Transmit Board; Circuit Description; Alignmnent Procedure - Telex TR-200 Service Manual

TRANSMIT BOARD
CIRCUIT DESCRIPTION
From the headset connector the microphone signal
is first amplified by Q3 and Q4. This pair has a
gain of 12 dB. Depending upon which type of
microphone is used, S1 is set by the userto dynamic
or electret. The electret position provides an l l d B
drop in gain to compensate for the extra output. R9
is the user adjusted microphone gain control. UlD,
Q1 and Q2 form a peak limiting amplifier which
responds to negative peaks exceeding about,.84
volt in amplitude. When such peaks are en-
countered, Q2 conducts and charges C12, driving
Q1 toward pinchoff. This reduces the gain momen-
tarily until the input signal is reduced. Visual in-
dication of peak limiting is provided by LED 102
which is driven by U103 when the voltage at pin 3
exceeds the 1.3 volt reference. U103 is also used
to sense the end of battery life and lights LED 102
continuously when this occurs.
D 1 (version 2 and later) isolates the time constant
of the amplifier from that of the LED driver. The
latter is much shorter than the former, and causes
the LED to extinguish immediately upon removing
the input signal. "O_n version 1
the^^^
will remain
!it for severalsconds.
U1C is an amplifier whose gain is controlled by the
level of audio entering the rectifier at pin 16. As the
level here increases, the gain is decreased and this
results in a 2: 1 compression characteristic. Pre-
emphasis is added by UlB. R23, R24, and C16 set
this at 115 pS. Q5 is used to turn the microphone
a n m d offand isdrivenby theqsh-to-talk latch
circuit. When SlOl is pressed in and held, Q106 is
driven on viaD103. This lights LED 101 and gates
on the microphone audio. Releasing S 101 turns off
the microphone. U104 does not change state under
these conditions because C151 is discharged when
the clock is pulsed. If, however, SlOl is depressed
twice in quick succession, C151 will be charged for
the second clock pulse and this will set U104. Now
,
the microphone is latched on via Dl04 and will
remain on until SlOl is depressed once again,
resetting U 104.
Q7 is a Colpitts oscillator with the collector tuned
to the third harmonic. L11, VVCl and VVC2 are
added in series with the crystal to allow frequency
modulation of the oscillator. The oscillator is
operated 10 KHz below the series resonant point of
the crystal. This improves linearity at the expense
of some stability. Q8 is a tripler and Q9
is
a straight
through amplifier. The carrier frequency is thus
nine times the crystal. C43, C44 and L7 function
as a low pass filter and matching network.
ALIGNMENT PROCEDURE
Equipment Required:
Deviation Meter
RF Power Meter
Frequency Counter
Audio Generator
DC Power Supply
1. Solder one end of a 50 ohm coaxial cable from
the transmit antenna pad to ground. Connect
the other end to the deviation meter, power
meter, and frequency counter.
2. Connect the audio generator from pin 1 (hot) to
pin 2 (ground) of the P.C. board connector. Set
the output to 10 mV at 1 KHz.
3. Set the power supply for 9 volts and connect it
from pin 11 (+) to pin 10
(-)
of the P.C. board
connector.
4. Turn R9 to maximum clockwise.
5. Depress SlOl twice quickly to latch on the
microphone. If aligning with boards separated,
ground pin 8 of the P.C. board connector.
6. Adjust L2, L3, L4, C39 and C42 for maximum
Dower.
NOTE: If adjustmenu are severly out of align-
ment and no output can be obtained, it may be
necessary to first adjust L2 and L3 for maximum
DC voltage at the emitter of Q8. Then adjust
LA
for maximum DC voltage at the emitter of Q9.
Now adjust C39 and C42 for maximum power.