Yaesu FT-901DM Maintenance Service Manual page 171

NB UNIT (PB-1994)
A portion of the 8987.5 kHz IF signal is fed to
pin 4 of the noise blanker unit and appears at
noise blanker mixer Q399 (3SK40M), where the
8532.5 kHz signal generated by Q2,, (2SC1815Y)
is mixed with the incoming signal to produce a
455 kHz signal. The 455 kHz signal is then ampli-
fied by Q219 (3SK40M).
When a carrier or a noise-free modulated signal is
received, the 455 kHz signal (with its correspond-
ing strength) is rectified by Dz92 and D293 (both
1N270) to charge C235. There is no discharge loop
for C335; therefore, signals which exceed the
charged voltage established by the reference
voltage on C35 will not pass through D292 and
D203. Accordingly, there will be no voltage drop
across R247, and Qzi2 (3SK51-03) will conduct
as the gate voltage approaches zero potential.
When Q,,2 conducts, the drain voltage at pin 2
of the printed board will drop.
The drain of Q 3,2 is directly connected to the
base of Q393 (2SC1815GR) inthe FILTER UNIT.
As the drain voltage of Q3,;2 drops, the base
voltage of Q393 drops, turning off Q3o3. The
collector voltage will then increase, producing
a forward bias to D39, (181007). As D3, con-
ducts, the signals will pass normally through the
circuit.
When pulse-type noise is received which exceeds
the charged reference voltage established by C235,
Dyo2 and D203 will permit negative-going pulses
to turn Q,,2 off. The drain voltage will rapidly
increase as it turns off.
As the drain voltage increases, Q393 will turn on
and the collector voltage will decrease. Accord-
ingly, D3o, will be biased to block the signal,
Thus, when pulse-type noise is received, the
signal passage will be blanked off momentarily.
IF UNIT (PB-1704C)
The IF signal from pin 14 is fed to the gate of the
IF first mixer. Qao1(3SK51-03)where the IF signal
is heterodyned with a 19.7475 MHz+ Af local signal
delivered trom crystal oscillator Qgos (2SC535A)
and butter amplifier Qsoe (2SC181SY),
resulting
in a signal of 10.76 MHz +Af.
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L P8-1994 NB UNIT