Crest Audio 3301 Service Manual

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SSCRESIALJDIO

4601 Professional Power Amplifier

Clip/Limit 88mm
Signal

Temp/DC
Active

Professional Series
Power Amplifiers

3301
4601
4801
6001
7001

Service Manual

CREST AUDIO lNC., 100 Eisenhower Drive, Paramus, New Jersey 07652 USA
T (201) 909-8700 FAX (201) 909-8666

CREST AUDIO EU ROPE, 5a Wilbury Grove, Hove, East Sussex BN3 SJQ England
T44+(0)273-325840 FAX 44+(0)273~775462

CREST AUDIO ASlA., 6001 Beach Road #17-03, Golden Mile Tower, Singapore 0719
fi65+(0)295-2328 FAX 65+(0)295-47D7

Release 1.1 4/93 Domestic

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the user manual.

This symbol, a lightning flash with arrowhead within an equilateral triangle, appears on the
amplifier chassis to warn the user that uninsulated dangerous voltaggggge present within the
enclosure that may pose a risk of electric shock. "

This symbol, an exclamation point within an equilateral triangle, appears on the amplifier
chassis to warn the user to follow important operating procedures and precautions detailed in

This manual and the procedures detailed within are intended for qualified technicians
only! There are potentially lethal voltages present within an audio power amplifier, and it
is the responsibility of the technician to exercise common safety practices to protect him-

self, his co-workers, and the end user.

In particular, always exercise caution when working on an amplifier with the chassis
cover removed. Do not leave it unattended while it is connected to the Ac mains. Never
expose the amplifier or your test equipment to rain or moisture, and never work on an am-
plifier while you are standing in, sitting in or othenlvise in contact with, rain or moisture.
Before starting any repair or troubleshooting procedure, read and understand completely
the instructions for the procedure.

1 INTRODUCTION

#

This service manual includes tests, calibrations, schematics, a full parts list/order form, and troubleshoot-
ing tips to assist you in the repair and maintenance of Crest Audio Pro series 3301, 4601, 4801, 6001, and

7001 amplifiers.

The first step in troubleshooting is to identify the symptom. The next step is a complete visual inspection.
You may isolate the problem, trace it to a module, and swap it with a good module or continue trou-
bleshooting down to component level. If you follow this manual step-by-step it will be that easy. Good

luck!
Identify Visual Isolate Replace or
Symptoms Inspection Problem Troubleshoot

Glossary

Following is a list of terminology used in this manual.
Board A printed circuit board, or PCB.
Chassis The steel case or frame in which the modules, transformer, etc. are mounted.
Harness This term refers to all the wires, as a whole, which interconnect the boards carrying

specific voltages, signals etc.

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Fan Driver Board

Heatsink Board

Preamp Board

NOTE: The Crest Audio Professional Series Amplifiers discussed in this manual use
variable speed fan circuitry. This board is located in the cenme amplifier between
the power transformer and the output modules. It is mounféd on a metal shield.

Powered by the display board, this circuitry determines the speed of the fan. Anytime
the amplifier is in protect mode or if the temperature sensor is disconnected, the fan(s)
will run at its (their) highest speed. During normal operation the fan(s) will run at a
slower speed. With respect to ground, the voltage regulator 1C1 (LM317T) will read
approximately 25 volts DC at pin 3 when the fan(s) is/are running at the highest speed.
During normal operation pin 3 of 1C1 will read approximately 14 volts DC.

The fan speed will increase linearly with the temperature.
NOTE: 0n model 4801, the fan driver circuitry is located an the Power Supply Board.

These two boards, one per channel, contain the output transistors and their support
circuitry (bias, Class AB or H, temp. sensing etc.) and are located on the right side of
the amplifier chassis. Each is mounted to an aluminum heatsink. (See Figure 1-1.)

These two boards, one per channel, mate to the Heatsink Boards via a 17-pin Molex
connector, 3 metal shield, and mounting screws (see Figure l-l). There are two main
versions of the Preamp Board: The older versions (before June 1991; serial numbers
9106XXX and earlier) used a 6-pin Clairex CLM 5010 optocoupler for IGM current
limiting and have a board identification number with the prefix 845. The newer version
uses an NE5517AN operational transconductance amplifier (OTA) in place of the
optocoupler and has a board identification number with the prefix 26C 1 124 (from July
1991; serial numbers 9107XXX and later).

The Preamp board contains the following circuitry:

IGM (Instantaneous Gain Modulation) - This circuit, developed and used exclusively
by Crest, allows the amplifier to operate safely into any load. When the amplifier sees
a load condition that overstresses the output stage, IGM attenuates the input thus
reducing the output to a safe level. In amplifiers manufactured before June 1991, the
IGM circuit uses a Clairex CLMSOlO optocoupler (or opto for short), shown on the

Preamp board

Figure 1-1 - Output Module

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3 Set up the oscilloscope for viewing both channels of the scope. Use DC settings for each channel.
Use the horizontal and vertical position controls to set the oscilloscope chagrielsA & B to the 0
position, so the traces run horizontally across the center of the screen.

4 Models 4801, 6001, 7001: Locate the two 390-ohm 5-watt resistors at the back end (opposite the
17-pin header) of the heatsink board (R140, R141 see component layout #1). Keep in mind that the
positive rail reference point will be the outside lead of the 390 ohm resistor located on the right
(black or positive) side of the board. The negative rail reference point will be the outside lead of
the 390 ohm resistor located on the left (green or negative) side of the board.

Connect oscilloscope Channel A to the negative (-) rail of the heatsink board by attaching its probe to
the outside lead of the 390-ohm resistor on the left (green or negative) side of the board. This
connects directly to the emitters of the negative (-) rail transistors.

Models 3301 & 4601: Locate the two 22 mF capacitors at the back end (the end furthest away from
you-opposite the 17-pin header) of the heatsink board. Keep in mind that the positive rail
reference point will be the outside lead of the 22 mF capacitor located on the right (black or
positive) side of the board. The negative rail reference point will be the outside lead of the 22 mF
capacitor located on the left (green or negative) side of the board.

Connect oscilloscope Channel Bs probe to the output section at either lead of any of the .33-ohm
5-watt emitter resistors (R124, 126 etc.).

Note: During this test Channel 8s oscilloscope probe should remain connected to the output
section of the module under test. Channel As oscilloscope probe will be moved from the positive
rail to the negative rail as the test progresses.

Continue with subsection 4.4.2.
4.4.2 Full module test

Tools and materials needed:

V As in subsection 4.4.1
I/ Clamp-on current probe (Fluke Y801, Amprobe RS-l or equivalent)

Setup:

u/ Perform procedure in subsection 4.4.1.
t/ Set the digital multimeter to AC volts and connect it across the Variac output.
V Connect the clamp on the current probe across either wire connected to the circuit breaker.

Procedure:

1 Familiarize yourself with the heatsink board. Examine the positive and negative rail circuitry and
compare it to the respective schematic. Notice that the positive rail uses ZSC3281 N'PN (black)
power transistors. The negative rail uses 28A1302 PNP (green) power transistors. NOTE: 0n the
4801, 6001, 7001 the output relays are located on the power supply board. 0n the 3301 and
4601, the output relays are located on the heatsink board.

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Channel A on the oscilloscope should show an increase in negative (-) or positive (4-) DC voltage
(depending on which reference point you are connected to). For class AB 114% 3301 and 4601
you should see that the DC rails will remain horizontal and flat (Figure 433). For the class H
models 4801, 6001, and 7001 you should see the rails modulating in sequence with the respective
positive or negative peak of the output sine wave (Figure 4-4). If everything is ok, turn the Variac
off, wait a full minute and remove the Heatsink Board Test Module.

If any changes, other than those described, occur see Symptoms and Solutions, Subsection 5.1.2

Steps 7 and 8 are (1 Signal F low Test.

7 Connect the good Preamp board to the Heatsink board under test (via the 17-pin connector) and
connect Channel Bs input and display connectors (See Figure 4). Set the audio signal generator
for a 2 kHz sine wave with a 1 volt balanced output. Use
a male XLR input connector with pin 3 as (+)
non-inverting (hot), pin 2 as (-) inverting and pin 1 as
ground. For use with an unbalanced signal source, tie
the inverting (out-of-phase) input (pin 2) to ground (pin
1) inside the XLR plug. Apply the signal to Channel Bs
input XLR on the rear panel or the amplifier.

Set the mode select switch (on the back of the amplifier) to
the STEREO" position and Channel Bs attenuator
control to 0 dB (full clockwise).

8 Slowly increase the Variac output up to 40 volts while
carefully observing the oscilloscope screen.

Channel B on the oscilloscope should always remain
at zero in the center of the screen.

Channel A on the oscilloscope should show an increasing
positive DC voltage if it is connected to the positive rail
reference point or an increasing negative DC voltage if it
is connected to the negative rail reference point. If this is
so, bring the Variac up to full line voltage (120 VAC or
220/240 VAC).

As you near full line voltage, the relays should click on,
turning the amplifier on and passing the 2 kHz sine wave Figure 4_3_ mass H

on to oscilloscope Channel B. On models 4801, 6001,

and 7001, Channel A on the oscilloscope should now show the positive (+) or negative (-) high
rails turning on synchronously with the signal (depending on which reference point you are
connected to). This is Class H operation (Figure 4-4).

On models 3301 and 4601, channel A on the oscilloscope should now show the positive (+) or
negative (-) DC rails (depending on which reference point you are connected to) displayed
horizontally across the screen. This is class AB operation (figure 4-3).

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5 Turn the Variac off and wait a full minute for the power supply capacitors to discharge. Remove the
preamp board from the heatsink board. Using an ohm-meter or diode tester chiclgyr open or
shorted transistors or diodes. ~

Use this section, the block diagram, and the preamp board circuit description to isolate the
problem area on the preamp.

On the older preamp board version which uses an opto-coupler - (used up to the following serial
numbers; 4801-#9107KOO, 6001-#9109E00, 7001-#9107E00 ) NOTE: All 3301 and 4601
amplifiers use the new style preamp boards.

IC1: an NE5532 dual op-amp (originally this IC was an LF353; it was replaced in later production
runs by the NE5532 to improve performance), which is the active component in the actively
balanced input and the subsequent gain stage.

ICZ: an RC4559N dual op-amp, which lies in the servo circuitry and also detects signal at the channel
output.

IC3: also an RC4559N; it serves as the limit detector and the clip detector.
IC4: an LM393N dual comparator, which operates the current limiter circuitry.

0n the newer preamp board (#26C1124-X), which has an operational transconductance amplifier

lC'l: an NE5532 dual op-arnp; it is the active component in the actively balanced input and the
subsequent gain stage.

IC2: an RC4559N dual op-amp, which lies in the servo circuitry and also detects signal at the channel
output.

IC3: an LM339N quad comparator,which operates the current limiter circuitry, the attenuator driver,
and the clip detector.

IC4: an NE5517AN dual operational transconductance amplifier, which attenuates the signal when the
channel is clip limiting, in IGM, or in Auto-Ramp.

6 Using an ohmrneter or in-circuit transistor tester, check all the transistors and diodes in the faulty
section of circuitry, on the preamp. Transistors Q6, Q8, Q10, Q12. and Q14 are 2N5551 NPNs,
while Q7, Q9, Q11, Q13, and Q15 are 2N5401 PN'Ps. Diodes D18 and D19 are type SF14. Zener
diodes Z1 and 22 are type 1N53SSB, and Z3 and Z4 are type 1N5938B.

7 Remember to use an ohmmeter to test resistors for proper values as they may have shorted or burned
open.

If you turn the Variac up to full mains voltage and the channel stays in protect but the module output
shows no latch check for voltage between the chassis and the heatsink. If there is no voltage on the
heatsink, go back and thoroughly check the board which you are troubleshooting. Something may
have been overlooked.

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