Lectrosonics UM250b User Manual

UM250B

FREQUENCY-AGILE UHF

BELT-PACK TRANSMITTER

OPERATING INSTRUCTIONS

and trouble-shooting guide

LECTROSONICS, INC.

Rio Rancho, NM

TABLE OF CONTENTS

INTRODUCTION ..................................................................................................... 3

GENERAL TECHNICAL DESCRIPTION ............................................................... 4

CONTROLS AND FUNCTIONS ............................................................................. 6

BATTERY INSTALLATION ..................................................................................... 8

OPERATING INSTRUCTIONS ............................................................................... 8

OPERATING NOTES ............................................................................................. 9

ADJUSTING THE TRANSMITTER FREQUENCY ................................................ 9

MICROPHONE CORD TERMINATION ................................................................ 10

5-PIN INPUT JACK WIRING ............................................................................... 11

A6U UHF ANTENNA ............................................................................................ 13

TROUBLESHOOTING .......................................................................................... 14

SERVICE AND REPAIR ....................................................................................... 15

RETURNING UNITS FOR REPAIR ..................................................................... 15

SPECIFICATIONS AND FEATURES .................................................................. 16

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

The UM250B transmitter is FCC type accepted under Part 74: 470-608MHz and 614-806MHz

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UHF Belt-Pack Transmitter

INTRODUCTION

Thank you for selecting the Lectrosonics UM250B frequency agile, belt-pack transmit­ter. The UM250B combines over 80 years of engineering experience with the very
latest components, in a design that addresses the most demanding professional
applications.

The design of the UM250B was the direct result of numerous conversations with
users, staging and touring companies and dealers across the US. The specific
concerns and needs brought up in these conversations led directly to the develop­ment of the operational features offered on the UM250B. Two hundred fifty six
frequencies are user selectable in 100kHz steps to alleviate interference problems in
travelling venues, and the low frequency roll-off is user adjustable to adapt to varying
acoustic environments and preferences.

The UM250B is a rugged, machined aluminum package with a removable, spring
loaded belt clip. The input section provides a correct input tap for virtually any
microphone or line level audio source. 5 Volts of bias voltage is available to power
electret mics with either positive or negative bias. Level indicating LEDs are pro­vided on the control panel to make level settings quick and accurate, without having
to view the receiver. The battery compartment accepts any 9 Volt lithium or alkaline
battery and makes a positive connection via self-adjusting contacts. The antenna is
a detachable, locking 1/4 wavelength flexible bronze cable that connects to a 50
Ohm SMA port on the transmitter.

Only the UM250B transmitter is covered in this manual. Companion receivers are
covered in separate manuals. The UM250B will operate with any 200 Series Lec­trosonics receiver in the same frequency group.

Rio Rancho, NM – USA

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GENERAL TECHNICAL DESCRIPTION

The UM250B transmitters are comprised of a number of functional sub-systems as shown in the block diagram below.

GENERAL

The 200 series uses 75kHz wide deviation for an extremely high signal to noise ratio. The transmitter circuits are all
regulated to allow full output power from the beginning (9 Volts) to the end (7 Volts) of battery life. The input amplifier
uses a Motorola 33078 op amp for ultra low noise operation. It is gain controlled with a wide range input compressor
which cleanly limits input signal peaks over 30dB above full modulation.

DUAL BAND COMPANDOR

Traditionally, compandors have been a source of distortion in wireless microphone systems. The basic problem with
conventional systems is that the attack and decay times are always a compromise. If the time constants are fast, high
frequency transients will not be distorted, but this will cause low frequency distortion. If the time constants are slower,
low frequency audio distortion will be low, but high frequency transients will then be distor ted. The 200 system
introduces an entirely new approach to solving this basic problem, called “dual-band companding.”

There are actually two separate compandors in the 200 system, one for high frequencies and one for low frequencies.
A crossover network separates the frequency bands at 1kHz with a 6dB per octave slope, followed by separate high
and low frequency compandors. The attack and release times in the high frequency compandor are fast enough to
keep high frequency transient distortion at a low level, and the low frequency compandor uses slower time constants,
reducing low frequency distortion to well below that of a conventional compandor.

+5V BIAS

SHUNT

LIMITER

FREQ

SWITCHES

SUPPLY

MIC

JACK

1
2
3
4
5

PHASE LOCKED LOOP

LF
ROLL-OFF

INPUT
AMP

LIMIT

LED

DIVIDER

SET
LED

AUDIO
LEVEL

PEAK AUDIO

INDICATOR &

FILTER

LIMITER
DRIVER

PRESCALER

LOW PASS

FILTER

LP

COMPANDOR

Vref

BASS

TREBLE

COMPANDED

LP FILTER

HP FILTER

AUDIO

UM250B Block Diagram

PILOT
TONE

OSC

Vreg

+5VDC

Vreg

+3.6VDC

VOLTAGE

CONTROLLED

OSCILLATOR

COMPANDED AUDIO

PWR

TRANSMITTER

TO XMTR

DELAY

+9VDC

PWR

LED

4

UHF Belt-Pack Transmitter

NO PRE-EMPHASIS/DE-EMPHASIS

The signal to noise ratio of the 200 system is high enough to preclude the need for conventional pre-emphasis (HF
boost) in the transmitter and de-emphasis (HF roll off) in the receiver. Pre-emphasis and de-emphasis in an FM radio
system usually provides about a 10dB improvement in the signal to noise ratio of the system, but the high frequency
boost in the transmitter must be removed in a purely complementary manner or else the frequency response of the
original audio signal will be altered.

Pre-emphasis can also cause distortion in the receiver. As this signal is passed through the IF filters in the receiver,
distortion can be produced, most noticeable at full modulation. De-emphasis cannot be applied until the signal is
converted into audio, so there is no way around this problem short of eliminating pre-emphasis altogether. Neither of
these problems occur in the 200 system. The dual-band compandor in the 200 Series system essentially provides a
dynamic pre-emphasis/de-emphasis function with extremely low distortion.

PILOT TONE SQUELCH

The 200 system utilizes an ultrasonic tone modulation of the carrier to operate the receiver squelch. This “pilot tone”
consists of a 32kHz signal mixed with the audio signal following the microphone preamp, just after the compandor, to
control the audio output muting of the receiver. The pilot tone is filtered out of the audio signal immediately after the
detector in the receiver so that it does not influence the compandor or various gain stages. The basic benefit of the
pilot tone squelch system is that the receiver will remain muted until it receives the pilot tone from the matching
transmitter, even if a strong RF signal is present on the carrier frequency of the system. This is extremely important in
applications that include an automatic microphone mixer.

WIDE-BAND DEVIATION

±75kHz deviation improves the capture ratio, signal to noise ratio and AM rejection of a wireless system dramatically,
compared to the more commonly used ±15kHz deviation.

LONG BATTERY LIFE

High efficiency circuits throughout the design allow over 2 hours of operation using a single 9 Volt alkaline battery. (A
9V lithium battery will provide over 7 hours of operation.) The battery compartment is a unique mechanical design
which automatically adjusts to fit any brand of battery. The battery contacts are spring loaded to prevent “rattle” as the
unit is handled.

FREQUENCY AGILITY

The transmitter section uses a synthesized, frequency selectable main oscillator The frequency is extremely stable
over a wide temperature range and over time.

Two rotary switches, located on the side panel of the unit, provide 256 frequencies in 100kHz steps over a 25.5MHz
range. This alleviates carrier interference problems in mobile or travelling applications.

ANTENNA

At UHF frequencies, where wavelengths and antennas are shorter than at VHF frequencies, a resonant length wire is
preferred over using the microphone cable as the antenna. The antenna on the UM250B consists of a flexible 1/4
wavelength bronze cable, detachable via an SMA connector. The impedance of this connector is 50 Ohms.

Rio Rancho, NM – USA

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LECTROSONICS

0

1

2

3
4
5

6

7

8

9

A

B

C

D

E

F

0

1

2

3
4
5

6

7

8

9

A

B

C

D

E

F

UM250

CONTROLS AND FUNCTIONS

FREQUENCY

1.6M

B

100K

0

–20

FREQUENCY

ADJUST

LF ROLL-OFF

ADJUST

75 Hz

OFF ON

AUDIO
LEVEL

ANTENNA

35 150

LF ROLL OFF

INPUT JACK

The input on the UM250B accommodates virtually every lavalier, hand-held or shotgun microphone available. Line level
signals can also be accommodated. Use a Switchcraft TA5F connector on the cord. See the separate sheet titled
“Transmitter 5-Pin Input Jack Wiring” regarding the correct connections for various microphones, and other sources.

ON/OFF SWITCH

Turns the battery power on and off. Even when the switch is turned off or on abruptly, the pilot tone muting system
prevents “thumps” or transients from occurring.

“ON” LAMP

Glows brightly when the battery is good and the ON/OFF switch is ON. A weak or dim lamp means that the battery is
weak. When the light goes out there is about 15 to 30 minutes of operation left with alkaline batteries and about 2
hours left with lithium. If the lamp fails to light, the battery should be replaced.

The “ON” lamp 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 or lithium battery. As the battery voltage drops during use, the LED
brightness will also decrease. After about 7 hours of operation (with a lithium battery) the battery voltage will be
about 7 Volts and the LED will be nearly out.

FREQUENCY ADJUST

These two rotary switches adjust the center frequency of the carrier. The 1.6M is a coarse adjustment and the 100K
is the fine adjustment. Each transmitter is factory aligned at the center of its operating range. The default position of
the frequency select switches is in the center of the transmitter’s range.

Since the internal circuits are all tightly regulated and the RF output stage has a separate discrete regulator, the
transmitter will continue to operate to a battery voltage of 7 Volts. From 7 Volts to 6 Volts, the transmitter will still
operate, but with degraded performance. Please note that a weak battery will sometimes light the POWER LED
immediately after turn on, but will soon discharge to the point where the LED will go out, just like a flashlight with
“dead” batteries.

The combination of an accurate battery condition indicator and regulation of all internal circuits provides much longer
battery life, as well as consistent performance over the life of the battery.

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