Yaesu FR-50B, Instruction Manual

Page: 4 / 22

Page 4 of 22
Figure 1. Receiver Block diagram {V8 changed from 6AW6 to 6BM8}
CIRCUIT DESCRIPTION
1. RF Amplifier
The high Gm tube V1 (6BZ6) provides a minimum of cross modulation.
It is a semi-remote cutoff pentode and has an excellent AGC
characteristic. A 5 MHz trap is inserted to reject a spurious
signal which could enter directly into the first I.F. stage. To
avoid complexity, two coils in each of the grid and plate tuning
circuits, together with switched capacitors, cover the full range.
A FM type variable condenser is used with electronic band spread
for pre-selector tuning. The centre of each amateur band is
approximately at the centre of the range of the pre- selector
control. On the 3.5 MHz band, the pre-selector covers 3.5 - 3.8
MHz.
2. 1st Mixer
The amplified signal and local oscillator output are mixed by
introducing to the grid and cathode of triode mixer V2, 12AT7. The
triode mixer is very effective because of its low conversion
noise. The first I.F. is 5 173.9 kHz.
3. VFO
Silicon transistor Trl, type 2SC373, is employed on a colpitts
oscillator circuit. Oscillator frequency range is selected by
switching a coil for each band. C61, C63, C65, C67, C69, C7l and
6BM8