Ramsey Electronics HR40 - Manual (Page 6)

HR40
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6
called “microphonics” which is a phenomenon where almost anything in the
physical circuitry of the receiver can act as a sort of microphone or audio
sound pickup. Touch or bump such radios and you will hear a thump or ring in
the speaker or earphones. Still another problem is that of AC line hum
whenever an unregulated power supply is used rather than batteries.
The Ramsey HR40’s use of the SA602 integrated circuit chip offers a circuit
configuration that is as immune as any simple superhetrodyne to the classic
problems with Direct Conversion receivers. The receiver is not as easily
overloaded by the VOA or Radio Moscow broadcasts, and there are few
annoying ”microphonics” or incurable AC hum.
Circuit Description:
U1 combines a double-balanced active mixer and oscillator in a single 8-pin IC
chip. L1 peaks the RF input to pins 1 and 2 of U1. L2, with varactor diode D1
and R2, C1, C2, and C4, control the resonant frequency of U1’s internal
oscillator. Rotating R2 gives about a 250 KHz tuning range. The audio output
is fed from pins 4 and 5 through R3 (volume control) directly to the LM 386
audio amplifier. R1 controls RF gain. C9 boosts the gain figure of the LM 386
from 20 to 50.