Vectronics VEC-1012K - Manual (Page 25)

VEC-1012K Owner’s Manual
25
pin 23 of U1, the schematic shows that the radio's voltage-tuning circuitry is
connected here. You might look for defective or misplaced components on that
portion of the circuit board as a potential cause. If, despite your best effort, you
cannot solve a problem with your radio, kit repair services are available through
Vectronics. See the warranty on the inside front cover for complete
instructions.
THEORY OF OPERATION AND SPECIFICATIONS
Technical Circuit Description:
The VEC-1012K FM receiver is a sensitive voltage-tunable dual-conversion
NBFM receiver that includes an effective squelch circuit and gated AF-amplifier
IC for speaker operation. Incoming signals are filtered through a bandpass filter
at L1/L2 to reduce out-of-band interference, then boosted by low-noise
preamplifier Q1. Q1 is series-matched to the input of U1. U1 is a Motorola
device that perform nearly all receiver functions for the radio. The first LO
(local oscillator) is voltage-tuned by a temperature-compensated varactor diode
built into the receiver chip. Signals are converted in the receiver's DBM 1st-
mixer stage down to 10.7 MHz, the radio's 1st-IF. Here, they are fed through a
roofing filter (FL1) to reduce in-band interference. The second LO, crystal-
controlled at 10.245 MHz, drives the 2nd mixer for converting signals to 455
kHz. FL2 establishes the radio's message-channel bandwidth prior to 100-dB of
signal amplification and limiting in the 2nd IF. Signals are demodulated by
quadrature detection, and recovered audio signals are preamplified to line level
by U1's output stage. Audio is then sent to the receiver's volume control and to
AF amplifier U2.
In addition to processing FM signals, U1 also measures signal strength through
log-amp RSSI circuitry. RSSI output is sent to a built-in op-amp comparator,
where signal strength is compared to a reference level set by the radio's squelch
control. The comparator then generates a logic signal to operate the "mute" pin
of audio amplifier U2 for squelch action.
Audio is supplied to U2 via the radio's volume control. Here, audio signals are
contoured for frequency response and amplified to speaker-level. The output of
U2 is fed to a speaker/phone jack.
Specifications:
Tuning Range ................................... 154-158 MHz (may be altered by retuning)