Circuit Description; Audio Section - Shure T1 Service Manual

Shure T1 Body-Pack Transmitters

Circuit Description

Audio Section

Section
RF
Characteristics
The T1 transmitter contains one circuit board which comprises an
audio and an section. It is intended for use with the matching T3 and
RF
T4 receivers.
Input: Audio signals enter via a
the tip and the ground on the ring (
discontinued model 1
T P
S
Pin 1: Ground
S
Pin 2: Supplies regulated 5 Vdc bias for electret condenser mi-
crophones
S
Pin 3: Audio input
S
Pin 4: 20 kΩ load resistor connected to pin 3 for Shure electret
microphones
Preamplifier Stage: This is centered in one section of the operation-
al amplifier (U102C). An externally accessible potentiometer (R125) ad-
justs the voltage gain of this stage over a 40 dB range.
Passive Pre-emphasis Network and Compandor: The network
(R145, C110, C111, R112, and R115) has a pole at 63 microseconds and
a zero at 1 microsecond. The
(U101A) provides a 2:1 logarithmic compression of the audio signal.
Noise and Distortion: U102A lowers the noise floor, and an internal
potentiometer (R130) nulls the system audio distortion. Operational am-
plifier U102B, operating as a two-pole, active, low-pass filter, restricts the
bandwidth of the system to audio frequencies.
Limiting: transistors Q103 and Q104 limit the level of the audio
PNP
signal leaving the audio section via U102B. Beginning in July 1995, this
section was removed from the "A," "B," and "C" boards but left in a newly
designated "T" board.
5 Vdc Bias and
LED
channel (U101B) supplies regulated, low-noise 5 Vdc bias to various
audio and circuit points. Transistor Q105 provides "reverse battery
RF
protection" to the circuit. Q106 drives
drives D102 ("Low Battery").
LED
Audio Input: Processed audio enters R217, an internal potentiom-
eter that is adjusted for 15 kHz deviation (100% modulation) when the
audio section provides a –2.2 dBV, 1 kHz tone.
Oscillation: The audio then goes to varactor diode D201, which is
part of the modulated oscillator-tripler stage (Q201). The latter's base-
emitter circuit operates as a crystal-controlled Colpitts oscillator in the
20 z region. Fundamental-mode crystal Y201 is tuned 10 kHz below
MH
series resonance by the series combination of frequency-netting coil
1
/ -in. phone jack, with the signal on
4
1 ), an attached microphone (the
T G
), or a four-pin, Tini
Q-G
571 integrated circuit compandor
NE D
Drive Circuits: The
NE
D101 ("Power On"), and Q107
LED
2

, connector (T1):
571 's identical second
D
25C1016 (CC)