Heathkit IM-4180 - Page 20

Page 57
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A number series has been assig ned to the « circuit com-
|
au
Wananta mast
poncnis Mounts
boards. These numbers are veferred to throughout the
sections of the Manual to help you locate and identify
the components. These components are grouped as
1-99 Parts mounted on the chassis.
101-199 Parts mounted on the main
circuit board
201-299 Parts mounted on the converter
circuit board.
Refer to the Block Diagram (Illustration Booklet, Page
10) and to the fold-in Schematic Diagram as you read
this “Circuit Description.”’
GENERAL
RF signals from a signal generator or atransmitter are
coupled to the RF Input jack on the FM Deviation
Meter by shielded cable, a wire, or through an acces-
sory antenna. The RF signals are routed into the con-
verter assembly where they are mixed with a local
ency af warhinh ta 9 MALT
ency Of which 18 .4 MiniZ
necillatar sian frequ
VWOULLIGLUL signal, the icyu
above or below the incoming signal. The two signals
are mixed with the output of a harmonic generator
and are coupled through a preamplifier and an IF
The amplified IF signal is then coupled through a
jimiter and a waveshaper circuit in which the signal
is formed intoa square wave to driv
Sane
The trigger output feeds into a monostable multivib-
rator whose duty cycle varies according to the devia-
tion frequency. The output ofthe multivibrator passes
through two stages of low-pass filters to produce a
demodulated audio signal. The audio signal is then
routed in two directions. In one direction, the signal
goes through a switch attenuator network, a peak
detector, and is coupled to the output meter. Routed
in the other direction, the audio signal is passed
through a de-emphasis network, across a level con-
at of tha
t Ul LLiv
eatrigoear circuit
a trigger circult.
tral and Intan an andin amniifier The nit
CEL, GALE LLILY Gil GULLY Gilt priliivl. Like VUUL
audio amplifier is coupled directly to a speaker (or
head-phone) jack on the front panel.
Power is supplied to the Meter circuits from ten
NIPNA ‘Toren c A) Lhattewinn aw far D attancr
IN EGLI DL Llype 15 (AA) VALLOLLIOSO Wi ib om a QLLUL y
Charger/Eliminator Accessory. Fifteen-volt power is
taken directly from the power source. Eight-volt
power for the integrated circuits is obtained from an
a filter. nilatar and
Lene eo 2 BLAU
ory
VEusaruL
A dual-gate MOSFET, Q202, is a variable frequency
oscillator. Coil L202B resonates with tuning capacitor
C919 tn datarminae tha nerillata frequency Cri
waid LO wd ine tne OSCiisiatcor iT cquency. WOlk
L202B provides positive feedback to Q202 gate 1 (G1).
Fine tuning is provided by control R3 and variable
capacitance diode D201.
Transistor Q201 is a harmonic generator. The oscil-
lator signal from the drain (D) of Q202 is DC coupled
into the base (B) of Q261. Capacitor C201 in the emit-
ter (E) circuit of Q201 stores a charge to keep the
transistor turned or the greater part of each cycle. On
negative excursions of the signal at the base of Q201,
the collec tor conaucts snort, high- -harmonic current
pulses.
A hot-carrier diode, D202, is a frequency converting
mivar Tha nurrant nulana fram the nnllartar nf M9n1
BLILAUGL. LLiG UULIOLIL PuUstovoe ALULIL LiL UULIGULUL UL VWeieoud
are coupled tothe anode of D202 and the incoming RF
signals are coupled to its cathode. The derived inter-
mediate frequency (IF) signai is created in a low-pass
filter consisting of enil 1.901
capacitor C202.
fat
nals are coupled from the output of the conver-
ter circuit to the (IF) Gain control. The attenuated IF
signal is then routed into the base of preamplifier
transistor Q101 on the main circuit board, Transistor
M1014 ta aq laweona en than wartth a gain nf 4in Tea
Wiul iS a4 10OW-fO18e Pre amplifier with 4 Saini OF 1uU. 1tS
collector output is coupled through capacitor C104
and into pin 3 of operational IF amplifier integrated