Lectrosonics M185 User Manual

M185

BELT-PACK TRANSMITTER

OPERATING INSTRUCTIONS

and trouble-shooting guide

LECTROSONICS, INC.

Rio Rancho, NM

INTRODUCTION

Thank you for selecting the Lectrosonics M185 transmitter. This transmitter represents over 70 years of
combined experience in the design of RF devices and sets new standards for operational convenience,
flexibility and mechanical ruggedness. The M185 transmitter features all metal construction and
operates on high band frequencies from 150mHz to 216mHz. The M185 was designed primarily for
"belt-worn" applications, however it works equally well for other applications and with virtually any
mounting configuration.

The M185 uses the microphone cord as the antenna, eliminating an unsightly "dangling wire". This
transmitter may be used with a wide variety of audio sources. The multi-pin input jack allows the use of
"phantom powered" lavalier microphones with either positive or negative bias voltages. The M185 will
also match low impedance dynamic microphones and high impedance inputs such as musical
instruments or tape decks.

Only the M185 transmitter is covered in this manual. Receivers are described in separate manuals.
The M185 operates with any Lectrosonics 185 or 170 Series VHF receiver. A matched
transmitter/receiver combination makes up a wireless "system".

TABLE OF CONTENTS

INTRODUCTION .......................................... 1

GENERAL TECHNICAL DESCRIPTION .......................... 2

CONTROLS AND FUNCTIONS ................................ 3

BATTERY REPLACEMENT ................................... 5

OPERATING INSTRUCTIONS ................................. 6

M185 INPUT JACK WIRING .................................. 7

TROUBLESHOOTING ....................................... 8

SPECIFICATIONS AND FEATURES ............................ 9

SERVICE AND REPAIR ..................................... 10

RETURNING UNITS FOR REPAIR ............................. 10

WARRANTY ........................................ Back cover

The M185 transmitter is FCC type accepted under the following Parts:

Part 90 (169-172 mHz)
Part 74 (174-216 mHz)

1

GENERAL TECHNICAL DESCRIPTION

The M185 transmitter is comprised of four major functional subsystems: the input compressor, the mic
preamp/gain control, the compandor, and the RF transmitter (see block diagram below).

The input compressor is a low distortion shunt FET compressor operating before the mic preamp.
Control signals for the compressor are derived after the mic preamp to eliminate any possibility of
overload under any signal condition up to a maximum of 30 Volts input at the pin 5 (line level) input
tap. The range of limiting action before distortion occurs is 20 dB. Audio signal level and compressor
action are indicated by two LEDs on the control panel.

The mic preamp is an ultra-low noise NE5534 type op-amp. Aside from gain control, this stage also
adds high frequency pre-emphasis to the audio signal. This insures the highest signal-to-noise ratios
under varying signal conditions. The gain control is semi-logarithmic to provide smooth action.

The M185 audio section includes a compandor circuit compatible with Lectrosonics 185 Series VHF
receivers. This compandor compresses the dynamic range by 2 to 1. A complementary system in the
receiver expands the original dynamics 1 to 2 for full audio quality. Compression and expansion ratios
are fully complementary. High frequency pre-emphasis is implemented in the transmitter to provide
another 10 dB signal-to-noise ratio improvement. Matching de-emphasis is provided in the receivers.

The RF transmitter is composed of a crystal stabilized main oscillator followed by a frequency tripler
and two frequency doublers. All three stages are double tuned. Double tuning provides high
attenuation of spurious signals, which in turn minimizes the possibility that a transmitter would interfere
with another transmitter/receiver system on another frequency.

All RF stages are biased from a regulated internal power supply. The output stage has a separate
feedback regulator which not only stabilizes its operating point, but also minimizes AM distortion.
These regulators keep the RF and audio performance consistent from the beginning (9 Volts) to the
end (6.5 Volts) of battery life.

Schematics and alignment instructions will be provided to qualified repair personnel upon request.

1
2
3
4
5

LIMITER

20 dB

MIC PREAMP

LEVEL

LED

LIMIT

LED

COMPANDOR

+9V IN

15 MHZ XTAL
OSCILLATOR

(VCO)

BATTERY

SENSOR
CIRCUIT

FREQ.

TRIPLER

POWER
LED

45MHZ 90MHZ 180MHZ

FREQ.

DOUBLER

VOLTAGE

REGULATOR

FREQ.

DOUBLER

FEEDBACK

REGULATOR

Figure 1 - M185 Block Diagram

RF
OUT

2

CONTROLS AND FUNCTIONS

LECTROSONICS

M185

ON

MUTE

OFF

LEVEL

LIMIT

MIC LEVEL

Figure 2 - M185 Front Panel

INPUT JACK

The input on the M185 accommodates virtually every lavalier, hand-held or shotgun microphone
available. Use a Switchcraft TA5F connector on the cord. See the separate sheet titled "M185 Input

Jack Configuration" regarding the correct connections for various microphones, and other
sources.

ON/MUTE/OFF SWITCH

Turns the battery power on and off. The center position is an "audio mute" which should be used when
setting the MIC LEVEL control to turn off the sound from the microphone during this adjustment. When
turning the transmitter on, pause for a moment in the MUTE position to prevent a turn-on surge from
occurring (a "thump" sound) in the audio.

POWER ON/OFF LED

Glows brightly when battery is good. A weak or dim lamp means that the battery is weak, and has
about an hour of operation left (maybe a little more). If the lamp fails to light, the battery should be
replaced. The power lamp should light up in both the "mute" and "on" positions of the POWER/MUTE
SWITCH.

The POWER LED is connected to a precision battery test circuit that continuously monitors battery
voltage. The LED is at full brightness with a new 9 Volt alkaline battery. As the battery voltage drops
during use, the LED brightness will also decrease. After 12 to 15 hours the battery voltage will be
about 7 Volts and the LED will be completely out.

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