plus bias circuit [pin number 7 of IC3, TX mixer] thus reducing the gain of the TX mixer as high level
RF signal is observed at L12. This circuit is disabled in the AM or FM mode of transmission. In
summary, the ALC circuit [both audio and RF] accomplishes very important function, not only
preventing overmodulation, but in the view of harmonie and spuirous suppression [especially in the
SSB transmit mode].
Antenna Transmission Line
The lowpass filter between the antenna and collector of Q10 serves to pass the 27 MHz signais,
attenuating higher frequency signais.
It also acts to match the antenna impédance to the output
impédance of the transmitter output stage, this nominally being 50 Ohm.
Receiver
The RF signal, at a frequency between 26.965 to 28.305 MHz, feeds from the antenna through L13,
12, 11, and T7 to the 27 MHz RF amplifier Q20. Then the amplified output signal from Q20 is
coupled through T9 to first mixer Q22 where it is beat with an injection signal from the VCO, IC2,
through the VCO buffer Q2.
The frequency of the injection signal from IC2 dépends on the channel being selected, as a signal
of the 37 MHz range is programmed by the Channel Selector. The output of Q22 is therefore, 10.695
MHz in the AM/FM/USB modes, and 10.692 MHz in the LSB mode, the first intermediate frequency
and is the resuit of the R F input and mixing of IC2 VCO signais.
In case of the AM or FM receive mode, this 10.695 MHz first IF signal is then fed to the second
mixer, balanced D22 and D23. Also fed to the second mixer is the second local oscillator signal,
10.24 MHz, from Q3. Mixing of these two signais resuit in a signal at a frequency of 455 kHz in T14.
This is the second intermediate frequency for AM or FM mode of réception. In AM mode, the 455
kHz signal passes through the ceramic bandpass filter CF, and fed to IF amplifiers Q27, 28, and 29,
which include IF transformer T15. The output of Q29 is applied to D25, the AM diode detector,
while in FM receiving mode, the 455 kHz signal amplified only through Q27 is led to FM demodulat-
ing IC, IC3 on the PTOS006 through T4. Résultant demodulated audio is acheived from the IC pin
#12 and input to AF gain control VR, VR1. In the SSB mode of réception, the signal obtained as a
resuit of the mixing of the RF input and IC2 VCO signais, 10.695 MHz in the USB, and 10.692 MHz
in the LSB mode, is not converted down to lower intermediate frequency, but is passed through the
crystal filter, XF, and fed to the SSB IF amplifiers, Q14, 16, and 17, which includes T11, and 12.
The signal at the secondary side of T12 is fed to Q19, the product SSB detector and beat with the
BFO [beat frequency oscillator] signal from the Q12 and finally rectified to audio frequency signal.
The audio signal output from detectors (for AM [D25], FM [IC2], and SSB [Q19]), is passed through
the AF Gain Control, VR1, to the input of the audio amplifier, IC5. The audio output is transformer-
coupled to the internai speaker, or to an external speaker through External Speaker jack, J3.
RX Diagram
ANT
Squelch
Q32, 33, and 34 are the squelch amplifier transistors. At low [or no] signal levels Q34 collector con-
ducts to ground and its output connected to pin number 6 of IC5 results in no signal output from the
audio amplifier. As the incoming RF signal increases it results in opening up the AF amplifier and
output is activated. The point at which Q34 cuts o ff is determined by setting of the Squelch Control,
VR2.
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