Crown CT-400 Service manual

Com-Tech 400 Amplifier Service Manual

POWER AMPLIFIER

SERVICE MANUAL

COM-TECH® 400

Model CT400B

©1995 by CROWN INTERNATIONAL, INC.

Mailing Address:

P.O. Box 1000

Elkhart, IN U.S.A. 46515-1000

Shipping Address:

57620 C.R. 105

Elkhart, IN U.S.A. 46517

Com-Tech®, ODEP® and Crown® are registered trademarks of Crown International, Inc.

K-SVCCT400

12-95

1

Com-Tech 400 Amplifier Service Manual

The information furnished in this manual does not include all of the details of design, production, or variations
of the equipment. Nor does it cover every possible situation which may arise during installation, operation or
maintenance. If you need special assistance beyond the scope of this manual, please contact the Crown
Technical Support Group.

Mail: P.O. Box 1000 Elkhart IN 46515-1000

Shipping: 57620 C.R. 105 Elkhart IN 46517

Phone: [phone]

FAX: [phone]

CAUTION

TO PREVENT ELECTRIC SHOCK DO

NOT REMOVE TOP OR BOTTOM
COVERS. NO USER SERVICEABLE
PARTS INSIDE. REFER SERVICING

TO QUALIFIED SERVICE

PERSONNEL. DISCONNECT

POWER CORD BEFORE REMOVING

REAR INPUT MODULE TO ACCESS

GAIN SWITCH.

WARNING

AVIS

À PRÉVENIR LE CHOC

ÉLECTRIQUE N’ENLEVEZ

PAS LES COUVERTURES.

RIEN DES PARTIES

UTILES À L’INTÉRIEUR.

DÉBRANCHER LA BORNE

AVANT D’OUVRIR LA

MODULE EN ARRIÈRE.

TO REDUCE THE RISK OF ELECTRIC

SHOCK, DO NOT EXPOSE THIS

EQUIPMENT TO RAIN OR MOISTURE!

2

Com-Tech 400 Amplifier Service Manual

Table of Contents

Introduction ............................................................ 4

Parts Information .................................................... 5

Specifications......................................................... 6

Theory .................................................................... 7

Electrical Checkout Procedures........................... 12

Parts List (Non Module) ....................................... 16

Module and Schematic Information ..................... 19

Q42762-7 Control Module Parts List .................... 20

Q43214-8 Control Module Parts List .................... 21

Q43242-9 Control Module Parts List .................... 22

Q42706-4 Display Module Parts List.................... 23

Q42716-3 Output Module Parts List .................... 24

Q42888-0 Output Module Parts List .................... 25

Q42969-8 Output Module Parts List .................... 26

Q43188-4 Output Module Parts List .................... 27

Q43012-6 Main Module Parts List........................ 28

Q43043-1 Main Module Parts List........................ 31

Q43129-8 Main Module Parts List........................ 34

Q43238-7 Main Module Parts List........................ 37

3

Com-Tech 400 Amplifier Service Manual

Introduction

This manual contains service information on Crown
power amplifiers. It is designed to be used in conjunc­tion with the applicable Owner's Manual. However,
some important information is duplicated in this Ser­vice Manual in case the Owner's Manual is not readily
available.

NOTE: THE INFORMATION IN THIS MANUAL IS INTENDED
FOR USE BY AN EXPERIENCED TECHNICIAN ONLY!

Scope

This Service Manual in intended to apply to all versions
of the CT-400B amplifier including the Amcron ver­sion. The Parts Listings include parts specific for the
US version and the European version (CT-400BE13).
For parts specific only to other versions contact the
Crown Technical Support Group for help in finding
part numbers.

This Service Manual includes several sections. These
sections include Parts Information, Specifications,
Voltage Conversion, Circuit Theory, Electrical Test
Procedures, Non-Module Parts Lists, and Module
Parts Lists. Schematics are attached. Note that com­ponent parts with circuit board comprise a complete
module. Module part numbers are always associated
with a specific circuit board, although an unpopulated

circuit board may be built up with different parts to
create different modules. Note that Crown does not
sell blank (unpopulated) circuit boards.

Each of the compact audio power amplifiers are
designed for professional or commercial use. Provid­ing high power amplification from 20Hz to 20KHz with
minimum distortion, they feature balanced inputs with
bridged and parallel monophonic capability. Specific
features vary depending on model family.

Warranty

Each Owner's Manual contains basic policies as re­lated to the customer. In addition it should be stated
that this service documentation is meant to be used
only by properly trained service personnel. Because
most Crown products carry a 3 Year Full Warranty
(including round trip shipping within the United States),
all warranty service should be referred to the Crown
Factory or Authorized Warranty Service Center. See
the applicable Owner’s Manual for warranty details. To
find the location of the nearest Authorized Service
Center or obtain instructions for receiving Crown Fac­tory Service please contact the Crown Technical Sup­port Group (within North America) or your Crown/
Amcron Importer (outside North America).

Crown

Technical Support Group

Factory Service

Parts Department

Mailing Address:

PO Box 1000

Elkhart, IN USA 46515-1000

Shipping Address:

57620 C.R. 105

Elkhart, IN USA 46517

Phone: [phone]

Toll Free: [phone]

FAX: [phone]

4

Com-Tech 400 Amplifier Service Manual

Parts Information

General Information

Later sections include both mechanical and electrical
parts lists for this product. The parts listed are current
as of the date printed. Crown reserves the right to
modify and improve its products for the benefit of its
customers.

Part Numbering System

As of the printing of this manual, Crown is using two
numbering systems. The elder system always uses
eight characters. The first character is a letter. Com­mon letters used are C, D, H, M, P, and Q. The second
through sixth characters are numbers. The numbers
build sequentially (for each prefix letter) as new parts
are added to our parts inventory system. (In some
cases there will be a space then a four character
number after the prefix letter; the space is considered
a character.) The seventh character is usually a hy­phen, though it may be a letter to indicate a revision or
special note. The last character is called a check-digit,
and is useful to Crown for internal tracking.

Crown is in the process of converting to a new part
number system. Length may vary from eight to twelve
characters. There is still a letter prefix, then five
numbers. These five numbers identify a type of part.
The seventh character is a hyphen. Remaining char­acters identify the details of the type of part identified
by the first part of the number.

Standard and Special Parts

Many smaller electrical and electronic parts used by
Crown are stocked by and available from electronic
supply houses. However, some electronic parts that
appear to be standard are actually special. A part
ordered from Crown will assure an acceptable re­placement. Structural items such as modules and
panels are available from Crown only.

shipment on a C.O.D. or pre-payment (check or credit
card) basis.

Terms

Normal terms are pre-paid. Net-30 Days applies to
only those firms having pre-established accounts with
Crown. If pre-paying, the order must be packed and
weighed before a total bill can be established, after
which an amount due will be issued and shipment
made upon receipt of pre-payment. New parts re­turned for credit are subject to a 10% re-stocking fee,
and authorization from the Crown Parts Department
must be obtained before returning parts for credit.

Crown is not a general parts warehouse. Parts sold by
the Crown Parts Department are solely for servicing
Crown/Amcron products. Part prices and availabil-

ity are subject to change without notice.

Crown

Parts Department

Mailing Address:

PO Box 1000

Elkhart, IN USA 46515-1000

Shipping Address:

57620 C.R. 105

Elkhart, IN USA 46517

Ordering Parts

When ordering parts, be sure to give the product
model, and include a description and part number
(CPN/DPN) from the parts listing. Price quotes are
available on request.

Shipment

Shipment will be normally made by UPS or best other
method unless you specify otherwise. Shipments are
made to and from Elkhart, Indiana USA, only. Estab­lished accounts with Crown will receive shipment
freight prepaid and will be billed. All others will receive

Phone: [phone]

or: [phone]

Toll Free: [phone]

FAX: [phone]

5

Com-Tech 400 Amplifier Service Manual

Specifications

Unless noted otherwise, all specifications are based
on driving an 8 ohm load per channel, both channels
driven, the sensitivity switch in the 26dB position, the
AC supply is 120VAC at 60Hz. Crown specifications
are guaranteed through the warranty period (normally
3 years). Because our testing methods are more strin­gent than our published specifications, every Crown
amplifier will exceed its published specifications.

Power

Power

8 Ohm Stereo—230W/Ch
4 Ohm Stereo—255W/Ch
70-V Line Stereo—225W/Ch
8 Ohm Bridge Mono—470W
4 Ohm Parallel Mono—445W
2 Ohm Parallel Mono—450W
Bridge Mono, 140-V Line—450W
Parallel Mono, 70-V Line—455W

Load Impedances: Rated for 16, 8, 4, 2, Ohm, and 70­V use. Safe with all types of loads, even totally reactive
loads.

AC Mains: 120VAC at 60 Hz with standard three-wire
grounded connector for North American units; 100VAC,
120VAC, 220VAC, and 240VAC at 50 or 60 Hz units
are available when ordered country specific. Com­Tech amplifiers are not voltage convertable.

Performance

Frequency Response: ±0.1dB from 20 Hz to 20 kHz at 1

Watt.

Phase Response: ±10° from 10 Hz to 20 kHz at 1 Watt.
Signal to Noise Ratio: A-weighted, better than 105 dB

below full rated output. Better than 100 dB below full
rated output from 20 Hz to 20 kHz.

Total Harmonic Distortion (THD): <0.05% from 20 Hz to
1 kHz, increasing linearly to 0.1% at 20 kHz at 500W.

I.M. Distortion: <0.05% from less than 164 milliwatts to
520 W at 26 dB gain.

Slew Rate: >13V per microsecond. (Slew rates are
limited to useful levels for ultrasonic/RF protection.)

Damping Factor: >1000 from 10 Hz to 400 Hz.
DC Offset: <10 millivolts.

Input Impedance: Nominally 20K ohms balanced; 10K

ohms unbalanced.
Output Impedance: <10 milliohms in series with <2

microhenries.
Protection Systems: Output Device Emulation Protec-

tion (ODEP) limits drive in the event of dangerous
dynamic thermal conditions without interrupting power.
Current limiting for shorted load protection. DC/LF and
common mode output current Fault circuitry to mute
audio. Delay of 4 seconds from turn on mutes amplifier
to prevent dangerous turn-on transients.

Mechanical

Input Connectors: Balanced three-terminal barrier block

for each channel on standard P.I.P.-BB module (in­cluded)

Output Connectors: Four-terminal barrier block.
Front Panel Controls: A front panel rocker switch used

to power the amplifier on and off.
Back Panel Controls: A three-position switch which

selects Stereo, Bridge-Mono, or Parallel-Mono mode.
A rotary potentiometer for each channel used to
control output level. A two position recessed switch for
each channel selects between 8/4 ohm and 70-V
modes. A push button circuit breaker used to protect
the power supply.

Internal Controls: A three-position switch selects 0.775V,

1.4V, or 26 dB voltage gain input sensitivity.
Indicators: Amber Enable indicator shows on/off status

of low-voltage power supply. A green indicator for
each channel shows the reserve energy status. If no
reserve energy is available the indicator will dim in
proportion to ODEP limiting. A yellow indicator for
each channel flashes in the event of distortion. And
green SPI (signal presence indicator) indicators show
the presence of output signal.

Construction: Black splatter-coat steel chassis with
specially designed flow-through ventilation system.

Mounting: Standard EIA 310 front-panel rack mount
with supports for supplemental rear corner mounting.

Dimensions: 19 inches wide, 3.5 inches high, 16 inches
deep behind front mounting surface.

Weight: 31 lbs, 12.5 oz. (14.4 kg)

6

Com-Tech 400 Amplifier Service Manual

Theory

Overview

It should be noted that over time Crown makes im­provements and changes to their products for various
reasons. This manual is up to date as of the time of
writing. For additional information regarding these
amplifiers, refer to the applicable Technical Notes
provided by Crown for this product.

This section of the manual explains the general opera­tion of a typical Crown power amplifier. Topics cov­ered include Front End, Grounded Bridge, and ODEP.
Due to variations in design from vintage to vintage
(and similarities with other Crown products) the theory
of operation remains simplified.

Features

Com-Tech amplifiers utilize numerous Crown innova­tions including grounded bridge and ODEP technolo­gies. Cooling techniques make use of the what is
essentially air conditioner technology. Air flows bot­tom to top, and front to side. Air flows a short distance
across a wide heatsink. This type of air flow provides
significantly better cooling than the “wind tunnel”
technology used by many other manufacturers. Out­put transistors are of the metal can type rather than
plastic case. This allows for a significantly higher
thermal margin for the given voltage and current
ratings. All devices used are tested and graded to
ensure maximum reliability. Another electronic tech­nique used is negative feedback. Almost all power
amplifiers utilize negative feedback to control gain
and provide stability, but Crown uses multiple nested
feedback loops for maximum stability and greatly
improved damping. Most Crown amplifiers have damp­ing in excess of 1000 in the bass frequency range. This
feedback, along with our compensation and ultra-low
distortion output topology, make Crown amplifiers
superior.

For additional details refer to the specification section,
or to the applicable Owner’s Manual.

Front End Operation

The front end is comprised of three stages: Balanced
Gain Stage (BGS), Variable Gain Stage (VGS), and
the Error Amp. Figure 1 shows a simplified diagram of
a typical front end with voltage amplification stages.

Balanced Gain Stage (BGS)

Input to the amplifier is balanced. The shield may be
isolated from chassis ground by an RC network to
interrupt ground loops via the Ground Lift Switch. The
non-inverting (hot) side of the balanced input is fed to
the non-inverting input of the first op-amp stage. The
inverting (negative) side of the balanced input is fed
to the inverting input of the first op-amp stage. A
potentiometer is provided for common mode rejection
adjustment. Electrically, the BGS is at unity gain.
(From an audio perspective, however, this stage
actually provides +6dB gain if a fully balanced signal
is placed on its input.) The BGS is a non-inverting
stage. It’s output is delivered to the Variable Gain
Stage.

Variable Gain Stage (VGS)

From the output of the BGS, the signal goes to the VGS
where gain is determined by the position of the Sen­sitivity Switch, and level is determined by the level
control. VGS is an inverting stage with the input being
fed to its op-amp stage. Because gain after this stage
is fixed at 26dB (factor of 20), greater amplifier sensi­tivity is achieved by controlling the ratio of feedback to
input resistance. The Sensitivity Switch sets the input
impedance to this stage and varies the gain such that
the overall amplifier gain is 26 dB, or is adjusted
appropriately for 0.775V or 1.4V input to attain rated
output.

Features specific to the Com-Tech Series’ include
slew rate limiting, and audio muting for delay or
protective action. This amplifier can operate in either
a Bridged or Parallel Mono mode as well as dual
(stereo). A sensitivity switch allows selection of input
voltage required for rated output. Level controls are
mounted on the rear panel and are of the rotary type.
Front panel indicators let the user know the status of
the low voltage power supply (enable), signal pres­ence, distortion, and an ODEP indicator for each
channel which shows the reserve energy status. In
general, the packaging of this model is designed for
maximum watt/price/weight/size value with user friendly
features.

Error Amp

The inverted output from the VGS is fed to the non­inverting input of the Error Amp op-amp stage through
an AC coupling capacitor and input resistor. Amplifier
output is fed back via the negative feedback (NFb)
loop resistor. The ratio of feedback resistor to input
resistor fixes gain from the Error Amp input to the
output of the amplifier at 26 dB. Diodes prevent
overdriving the Error Amp. Because the Error Amp
amplifies the difference between input and output
signals, any difference in the two waveforms will
produce a near open loop gain condition which in turn
results in high peak output voltage. The output of the

7

Com-Tech 400 Amplifier Service Manual

Theory

Error Amp, called the Error Signal (ES) drives the
Voltage Translators.

Voltage Amplification

The Voltage Translator stage separates the output of
the Error Amp into balanced positive and negative
drive voltages for the Last Voltage Amplifiers (LVAs),
translating the signal from ground referenced ±15V to
±Vcc reference. LVAs provide the main voltage ampli­fication and drive the High Side output stages. Gain
from Voltage Translator input to amplifier output is a
factor of 25.2.

Voltage Translators

A voltage divider network splits the Error Signal (ES)
into positive and negative drive signals for the bal­anced voltage translator stage. These offset reference
voltages drive the input to the Voltage Translator
transistors. A nested NFb loop from the output of the
amplifier mixes with the inverted signal riding on the
offset references. This negative feedback fixes gain at
the offset reference points (and the output of the Error
Amp) at a factor of -25.2 with respect to the amplifier
output. The Voltage Translators are arranged in a
common base configuration for non-inverting voltage
gain with equal gain. They shift the audio from the
±15V reference to VCC reference. Their outputs drive
their respective LVA.

Also tied into the Voltage Translator inputs are ODEP
limiting transistors and control/protection transistors.
The ODEP transistors steal drive as dictated by the
ODEP circuitry (discussed later). The control/protec­tion transistors act as switches to totally shunt audio to
ground during the turn-on delay, or during a DC/LF or

Fault protective action.

Last Voltage Amplifiers (LVAs)

The Voltage Translator stage channels the signal to
the Last Voltage Amplifiers (LVA's) in a balanced
configuration. The +LVA and -LVA, with their push-pull
effect through the Bias Servo, drive the fully comple­mentary output stage. The LVAs are configured as
common emitter amplifiers. This configuration pro­vides sufficient voltage gain and inverts the audio. The
polarity inversion is necessary to avoid an overall
polarity inversion from input jack to output jack, and it
allows the NFb loop to control Error Amp gain by
feeding back to its non-inverting input (with its polarity
opposite to the output of the VGS). With the added
voltage swing provided by the LVAs, the signal then
gains current amplification through the Darlington
emitter-follower output stage.

Grounded Bridge Topology

Figure 2 is a simplified example of the grounded
bridge output topology. It consists of four quadrants
of three deep Darlington (composite) emitter-follower
stages per channel: one NPN and one PNP on the
High Side of the bridge (driving the load), and one
NPN and one PNP on the Low Side of the bridge
(controlling the ground reference for the rails). The
output stages are biased to operate class AB+B for
ultra low distortion in the signal zero-crossing region
and high efficiency.

High Side (HS)

The High Side (HS) of the bridge operates much like
a conventional bipolar push-pull output configuration.

Audio

Inputs

8

BGS VGS Error

Amp

+

-

+

-

+

-

Figure 1. Typical Amplifier Front End and Voltage Amplification Stages.

+15V

Voltage Divider

+

-15V

-

ODEP

Q100

Q103

Mute

Q121

Q122

NFb Loop

Voltage

Translators

Q101

Q102

+VCC

Q105

NPN Outputs (+HS)
PNP Outputs (-HS)

Q110

-VCC

LVA's

Com-Tech 400 Amplifier Service Manual

Theory

As the input drive voltage becomes more positive, the
HS NPN conducts and delivers positive voltage to the
load. Eventually the NPN devices reach full conduc­tion and +Vcc is across the load. At this time the HS
PNP is biased off. When the drive signal is negative
going, the HS PNP conducts to deliver -Vcc to the load
and the HS NPN stage is off.

The output of the +LVA drives the base of predriver
device. Together, the predriver and driver form the
first two parts of the three-deep Darlington and are
biased class AB. They provide output drive through
the bias resistor, bypassing the output devices, at
levels below about 100mW. An RLC network between
the predriver and driver provide phase shift compen­sation and limit driver base current to safe levels.
Output devices are biased class B, just below cutoff.
At about 100mW output they switch on to conduct high
current to the load. Together with predriver and driver,
the output device provide an overall class AB+B
output.

The negative half of the HS is almost identical to the
positive half, except that the devices are PNP. One
difference is that the PNP bias resistor is slightly
greater in value so that PNP output devices run closer
to the cutoff level under static (no signal) conditions.
This is because PNP devices require greater drive
current.

HS bias is regulated by Q18, the Bias Servo. Q18 is a
Vbe multiplier which maintains approximately 3.3V
Vce under static conditions. The positive and negative
halves of the HS output are in parallel with this 3.3V.
With a full base-emitter on voltage drop across
predrivers and drivers, the balance of voltage results
in approximately .35V drop across the bias resistors in
the positive half, and about .5V across the bias resistor
in the negative half. Q18 conduction (and thus bias) is
adjustable.

A diode string prevents excessive charge build up
within the high conduction output devices when off.
Flyback diodes shunt back-EMF pulses from reactive
loads to the power supply to protect output devices
from dangerous reverse voltage levels. An output
terminating circuit blocks RF on output lines from
entering the amplifier through its output connectors.

Low Side (LS)

The Low Side (LS) operates quite differently. The
power supply bridge rectifier is not ground refer­enced, nor is the secondary of the main transformer.
In other words, the high voltage power supply floats
with respect to ground, but ±Vcc remain constant with
respect to each other. This allows the power supply to
deliver +Vcc and -Vcc from the same bridge rectifier
and filter as a total difference in potential, regardless
of their voltages with respect to ground. The LS uses

+

+Vcc (Positive Rail)

Input
signal

HIGH SIDE LOW SIDE

Load

(speaker)

-Vcc (Negative Rail)

Inverting Op-amp

-

Figure 2. Crown Patented Grounded Bridge Topology

9

Com-Tech 400 Amplifier Service Manual

Theory

inverted feedback from the HS output to control the
ground reference for the rails (±Vcc). Both LS quad­rants are arranged in a three-deep Darlington and are
biased AB+B in the same manner as the HS.

When the amplifier output swings positive, the audio is
fed to an op-amp stage where it is inverted. This
inverted signal is delivered directly to the bases of the
positive (NPN) and negative (PNP) LS predrivers. The
negative drive forces the LS PNP devices on (NPN
off). As the PNP devices conduct, Vce of the PNP
Darlington drops. With LS device emitters tied to
ground, -Vcc is pulled toward ground reference.
Since the power supply is not ground referenced (and
the total voltage from +Vcc to -Vcc is constant) +Vcc
is forced higher above ground potential. This contin­ues until, at the positive amplifier output peak, -Vcc =
0V and +Vcc equals the total power supply potential
with a positive polarity. If, for example, the power
supply produced a total of 70V from rail to rail (±35VDC
measured from ground with no signal), the amplifier
output would reach a positive peak of +70V.

Conversely, during a negative swing of the HS output
where HS PNP devices conduct, the op-amp would
output a positive voltage forcing LS NPN devices to
conduct. This would result in +Vcc swinging toward
ground potential and -Vcc further from ground poten­tial. At the negative amplifier output peak, +Vcc = 0V
and -Vcc equals the total power supply potential with
a negative polarity. Using the same example as above,
a 70V supply would allow a negative output peak of ­70V. In summary, a power supply which produces a
total of 70VDC rail to rail (or ±35VDC statically) is
capable of producing 140V peak-to-peak at the ampli­fier output when the grounded bridge topology is
used. The voltage used in this example are relatively
close to the voltages of the PB-1/460CSL.

perform the same function as the HS flybacks. The
output of the LS is tied directly to chassis ground via
ground strap.

Output Device Emulation Protection (ODEP)

To further protect the output stages, a specially devel­oped ODEP circuit is used. It produces a complex
analog output signal. This signal is proportional to the
always changing safe-operating-area margin of the
output transistors. The ODEP signal controls the Volt­age Translator stage by removing drive that may
exceed the safe-operating-area of the output stage.

ODEP senses output current by measuring the volt­age dropped across LS emitter resistors. LS NPN
current (negative amplifier output) and +Vcc are
sensed, then multiplied to obtain a signal proportional
to output power. Positive and negative ODEP voltages
are adjustable via two potentiometers. Across ±ODEP
are a PTC and a thermal sense (current source). The
PTC is essentially a cutoff switch that causes hard
ODEP limiting if heatsink temperature exceeds a safe
maximum, regardless of signal level. The thermal
sense causes the differential between +ODEP and –
ODEP to decrease as heatsink temperature increases.
An increase in positive output signal output into a load
will result in –ODEP voltage dropping; an increase in
negative output voltage and current will cause +ODEP
voltage to drop. A complex RC network between the
±ODEP circuitry is used to simulate the thermal barri­ers between the interior of the output device die
(immeasurable by normal means) and the time delay
from heat generation at the die until heat dissipates to
the thermal sensor. The combined effects of thermal
history and instantaneous dynamic power level result
in an accurate simulation of the actual thermal condi­tion of the output transistors.

The total effect is to deliver a peak to peak voltage to
the speaker load which is twice the voltage produced
by the power supply. Benefits include full utilization of
the power supply (it conducts current during both
halves of the output signal; conventional designs
require two power supplies per channel, one positive
and one negative), and never exposing any output
device to more than half of the peak to peak output
voltage (which does occur in conventional designs).

Low side bias is established by a diode string which
also shunts built up charges on the output devices.
Bias is adjustable via potentiometer. Flyback diodes

10

Com-Tech 400 Amplifier Service Manual

Theory

BALANCED

INPUTS

+VOLTAGE

TRANSLATOR

VGSBGS

-1

ERROR

AMP

OUTPUT

DEVICE

EMULATION

PROTECTION

-VOLTAGE

TRANSLATOR

+LVA

-1

HIGH SIDE

BIAS

SERVO

-LVA

-1

MAIN NEGATIVE FEEDBACK (NFb) LOOP

LOW SIDE

-1

INVERTING

BRIDGE

BALANCE

BIAS

DIODE

STRING

POSITIVE

HIGH SIDE

OUTPUT

NPN STAGE

NEGATIVE
HIGH SIDE

OUTPUT

PNP STAGE

POSITIVE

LOW SIDE

OUTPUT

NPN STAGE

NEGATIVE
LOW SIDE

OUTPUT

PNP STAGE

Figure 3. Typical Crown Amplifier Basic Block Diagram (One Channel Shown)

11

Com-Tech 400 Amplifier Service Manual

Electrical Checkout Procedures

General Information

The following test procedures are to be used to verify
operation of this amplifier. DO NOT connect a load or
inject a signal unless directed to do so by the proce­dure. These tests, though meant for verification and
alignment of the amplifier, may also be very helpful in
troubleshooting. For best results, tests should be
performed in order.

All tests assume that AC power is from a regulated 120
VAC source. Test equipment includes an oscillo­scope, a DMM, a signal generator, loads, and I.M.D.
and T.H.D. noise test equipment.

Standard Initial Conditions

Level controls fully clockwise.
Stereo/Mono switch in Stereo.
Sensitivity switch in 26 dB fixed gain position.
70-V mode switches in the 8/4 ohm position.
It is assumed, in each step, that conditions of the
amplifier are per these initial conditions unless other­wise specified.

Test 1: DC Offset

Spec: 0 VDC, ±10 mV.
Initial Conditions: Controls per standard, inputs shorted.
Procedure: Measure DC voltage at the output connec-

tors (rear panel). There is no adjustment for output
offset. If spec is not met, there is an electrical malfunc­tion. Slightly out of spec measurement is usually due
to U104/U204 out of tolorance.

Test 2: Output Bias Adjustment

Spec: 300 to 320 mVDC.
Initial Conditions: Controls per standard, heatsink tem-

perature less than 40°C.
Procedure: Measure DC voltages on the output module
across R31 (15 ohm), adjust R26 if necessary. Mea­sure DC voltages on the output module across R32 (15
ohm), adjust R23 if necessary. Repeat for second
channel.

Test 3: ODEP Voltage Adjustment

Spec: Bias Per Chart, ±0.1V DC.
Initial Conditions: Controls per standard, heatsink at

room temperature 20 to 30°C (68 to 86°F). Note: This
adjustment should normally be performed within 2
minutes of turn on from ambient (cold) conditions. If
possible measure heatsink temperature, if not mea­sure ambient room temperature. Use this information
when referencing the following chart.

The following list of ODEP bias voltages VS. tempera­ture is based on the use of Main Modules built on any
circuit board up to and including the D 7993-5 board.
°F °C V

–ODEP

V

+ODEP

66 18.9 –10.31 11.41
68 20.0 –10.26 11.36
70 21.1 –10.20 11.30
72 22.2 –10.14 11.24
74 23.3 –10.09 11.19
76 24.4 –10.03 11.13
77 25.0 –10.00 11.10
78 25.6 –9.97 11.07
80 26.7 –9.91 11.01
82 27.8 –9.86 10.96
84 28.9 –9.80 10.90
86 30.0 –9.74 10.84
88 31.1 –9.69 10.79
90 32.2 –9.63 10.73
92 33.3 –9.57 10.67
94 34.4 –9.51 10.61

The following list of ODEP bias voltages VS. tempera­ture is based on the use of Main Modules built on any
circuit board after the D 7993-5 board.
°F °C V

–ODEP

V

+ODEP

66 18.9 –10.31 10.31
68 20.0 –10.26 10.26
70 21.1 –10.20 10.20
72 22.2 –10.14 10.14
74 23.3 –10.09 10.09
76 24.4 –10.03 10.03
77 25.0 –10.00 10.00
78 25.6 –9.97 9.97
80 26.7 –9.91 9.91
82 27.8 –9.86 9.86
84 28.9 –9.80 9.80
86 30.0 –9.74 9.74
88 31.1 –9.69 9.69
90 32.2 –9.63 9.63
92 33.3 –9.57 9.57
94 34.4 –9.51 9.51

–ODEP Procedure: Measure pin 3 of J500 and, if neces­sary, adjust R121 to obtain V

as specified above.

–ODEP

Measure pin 3 of J500 and, if necessary, adjust R221
to obtain V

as specified above.

–ODEP

+ODEP Procedure: Measure pin 4 of J500 and, if neces­sary, adjust R132 to obtain V

as specified above.

+ODEP

Measure pin 4 of J500 and, if necessary, adjust R232
to obtain V

as specified above.

+ODEP

12