Crest Audio 4801 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

Page 2

lb |>

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.

Crest Audio Professional Series Power Amplifiers Page 1-1

Service Manual Release 1.1 4/93

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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

Crest Audio Professional Series Power Amplifiers Page 1-3
Service Manual Release 1.1 4/93

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Crest Audio Professional Series amplifiers use female XLR connectors on the rear panel for inputs. Pin 1
on each XLR is signal ground, pin 2 is the inverting input and pin 3 the non-invggigg, or hot," input.
From the XLRs, the input signals are routed through the octal accessory sock-e15, which are also mounted
on the rear panel. If no accessories are being used in the sockets, small jumpers must be used to connect
pin 1 to pin 8 for the inverting input and pin 6 to pin 7 for the non-inverting input. These jumpers are fac-
tory-installed. If they are lost or damaged you can make new ones from #10 AWG (4 mm) solid, tinned

'_ 5 copper wire or order the Crest authorized replacement parts from the factory.

After the octal accessory sockets, the signal continues to the mode select switch. This switch, located just
above die input connectors, sets the amplifier to one of three operating modes: stereo, parallel," or
bridged. The stereo" mode is normal two-channel operation, in which a signal at Channel As input pro-
duces an analogous amplified signal at Channel As output, and likewise, a signal at Channel Bs input pro-
duces an amplified signal at Channel Bs output. In parallel operation, both outputs are driven from
Channel As input; Channel Bs input is unused. NOTE: Parallel mode does not imply that the outputs
of the amplifier can be wired in parallel In bridged operation, both outputs are also driven from
Channel A, except that the mode switch reverses the polarity of the signal going into Channel Bs circuitry;
this makes the two channels work in opposition, effectively making the two channels into a single monau-
ral amplifier with double the voltage swing . To bridge, the load must be connected between Channel A
and Bs red output terminals. The mode select switch only acts on Channel B and never affects the signal
on Channel A.

The first active circuitry in the signal path, assuming no octal socket accessories are in use, is the balanced
transforrnerless amplifier formed by operational amplifier (op-amp) lC 1B, along with five resistors (R9A,
R9B, R9C, R9D, and R10 on the old version, and SIPIA, SIPlB, SIPlC, SIPlD, and R12 on the newer ver-
sion implemented in late 199l), and two capacitors (old version: C3 and C4; new version: C4 and C6).
Shielded-pair cable in the wiring harness carries the signal from the input board to the preamp board,
where the input stage lies. Note that in this differential configuration, the non-inverting input of the power
amplifier enters the inverting input of the op-amp, and vice-versa. The two paralleled resistor-capacitor
pairs provide a 6-dB per octave high-frequency rolloff starting at 500 kHz to filter RF interference. Four di-
odes protect the input stage from excessive differential and common-mode input voltages. The input stage
presents a balanced input impedance of 20 kilohms.

In the middle

The signal leaves the preamp board and is routed to the input attenuators, or volume controls, located on
the display board. It returns to op-amp IClA, set up as a +6 dB gain stage. This is also where the clip limit-
ing, IGM, muting, and Auto-Ramp circuits act on the signal. On the older preamp boards, the attenuation
was performed by the photoresistor section of a Clairex CLM5010 opto-isolator connected across the feed~
back resistor R16, with the op-amp configured as an inverting amplifier. When no attenuation is taking
place, the resistance of the photoresistor is high enough that it is essentially out of the circuit and thus has
no effect on the signal. When clip limiting, IGM, muting, or Auto-Ramp is triggered, their respective con-
trol circuits draw current through the internal LED in the CLM5010. The light reduces the resistance of the
photoresistor and thus also reduces the signal gain. On die newer preamp board version, an operational tran-
sconductance amplifier, 1C4, handles attenuation when necessary by shunting some of the signal off resis-
tor R13, which is in series with the input of IClA, set up as a non-inverting amplifier. When the OTA
receives no control current from the clip limit, IGM, mute, or Auto~Rarnp control circuits, it has zero gain
and draws no signal, presenting an ultra-high impedance at lClAs non-inverting input, so the signal
passes through R13 with no attenuation.

Page 2-2 Crest Audio Professional Series Power Amplifiers
Service Manual Release 1.1 4/93

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7 Check the insulator underneath the transistor for rips, holes, or debris. Clear away anything that may
have caused a puncture in the insulator. :2;-

: 8 Replace the coolsil or mica. If a mica washer is used, coat both the heatsink area and the device with
a thin film of silicone thermal grease first.

9 Replace the transistors mounting screw. Tighten the mounting screws of the large power transistors
to 15 lb.-in (1.7 N-m). Over tightening will damage the mica and/or deform the coolsil insulator.
Carefully hand- tighten the mounting screws of the smaller devices enough to hold firmly without
crushing the shoulder washer, which would short the device to the heatsink. Replace the shoulder
washer if it is damaged. (Figure 5-7)

Step 10 is necessary because more than one transistor may be shorted to the heatsink

10 Repeat Steps 2 and 3 to test for voltage on the heatsink. Repeat this procedure as necessary until no
voltage remains on the heatsink.

Remember! There should be no DC, positive or negative voltage and no more than 10 volts AC
present on the heatsink with respect to chassis ground.

Tighten to 15 Ib-in
(1.7 N-m)

Hand tighten (do not crush 5 cu er washer)

an...

Figure 5-7. Transistor Mounts

; Crest Audio Professional Series Power Amplifiers Page 5-9
Service Manual Release 1.1 4/93

Page 45

RESIST. 2.7K OHM 1/4w 5% 74000034 $0.25 R5,6,9,10,15-18

x: RESIST. 4.7K OHM 1/4w 5% 74000037 T0277}, R25,26,33,34
: RESIST. 5.1K OHM 1/4w 5% 74000038 $0.25 R19,20
RESIST. 10K OHM 1/4w 5% 74000042 $0.25 R3,4,13,14,29
RESIST. 100K OHM 1/4w 5% 74000051 $0.25 R7,8,38,39
RESIST. 390K OHM 1/4w 5% 74000057 3-5 $0.25 R31,32
RESIST. 33 OHM 2w 5% 77000006 $0.95 R42
RESIST. 5K OHM SW 10% 78000008 C-3-6 $1.25 R11,12
RESIST. 60.4K OHM 1/4w1% 73500141 SEL.4801 $0.50 R40,41
RESIST. 732K OHM 1/4wl% 73500140 SEL.6001 $0.50 R40,41
SOCKET 16 PIN 85000005 $0.50
TAB 1/4 90000003 ON BACK $0.50
TRANSISTOR 2N5551 94000003 $1.00 Q1-4.
TRIAC MAC97 94000013 C-3-6 $2.00 TR1,2 _..= "
TRANSISTOR MPSA75 94000024 B-2-7 $2.00 Q5.6
SPACER RD. 1/4 6-32 5/8 86000016 $1.00 SP1-4

2/93D