Heathkit HW 24HT Service Manual

Page 16

VHF-The receive signal is applied to RF coil
L406 and passes through an antenna circuit and RF
attenuator circuit. When forward-biased, the RF at-
tenuator switch tums on and diode Q418 allows both
direct current and RF current to be passed to the RF
attenuator circuit. The receive signal is attenuated ap-
proximately 20 dB.

The receive signal in L406 is amplified by RF
amplifier FET Q417. The amplified receive signal is
applied to the gate of first mixer FET Q415, then
passing through the two-stage band-pass filter con-
sisting of L405 and L404.

UHF-~The receive signal is applied to RF coil
L413 and passes through an antenna switching circuit
and RF attenuator cirmrit. When forward- biased by
turning on the RF attenuator switch, diode Q429 al-
lows both direct current and RF current to be passed
to the RF attenuator circuit. The receive signal is at-
tenuated by approximately 20 dB.

The receive signal in M13 is amplified by RF
amplifier Q426. The amplified receive signal is then
applied to the base of first mixer transistor Q424.

First Mixer/IF

The first IF is applied to a matched pair of 55.05
MHz crystal filter circuits consisting of L204, F201,
L205, F202, and L206. and passes through forward-
biased Q414 or Q423 and a switching diode. The sig-
nal is then filtered in the crystal filter circuit. obtain-
ing excellent cross modulation

VHF-The filtered signal from MOS FET Q417 is
applied to the gate of MOS FET Q415. the first
mixer, while the receivers PLL local signal (199.05
MHz to 203.04 MHz) is simultaneously injected at
the source of Q415. The two signals are heterodyned
against each other, producing a first IF of 55.05 MHz
on the drain of Q415.

Page 15

UHF-The filtered signal from transistor Q426 is
applied to the base of transistor Q424, the first mixer,
while the receivers PLL local signal (382.95 MHz to
394.94 MHz) is simultaneously injected at the base
of Q424. The two signals are heterodyned against
each other, producing a first IF of 55.05 MHZ on the
collector of Q424. After filtering, the first IF signal is
amplified by first IF amplifier Q236 and applied to
IC Q201 pin 20.

Second IF and Discriminator

Q201 is a multifunction IC consisting of the follow-
ing circuits: second local oscillator, second mixer,
second IF amplifier, detector, and squelch switching
circuit.

The amplified 55.05 MHz first IF is applied to the
second mixer section of Q236. which in turn hetero-
dynes it against the second local oscillator frequency.

A second local oscillator frequency is generated at
54.595 MHz crystal X202, using low side-injection.
The second IF, 455 kHz at Q201 pin 4, is applied to
ceramic filter F203, with bandwidth characteristics.
After filtering. the second IF is fed to a limiter,
quadrature detector. and amplifier in the 1C. The
frequency-modulated second [F signal is demodu-
lated into an audio signal in the quadrature detector.

Audio Circuit

Audio signals from the pin 11 of Q201 are applied to
the de-emphasis circuit consisting of R291, C263,
R292, and C264, which limits audio frequency re-
sponse to below 3 kHz, and to muting circuit Q248.
After filtering, the audio is applied to volume control
R295. The volume control determines the audio level
to audio power amplifier Q203. Q203 develops the
required level of audio drive to the speaker.

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Page 20

TERMINAL DESCRIPTIONS

Transceiver Board

Page 19

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Terminal Function Terminal Function
LOW Hi/Lo power detect line XT -

(high: open, low: ground) SQL OFF Squelch is on with high level and off
SPA AF power AMP OUT. with low level.
MIC Microphone line. LAS Ground by pressing lamp switch.
SPK Speaker line. LAMP Lamp line.
TV 4.3 V during transmission at VHF. DAC Analog data line for signal meter and
+13 Line through power switch. TX power meter. ~
TU 4.3 V during transmission at UHF. SQL :rniZYievi/lehfgliioagfitsenfid
RU 4.3 V during reception at UHF. detected.
RX - ULM Unlock signal line
RV 4.3 V during reception at VHF. (5 V When unlocked).
G Ground. EU Encoder UP signal.
IF 55.05 MHz IF signal output. ED Ema DOWN Sign
IFG 55.05 MHz IF signal ground. G G' ' .
ULR gxfififfifigfigim from PLL CK Zfi;§:§f§fifi:§§am is used).
VU 4.3 v during transmission at UHF. so $5$i$§i§i§§§§§§f°r 5
MOD VCO modulation line. (compatible when tone data is used).
W 4.3 V during transmission at VHF. [OE Data strobe signal for 5-volt power
SV 5 V with VHF operation. supply Shift resistor.
SU 5 V with UHF operation. PU PLL IC data strobe signal for UHF.
PV PLL IF data strobe signal for VHF. TE Data strobe signal for tone squelch.
PU PLL IF data strobe signal for UHF. PV PLL [C data strobe signal for VHF.
SO PLL [C data signal, signal for 5-volt BZ Buzzer signal line.

power supply shift resistor BST 100 Hz tone signal output line.

(compatible when tone data is used). MIC Microphone line.
CK golmpgtiibalffigritgflata is used). 5V SW0 regulator.
5V 5-volt regulator. T5 5 V during transmission.

SPK Speaker line.
G Ground.
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