the
FM carrier and is then radiated from the
broadcasting antenna.

Figure 14 shows the locations of

the different

components that modulate an FM stereo signal.

The “main channel”

15 kc. Monaural

signal is from 50 cps to

FM

tuners use only this part of

the signal, and the remaining parts are atten-

uated by the tuners de-emphasis network,

L+R

AUDIO

MULTIFLEX SIGNAL’

L-R

I

sca signals

lls

TTED
IONS 1

Figure 14

A 19 kc pilot signal is transmitted to give the
proper phasing for the demodulated
channel, The 38 kc

subcarrier

subcarrier

channel is

AM

modulated from 23 kc to 53 kc.
A second subcarrier signal is transmitted by

some stations at 67 kc. This is usually a com­mercial music signal. This signal is called the
SCA (Subsidiary Communications Authorization)
channel.

The signal that is used for stereo multiplex
operation is coupled from the ratio detector.
through capacitor C39 and the SCA filter, to
audio amplifier transistor Q8. The SCA filter,
which

consists of coil L6 and capacitors

C4O
and C41, removes the 67 kc SCA signal. These
signals are not used for stereo reception.

and main channel signal that comes from tran-

sistor Q9. Waveform

2 is the 38 kc oscillator

signal that is reinserted in the stereo signal at
the same phase and frequency as the original 38

kc carrier. Remember, this 38 kc oscillation

was locked at the correct frequency and phase by

the 19 kc pilot signal from

Q9.

The actual detection process takes place in the

following manner:

When waveform 3 is applied

to the switching detector transistors, Q12 only

conducts
ries the L waveform, Thus, only the

form 4 appears at its output. Transistor

only

on

that part of the waveform that car-

L

wave-

Qll

conducts on the R portion of the 38 kc
waveform, thus only the R waveform 5 appears
at its output. These are the left and right sig-

nals originating at the broadcasting station.

The complete stereo multiplex signal consists of
the

(L+R)

main channel, the (L-R) subchannel,

19

and the

kc pilot signal, The complete stereo
signal is amplified by transistor Q8 andcoupled
through capacitor C43 to

19

kc amplifier tran­sistor Q9. The collector circuit of transistor Q9
is tuned to 19 kc by

coil

L7 and capacitor C44.

Phase control R43 and capacitor C45 are con­nected across a portion of coil L7 so the phase
of the 19 kc signal can be adjusted, The 19 kc
signal is then coupled to the base of the 38 kc
oscillator transistor,

Q10.

where it locks the

38 kc oscillator in phase and frequency with the
transmitted 38 kc subcarrier signal,

The 38 kc oscillator signal from transistor QIO
is applied through transformer T7 to the base
circuits of switching detector transistors Q11
and Q12. At the same time, the main channel

(L+R) and subchannel (L-R) signals are coupled
from the emitter of transistor Q9 to the emitters
of transistors

Qll

and QI2,

When the main channel and subchannel signals
are combined with the 38 kc oscillator signal in
the switching detector

circuit,

the 38 kc carrier
that was removed at the transmitter (suppressed
carrier transmission) is reinserted into the

stereo signal (waveform 3 on Block Diagram),

in

Figure 16, the 38 kc signal is shown super-

imposed on the stereo signal. At each 38 kc

peak on the L waveform, Q12 conducts and Qll.

is cut off. At each 38 kc peak on the R wave­form, $12 is cut off and Qll conducts. The L
signal from transistor $12, charges capacitor
C54: the R signal from transistor

Qll,

charges

capacitor C55.

L WAVEFORM

Q12

CONDUCTS

Q11 cut-off

3

,o

Figure 15 shows the various waveforms that
are present in the switching detector circuit,

Waveform

1

is the suppressed carrier steres

?

WAVEFORM

(Q12 2 CUT-OFF

Cl11 CONDUCT5

F

igure

16

I--

RIGHT

WAVEFORk/’

1

IEF’

WAVEFGRN

LEFT

WAVEfGRh’

38

--~-

~

~~l/jIli~https://manualmachine.com/lIljI~I~ilI~

r

ICC

---- --.

i\l? CGNCMTS

ONLY Stv THESi

3e KC FEAKS

-.-- .~ ~~ -_

LFFT WAVEFORN

RIGH r’ WAVEFORM

31 i CONDUCTS

CNLY Oh THESE

38 KC PEAKS

SWITChcD

DFTFICTIChl

RIGHT WAVEFORM

I

-

----_

-

RlCdT

,,r\

‘\

WAVEcCRhr

.-

I

\

i

LEFT WAVEFORM

I

I

5

0

Figure

The left and right channel audio signals then are
applied to individual 38 kc PEC filters that
remove any remaining 38 kc signal. Proper
de-emphasis of each signal is provided by the
combinations of the PEC components and ca-

pacitors C57 and C58, The stereo signals are

then connected to the Left and Right channel
outputs.

When the Phase switch is in the “out” position,
a 38 kc bandpass filter is connected into the

circuit (coil L8 and capacitor C52. This circuit
allows only the

through to transistors

38

kc subcarrier channel

Qll

and Q12. The phase

to pass

control is used to adjust for proper phasing

75

between the reinserted 38 kc carrier and the

38 kc subcarrier signal, This insures maxi­mum separation from the receiver, (The Phase
control is adjusted by listening for maximum

sound in the subcarrier signal: the presence
of main channel sound would make this adjust­ment impossible.)

A small amount of the 19 kc signal is coupled

from coil L7, through capacitor C38, to the base

of stereo indicator amplifier transistor Q13.
This 19 kc signal is only present when a stereo
signal is being broadcast.

MONOPHONIC FM OPERATION

The monophonic signal is coupled from the ratio

detector circuit to

the

base of transistor Q8.

After amplification, this signal is coupled to
transistor Q9.
follower since no 19

Q9

acts only as an emitter

kc signal is present.

The monophonic signal is then coupled through

capacitor C52 to the emitters of transistor
and Q12. No
transistors Qll and

oscillator

Q12,

signal is coupled to

since the 38 kc oscil-

Q11

lator circuit is disabled by the Source switch.

LEFT CHANNEL AMPLIFIER

The

consists
channel

complete amplifier section of

of

a left

amplifier, The left

channel

amplifier and a right

and

the

Receiver

right amplifiers
are identical; therefore, in order to simplifv
this description, only

the

left channel amplifier

will be discussed,
The

FM

signal from the FM tuner section is

applied through resistor

switch contacts 6 and 3 to
sistor Q1. Resistor

R105 and Source

the

base of

R11

1 1s used to pro-

tran-

vide the proper load impedance for the phono

cartridge. ‘Transistor Ql operates as a high-

gain low-noise amplifier to increase the level

of the incoming signal,

The amplified signal from the collector of Ql

is applied directly to the ‘base of transistor
Q3. Transistor Q3 amplifies the signal again.
DC feedback is applied from the emitter of Q3 to

the base of Ql through resistor R117. From the

collector of Q3, for Phone operation, a portion of
the signal is applied through a frequency selec­tive network. which corisists of capacitors Cl05
and ClO9, resistor

Rl25,

and the Source switch.

This network provides RIAA equalization,

Transistors

Qll

and

Q12

will conduct when no
38 kc signal is applied to their bases. Then the
same monophonic signal is present at the col­lectors of transistors

Qll

and Q12. These signals
are then coupled through the de-emphasis cir­cuits to the Left and Right channel outputs,

The equalized signal from the collector of Q3
is applied through capacitor Cl07 to Volume
control R129. The amount of signal required to

produce the desired listening level is tapped

off by the slider portion of the Volume control,
This signal voltage is applied through the Bass
and Treble control circuits to the base of
amplifier transistor Q5.

The signal from Cl07 is also applied through iso-

lation resistors R181 and R183 to the Tape Cut-

put jack.

Q5

Transistor

further amplifies the signal.
A small portion of the signal from the collector
of Q5 is applied as feedback through capacitor
Cl17 and part of the tone control network to the
base of Q5. The output signal from the col­lector of Q5 is coupled through capacitor C119
and resistor R149 to the base of transistor

Q9

Q7. Transistors Q7 and

are direct-coupled

amplifiers which further amplify the signal.

The

of

amplified

Q9

is coupled through capacitor Cl25 to the

base of driver transistor
The amplified signal from the collector of

signal from the collector

Qll.

Qll

is applied to the bases of the output tran­sistors, Q13 and Q15. Diode DlOl determines
the AB operating point of the output transistors,

eliminates crossover distortion, and also pro­vides temperature stability.

Transistors Q13 and Ql5 are connected as a
push-pull output stage. The output signal from
this stage is applied through capacitor

C129 and

the switch contacts on the rear of the Bass

control to the speakers. The output signal is also

applied across a voltage divider network made
up of resistors R173 and R175, The voltage di-

vider applies a portion of the signal through

resistor R151 and capacitor Cl21 as overall
negative feedback to the base of transistor Q7.
If a stereo headphone set is plugged into the

Phone jack, the signal is applied to the head­phones. The speaker can be disconnected using
the switch on the Bass control.

POWER SUPPLY

The fused transformer-operated power supply
uses four silicon diodes, 0200 through D203, in

C201

a bridge rectifier circuit. Capacitor

filters
the supply voltage for the power output stages.
Resistor

and capacitor

C202

provide the

R201

supply voltage for the stereo indicator lamp.
An electronic filter, consisting of transistor

QlOO,

resistors R205, R206, and R207, and ca­pacitor
transistors, Q3, Q4, Q5, and

C205,

provide filtering for preamplifier

Q6.

The final

filtering for high-gain low-noise preamplifiers

Q1 and Q2, is accomplished by resistor R208

and capacitor

C207.

The supply voltage for the Tuner section is ob-

R201

tained through resistors

pacitors

C202

and

C203,

and R202. Ca-

provide filtering.

The DC voltage for the pilot lamps is obtained
from the junction of diodes D200 and D202.
Dropping resistor R200 lowers the voltage to
the proper lamp operating voltage. The switched
AC socket and the primary of the power trans­former are activated by the On-Off switch. The
unswitched

AC socket is connected directly
across the line and supplies power contin­uously.