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Product detector V6, 6BE6, has a very efficient performance for
SSB and CW detection. BFO voltage is derived from V7,6BA6, in a
self-excited oscillator circuit. BFO pitch is controlled by
trimmer condenser VC4.
10. Automatic Gain Control (AGC) Circuit
AGC voltage from the AM detector is applied to control grids of
Vl, V4 and V5. The AGC circuit keeps the output level of the
receiver almost constant regardless of input signal strength. In
the case of reception of DX (CW & SSB signals), the AGC circuit
may be switched off and RF GAIN control adjusted for good
reception quality.
{My receiver does not have a switch to turn the AGC off – I
suspect that this is legacy text from the first model of the
receiver (FR-50) and instead a slow/fast AGC switch took the place
of the AGC on/off switch on the FR-50B model}
11. Mute & Monitor Circuit
Adoption of a muting circuit enables smooth switching of transmit
and receive. RF GAIN control VRl, and monitor control VR2 are in
series to cathode return circuit of Vl and V4. For muting with a
transmitter, relay contacts must open on "transmit" and close on
"receive". Thus, on "receive", the mute terminal is grounded. On
"transmit", the monitor potentiometer increases the bias.
{The relay mentioned here is part of the FL-50B transmitter}
12. A.F. Amplifier
Output from the detector, through the mode switch (S3), is applied
to the triode section of V8 (6BM8), from which output is coupled
to the power amplifier, the pentode section of V8.
Since the same circuit is used for the speaker connection and
headphone jack, low impedance speaker or headphones, 4 - 8 ohms,
should be used. The output transformer (T2) also has a 600 ohm
winding to supply anti-trip signal for a VOX circuit.
13. Power Supply
HT supply consists of four silicon diodes (D5, D6, D7 & D8) in a
bridge circuit and with a filter choke performs with good
regulation. Supply for the transistor oscillator is stabilized by
a zener diode 1S225. Heater supply is 6.3 volts, same as that of
the FL-5OB.
{The original diode in my receiver is a 1N4740 which appears to be
an equivalent zener diode}
