CIRCUIT DESCRIPTION
1) VHF Reception
Incoming VHF signals are passed through a low-pass filter network, antenna switching diodes D20
(1SV268), D19 (1SS355) and D26 (DAN235E), and a high-pass filer network, and on to the RF amplifier
Q19 (3SK131). The amplified RF signal is passed through another RF amplifier Q18 (2SC5226) and
band-pass filtered again by varactor-turned resonators L46, L49, L51 and D28, D29, D30 (all HVU359),
then applied to the 1st mixer Q21 (3SK240) along with the first local signal from the PLL circuit.
The first local signal is generated between 122.3 MHz and 126.3 MHz by the VHF VCO, which consists
of Q9 (2SK508) and varactor diodes D10, and D11 (both 1SV282) according to the receiving frequency.
The 21.7 MHz first IF signal is applied to monolithic crystal filters XF and XF2 (both Q2175AD20)
which strip away unwanted mixer products, and the IF signal is applied to the first IF amplifier Q20
(2SC4618). The amplified first IF signal is then delivered to the FM IF subsystem IC IC3 (TK10931V),
which contains the second mixer, limiter amplifier, noise amplifier, and FM detector.
The second local signal is generated by 21.25MHz TCXO, producing the 450 kHz second IF signal
when mixed with the first IF signal within IC3.
The 450 kHz second IF signal is applied to the ceramic filter FL1 (ALFYM450E) which strips away all
but the desired signal, and then passes through the limiter amplifier within IC3 to the discriminator coil
L101, which removes any amplitude variations in the 450 kHz IF signal before detection of speech.
The detected audio then signal is amplified by IC9 (NJM2902V-B) passes through the de-emphasis
network, a high-pass filter consisting of IC9 (NJM2902V-A) and associated circuitry, and low-pass
filter consisting and associated circuitry. The filtered audio signal is switched by IC12 (BU4052), then
passes through the audio volume control IC IC13 (M511312FP) which adjusts the audio sensitivity to
compensate for audio level variations.
The audio signal is amplified by IC8 (LA4425A), then applied to the internal loudspeaker.
4 |
ACEC
