Furuno FR-2155, Service Manual

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The circuit made up of U850 (2/2) prevents the modulator from being fired by noise. The first
stage of the one-shot multivibrator U850 (1/2) operates immediately after the TX trigger pulse,
and next the second stage U850 (2/2). The output (#9) of U850 (2/2) is applied to the reset ter-
minal (#3) of the first stages of U850 (1/2). In this manner, the level at pin #6 of U850 (1/2) is
kept “L” while the level at pin #9 of U850 (2/2) is “L”. Therefore, even if noise appears after the
TX trigger pulse, Q853 is kept off.
The circuit made up of U851/U852 and U853 operates as a decoder to drive the relays
(K801/802/803) for changing the TX pulselength according to range and menu settings. Refer to
the truth table shown below.
Pulselength P/L B P/L A K801 K802 K803
S L L OFF OFF OFF
M1 L H ON OFF OFF
M2 H L ON ON OFF
L H H ON ON ON
ON: Short, OFF: open
The circuit made up of K851, U851 and U853 is the switching circuit for the magnetron heater
voltage. On short and middle ranges, relay K851 is not activated, and the voltage of 12 V (or 9
V). HOT “ is passed through R812/K851 and applied to the magnetron heater. On long ranges,
U851 becomes conductive since levels at pin #5 of U853 go high under TX condition, causing
the voltage of “12 V (or 9 V) HOT” to pass through R812/R811/K851 in order. As a result, lower
magnetron heater voltage is applied to the magnetron.
TX Trigger
Q850 Collector
U850 (1/2) #7
U850 (2/2) #9
U850(1/2) #6(TP1) SCR Trigger prohibited
Q854 Collector
Q853 Base
Q853 Collector
Modulator trigger pulse
Figure 2.16 Trigger sequence