Receive Board - Telex Radiocom BTR-200 Series Service Manual

RECEIVE BOARD
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CIRCUIT DESCRIPTION
PCB 70714000
This unit is a four channel single conversion
receiver consisting of one front end and four I.F.
strips. Signals entering the board proceed through
3 tuned circuits, Q30l, and 3 more tuned circuits
to the mixer. MX301 is a doubly balanced diode
mixer and receives +7 dBm from the oscillator.
Q302 is a Colpitts oscillator with the collector
tuned to the second harmonic. Y301 operates on
the fifth overtone with C319 and L308 acting as
mode suppressor. L310 is used to set the oscillator
on frequency. From the mixer the signal continues
through a broadband filter to line driver Q303.
VR309 is used to set the collector current to 20 rnA.
Q303 drives the four I.F. strips, beginning with
crystal filters FL30l-FL308. The four I.F. strips
are the same except for component values. Also,
the 10.7 MHz I.F. substitutes a ceramic filter for
two tuned circuits. For the remainder of the
description the parts in the 10.7 MHz I.F. will be
referred to. The I.F. signal is amplified by Q304 and
U301A, and limited and detected by U301B.
U301B contains a quadrature detector which is
tuned by T309. Squelch is of the amplitude
type, the D.C. voltage at pin 5 of U301 varying in
propOltion to the signal strength. With no signal
U302 pin 1is low and the audio outputis squelched.
When the received signal causes the voltage at pin
3 of U302 to exceed that of pin 2 of U302, pin 1
goes high and the squelch opens. VR30l sets the
threshold at pin 2 and thus the squelch. R320
provides hysteresis. Q305 is used to drive the LED
on the front panel. From U301 the audio is
amplified by U303A. R389, R328 and C354 set
the de-emphasis at 115uS. VR302 is used to com-
pensate for variations in detector output level.
U303B is a 3 pole ButterwOlth low pass filter with
a cutoff frequency of 10KHz.
3-9
PCB 750309
This unit is a four channel dual conversion receiver
consisting of one front end and four I.F. strips.
Signals entering the board proceed through 3 tuned
circuits, Q301, and three more tuned circuits to the
mixer. HY301 is a doubly balanced diode mixer
and receives +7 dBm from the oscillator. Q302 is
a Colpitts oscillator with the collector tuned to the
second harmonic on version 1 boards and the third
harmonic on version 2. Y301 operates on the fifth
overtone with C311 and L308 acting as mode sup-
pressor. L307 is used to set the oscillator on fre-
quency. From the mixer the signal continues
through Q303 to filtet drivers Q320, Q370, Q420
and Q470. On version 1 boards the signal is resis-
tively coupled to the filters. On version 2 boards it
is inductively coupled. The four I.F. strips are the
same except for component values. For the
remainder of the description the parts in the 10.7
MHz I.F. will be referred to. U320A mixes the
incoming signal with a 10.245 MHz oscillator to
produce the second I.F. of 455 KHz. (The other
three oscillators operate at 455 KHz above their
n~spective
input frequencies). U320B contains a
limiter and quadrature detector, which is tuned by
L322. U320C functions as a 94 KHz bandpass
filter for operating the squelch. With. no signal a
large amount of noise is present at U320 pin 13 and
a portion of this is rectified by Q321. If the emitter
rises above .7V U320D clamps pin
16
to ground,
muting the audio, and drives pin 15 low, extin-
guishing the LED on the front panel via Q322.
When a signal is received, the noise at U320 pin 13
drops and lowers the voltage at the emitter of Q321,
releasing the audio clamp and lighting the front
panel LED. Since the LED current flows through
R33l, the voltage drop acts on Q32l and provides
hysteresis. VR321 adjusts not only the noise
amplitude delivered to Q321 but also the D.C. bias,
and is used to set the squelch at 1 microvolt. From
U320 the audio is buffered and filtered by U321.
R328, C335, and VR320 set the de-emphasis at
l15uS. U321Ais a three pole Butterworth low pass
filter with a cutoff frequency of 10 KHz.