CBS VOLUMAX 4300 Manual

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V0LUMAX
Model
4300
Automatic .Peak Controller
OPERATING AND MAINTENANCE INSTR ~ IONS
OCTOBER 1974
PROFESSIONAL PRODUCTS
CBS LABORATORIE~
A Division of CBS Inc.
Stamford, Connecticut 06905

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TABLE OF CONTENTS
Page No.
SECTION I INTRODUCTION
1-1
General
1-1
1-2
Functional Description
1-1
1-3
Physical Description
1-4
1-4
Principles of Operation
1-5
1-5
Warranty
1-6
1-6
1
Factory Service and Repair
1-6
1-7
Specifications
1-6
SECTION II INSTALLATION
2-1 Unpacking 2-1
2-2
Physical Installation
2-1
2-3 Power Source
2-5
2-4
Electrical Connections
2-6
SECTION ill OPERATIONAL SET-UP PROCEDURES
• 3-1
General
3-1
3-2 Input
and Output Phasing
3-2
3-3
Adjustment for Maximum Modulation
3-6
3-4 Alternate Method of Phasing and Setting
3-8
Modulation Level
3-5 Headroom Consideration
3-9
3-6 Proof-of-Performance Measurements
3-11
3-7
Internal Trimming of Output Leve 1 3-12
SECTION IV
THEORY OF OPERATION
4-1 Block Diagram and Schematic
4-1
4-2 Input/Output
Circuitry and AGC Action
4-1
4-3 AGC Amplifier
4-3
4-4 Automatic Peak Phasing (A . P. P.) Circuit
4-5
SECTION V MAINTENANCE
5-1 General
5-1
5-2
Alignment Procedures and Tests
5-6
5-3 Troubleshooting Audio Input Section
5-20
• 5-4
Troubleshooting Phase Reversal Section
5-21
5-5
Troubleshooting Audio Output Section
5-21
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TABLE OF CONTENTS (Cont)
SE
CT ION V (Cont)
5-6
5-7
5-8
5-9
SECTION VI
6-1
6-2
6-3
6-4
6-5
6-6
SECTION VII
Figure
1-1
1-2
1-3
2-1
2-1
2-2
3-1
3-2
3-3
3-4
Troubleshooting AGC Section
Troubleshooting Automatic Peak Phasing
Section
Troubleshooting Phase Indicator Circuits
Troubleshooting Gain Meter Circuit
PARTS LISTS
General
Resistors
Transistors and Diodes
Manufacturers' Name Abbreviations
Main Assembly, Volumax 4300 ( CBS
Laboratories Part No. 962400)
Printed Circuit Board Assembly (Al)
DRAWINGS
LIST
OF ILLUSTRATIONS
Volumax Model 4300
Phase Switching, Comparison Waveforms
Volumax 4300 Rear View
Volumax Model 4300, Installation Drawings
(Sheet 1 of 2 Sheets)
Volumax Model 4300, Installation Drawings
(Sheet 2 of 2 Sheets )
Typical Volumax Installation Block Diagram
Volumax Model 4300 Front Panel Controls
Correctly Phased Modulated R-F Envelope
Incorrectly Phased Modulated R-F Envelope
Squared Tone Modulation (95 Percent)
Page No.
5-22
5-23
5-26
5-27
6-1
6
-1
6-1
6-2
6-3
6-5
7-1
Page No.
1-0
1-2
1-7
2-2
2-3
2-4
3-3
3-5
3-5
3-7
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Figure
3-5
3-6
4-1
5-1
5-2
5-3
5-4
5-5
5-6
7-1
7-2
Table
5-1
5-2
LIST OF ILLUSTRATIONS (Cont)
Correctly Phased Modulated R- F Envelope
(as Indicated by a Trapezoidal Pattern)
Incorrectly Phased Modulated R-F Envelope
(as Indicated by a Trapezoidal Pattern)
Volumax Model 4300 Block Diagram
Volumax Unit with Top Cover Removed
Factory Test Setup
Positive Limit Output Levels for the Three
Positions of S3
Release Time Waveform
A. P. P. Test Circuit
Integrated Circuit Pin Identification
Volumax 4300 Schematic Diagram
Circuit Board Al Parts Locations
LIST OF TABLES
Power Supply Voltages
Voltage Measurements
Page No.
3-10
3-10
4-0
5-0
5-7
5-10
5-13
5-19
5-24
7-2
7-3
Page No.
5-3
5-4
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INTRODUCTION
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Figure 1-1. Volumax Model 4300
1-0

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1-1. GENERAL
SECTION I
INTRODUCTION
This manual contains instructions for the CBS Laboratories Model
4300 VolumaxR unit, a
specialized professional audio peak limiter for AM
broadcasters that prevents overmodulation while providing maximum pro-
gram power in each watt of carrie r power. When used in conjunction with
an automatic l evel contro l such as the CBS Laboratories AudimaxR, it also
guarantees program-power gains several times greater than those obtained
by conventional methods.
The present Volumax model combines the achieve-
ments of earlier Volumax models with higher standards of quality and fidelity.
Among the
improvements is an excellent signal-to-noise ratio of at least 70 db
• with
respect to maximum output.
1-2. FUNCTIONAL DESCRIPTION
The Vo lumax unit combines a superio r dual-action peak limiter with
an automatic peak phaser to ensure that the higher amplitude peaks of a pro-
gram wave always modulate the carrier in the positive direction and that
negative peaks are precisely limited to 100 percent. Th e positive direction
is used to obtain modulation levels up to 125 percent as allowed by the FCC.
Audio
is passed without modification when there are no instantaneous peaks
that would cause overmodulation, but each such peak is silently limited as just
described. No audible clicks occur when a Volumax function is performed.
• ¾egistered trade mark
1-1

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INTRODUCTION
Designed
primarily for use with well controlled average input levels such as
provided by Audimax, the Model 4300 Volumax unit supersedes all other peak
limiters in the process of preventing carrier overmodulation.
A. Conventional Peak Limiters and Instantaneous Phase Switches
With conventional
peak limiters, the broadcaster must either
reduce the program level so much that the limiting of peaks is not severe
enough to cause "pumping" -- thus lowering average modulating power -- or
must tolerate the "pumping" and consequent distortion. Although conventional
limiters moderate "pumping" by using long recovery times, the intervals im-
mediately following high program peaks waste modulation capability; in these
times, the limiter is insensitive while it recovers from reduced gain.
The Volumax unit may be regarded as an instantaneous phase-
switch limiter. Limiters of this type may waste less power, but those other
than Volumax introduce audible clicks whenever phase switching occu..-s.
Volumax
is silent , as shown in figure 1-2.
SILENT SWITCHING The two photographs above show the word "content" displayed on an oscilloscope. The
photo on the left taken with the VOLUMAX
in operation, shows clearly that phase switching took place during
the momentary pause between syllables-completely and silently.
The photo on the right using an instantaneous
phase switch shows that the phase switching took place during the second syllable-and with an audible click.
Figure 1-2. Phase Switching, Comparison Waveforms
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INTRODUCTION
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absolute limits have a ratio of 1. 15 positive to 1.00 negative. If the 125
percent position of the switch is chosen, the Volumax AGC amplifier increases
this ratio to 1. 25 to 1.00. Because of the FCC ruling which places the limit
on positive modulation at 125 percent, the 125 percent position may be used
without violation. (See Section III. )
CAUTION
The front-panel LIMITER switch should be kept in
NORMAL position during regular broadcast opera-
tion.
1-3. PHYSICAL DESCRIPTION
• Packaged in CBS Laboratories' slimline design, the Volumax unit re-
•
quires only 1-3/4 inches of standard 19-inch rack height and only 15 inches of
depth behind the front panel. The weight is 6. 5 pounds. Its reliable solid-
state circuitry is packaged on a printed circuit board with appropriate test
points to speed maintenance tests and troubleshooting. Front panel controls
are conveniently arranged, and those requiring infrequent use (e. g. , INPUT
LEVEL, OUTPUT
LEVEL, AND O UTPUT MAX dbm) are screwdriver adjust-
ments. A front panel meter indicates the relative degree of gain reduction.
The unit can be operated from either a 115- volt or 230- volt single-
phase 50 or 60 Hz power source. On delivery, the unit is connected for 115-
volt operation. Wiring changes for 230- vo lt operation are explained in para-
graph 2-3 .
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INTRODUCTION
The unit can be rack mounted simply by means of its panel mounting
flanges
(ears), but an optional chassis slide accessory . kit is available (CBS
Laboratories p/n 962163-4).
1-4. PRINCIPLES OF OPERATION
NOTE
For a complete circuit description, see Section IV
of this manual.
The program input is fed to a differential amplifier allowing balanced
or unbalanced inputs. An AGC amplifier with fast attack and moderately fast
recovery times is then used to reduce the gain of the program signal on peaks
as described in paragraph 1-2. This action in conjunction with microsecond -
fast diode limiting ensures absolute control of the limit point.
Meanwhile,
the input program signal also drives a polarity detector
and a pause detector. These circuits sample the signal and determine from
its asymmetry and level whether the phase of the signal needs to be reversed
to allow the higher peaks to produce positive modulation of the carrier. They
also determine when that change should occur. The actual phase reversal is
accomplished by a clocked flip-flop -- an integrated circuit "borrowed" from
the computer industry. This flip-flop determines whether the program signal
will pass through an inverting or non- inverting path in the Volumax unit.
The operation of the automatic peak phasing circuit is predicated on
the fact that most speech waves are asymmetric; in other words, that the
amplitude of one side may be as much as 10 dB above the other. The Volumax
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INTRODUCTION
unit
can sense asymmetry factors as low as 3 dB and turn them into useful
positive supermodulation.
The OUTPUT LEVEL
control on the front panel of the unit provides
a wide range of adjustment of modulation level. This control is a 600-ohm*
variable H-pad attenuator. In con jun ction with an adjacent, switchable, 16-db
attenuator labelled OUTPUT MAX dbm, the OUTPUT LEVEL control can be
set anywhere in a 0-to-20 dB range with maximum output at 6 or 22 dBm.
1-5. WARRANTY
A warranty with a return postcard is included with yo ur Voluma.x
unit.
Fill out the postcard and return it to CBS Laboratories as soon as pos-
sible to validate your warranty .
1-6. FACTORY SERVICE AND REPAIR
If you should experience difficulty in installing, operating, or repair -
ing
your Voluma.x unit, please contact your distributor for assistance. If
necessary, call CBS Laboratories, Professional Products Department, Stam-
ford, Connecticut (Area Code 203, 327-2000).
1-7. SPECIFICATIONS
Input and output impedance
Input
level
Maximum output
600 ohms,
balanced or unbalanced;
150 ohms on
special order
-24 to +18 dBm
22 dBm
* 150 ohms
in units manufactured for the 150-ohm output impedance option.
(See Voluma.x
specifications in paragraph 1-7.)
1-6
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