QSC PLX-1202 Service manual
PLX™ Series
▲▲
▲ PLX 1202
▲▲
▲▲
▲ PLX 1602
▲▲
▲▲
▲ PLX 2402
▲▲
▲▲
▲ PLX 3002
▲▲
Technical Service Manual
TD-000078-00
TD-000078-00
+ PLX
+
PLX 1602
+
PLX 2402
1202
+
PLX 3002
PLX SERIES
SERVICE MANUAL
PLX
1202
PLX 1602
PLX 2402
PLX 3002
QSC Technical Services
Wats: I
Local:
Fax: 1-714-754-6173
QSC Technical Support BBS
QSC Audio Products, Inc.
1675 MacArthur Blvd.
Costa Mesa,
-%OO-772-2034
1-714-957-7150
I -714-668-7567
1-800-856-6003
CA
92626
http://www.qscaudio.com
Table of Contents
PLX Product Specifications ................................................................
Introduction ........................................................................................
Test and Calibration ............................................................................
Troubleshooting PLX ...........................................................................
PLX Parts List
PLX 1202 PCB Assembly (120V) 23
PLX 1202 PCB Assembly (230V) 25
PLX 1202 Chassis Assembly (120V) 28
PLX 1202 Chassis Assembly (230V) 28
PLX 1602 PCB Assembly (120V) 28
PLX 1602 PCB Assembly (230V) 31
PLX 1602 Chassis Assembly (120V) 34
PLX 1602 Chassis Assembly (230V) 34
PLX 2402 PCB Assembly (120V) 35
PLX 2402 PCB Assembly (230V) 38
PLX 2402 Chassis Assembly (120V) 41
PLX 2402 Chassis Assembly (230V) 41
.....................................................
.....................................................
................................................
................................................
.....................................................
.....................................................
................................................
................................................
.....................................................
.....................................................
................................................
................................................
1
2
3
13
PLX 3002 PCB Assembly (120V) 42
PLX 3002 PCB Assembly (230V) 45
PLX 3002 Chassis Assembly (120V) 48
PLX 3002 Chassis Assembly (230V) 49
.....................................................
.....................................................
................................................
................................................
PLX Semiconductors ..............................................................................
Chassis Drawings ...................................................................................
Schematics
PLX 1202 Main PCB Schematics 57
PLX 1602 Main PCB Schematics 62
PLX 2402 Main PCB Schematics 67
PLX 3002 Main PCB Schematics 72
.....................................................
.....................................................
.....................................................
.....................................................
PLX Series PCB Circut Board Drawings .................................................
50
53
73
PLX
1202
PLX 1602
PLX 2402
PLX Product Specifications
PLX
3002
Output
Power (per channel):
Continuous Average
8
ohms,
ohms,
Impedance
2OHz-2OkHz,
2OHz-2OkHz,
Average
2OHz - 2OkHz,
4
Con0nuous
8 ohms,
Voltage Gain (dB)
Sensitivity (for rated power @ 8 ohms)
Distortion
Frequency Response (LF Switched Off)
Damping Factor (1
Noise
Input
Dimensions
Faceplate Width
Faceplate Height
Chassis Depth
Weight
Net, Lbs/kg
Output
Power both channels driven:
0.03%
THD
0.05%
Output
kHz
THD
Power bridged mono
0.1% THD
and Below)
PLXI
PLXI
202
200watts
325
watts
ooeration:
700
iatts
32
1.00vrms
SMPTE-IM
2OHz to 2OkHz,
8Hz to 5OkHz, +Ol-3dB
Greater
106dB
6k unbalanced,
Standard 19” Rack Mounting
3.Y
13.2Y
2119.5 2119.5
Less
than
+I-0.2dB
than
500
below rated output (20 Hz to 20 kHz)
12k
balanced
602
300
waKs
500
watts
1100
watts 1500 watts
32
1.20 Vrms
0.01%
3.r
13.2v
PLX2402
425watts 550
700
watts 900watts
32
1.50 Vrms
3.5” 3.Y
13.2Y 13.2Y
2119.5
PLX3002
2000 watts
32
1.70
2119.5
watts
vrms
Introduction
This manual is prepared to assist service personnel with the repair and calibration of PLX power
amplifiers
and sophisticated audio test equipment.
Documentation
This manual contains schematics, printed circuit board (PCB) drawings, parts lists, and
mechanical assembly drawings. This information should be used in conjunction with the
test and troubleshooting guide.
The electrical and electronic components are identified by circuit identification numbers on
the schematics and the parts list. The test
designations shown in the schematics.
.
The procedures described in this manual require advanced technical experience
CAUTION: To reduce the risk of electric shock,
do not remove the cover. No user-serviceable
parts inside. Refer servicing to qualified
personnel.
WARNING: To prevent fire or electric shock, do
not expose this equipment to rain or moisture.
&
troubleshooting sections refer to
CAUTION
Equivalent Parts
Although many of the electronic components used in this product may be available from
WC.
electronic suppliers, some components are specially tested and approved by
product repaired with
Repairs performed using non-QSC parts may void the product warranty.
you may contact
Parts orders to QSC should include the product model number, the part description, and
the QSC part number (from the parts list in this manual). Parts will be shipped via UPS,
F.O.B. Costa Mesa, California. Shipping, handling and COD charges may be added to
the cost of the parts.
non-WC
QSC
Technical Services for assistance.
supplied components may not meet factory specifications.
men
A
in doubt,
Factory Repair
It may become necessary to return a product to the factory for repair. Call QSC Technical
Services for return instructions. QSC Technical Services may be reached at (800)
772-2834.
Test and Troubleshooting Equipment
-Audio
-
Distortion Analyzer capable of 0.01% THD+N
-
High Power Load Bank
-
Function Generator 8 Digital Multimeter
-
2OMHz
Oscilloscope
-
Variac (0-140
VAC, 30-40A)
(8,4,8
2
ohms)
Precision - System One
+
Thermometer
2
Test & Calibration
PLX 1202 Test Procedure
.
SET-UP
1.
Connect a test load to the output terminals of the amplifier.
-
2. Make sure Mode Switches 1
3. Connect a distortion analyzer with a resolution of
output terminals of the amplifier. Enable the
4. Connect a dual-channel oscilloscope to the following test points:
Chl
-
a
IOX
(vertical sensitivity -
Ch2
-
a IX scope probe (vertical sensitivity -
output.
5.
Set amp gain pots fully clockwise and turn on power switch.
6. Connect the output of the signal generator to the input terminals of the amplifier and
select an output of 1.30
7. Plug the amplifier into a variac and set up an AC line current monitor.
. POWER UP 8 MUTE DELAY TEST
CAUTION: To avoid damage to the main printed circuit board, place a 50 ohm
240VAC) in series with the high
supply has a shorted device at initial power up, this AC resistor pad will help prevent undue damage. After
the amplifier has been fully powered up via the variac, confirm that the amplifier has achieved
stable operation during idle. Remove AC power from the amplifier and disconnect the series
resistor for normal operation. Continue with the test
10 are in the default position (1 on, 10 on, all others off).
O.Ol%, 20-2OkHz
8OkHz
low pass filter.
2V/cm)
scope probe to the channel speaker output.
O.lV/cm)
VRMS,
1
kHz
sine wave.
(+)
lead on the AC cable during variac ramp up. If the switching power
&
calibration process.
to the distortion analyzer
(or better) to the
225W
resistor (100 ohm,
I. Slowly raise the variac voltage and watch for excessive current draw (line current
greater than
0.5A
a.c. at 60 Volts). This is slightly less for 240V.
Pause at
(2OOVAC European) for three seconds until the mute I protect circuit disengages.
Continue to
12OVAC
(240V European).
2. Verify that the fan is operating at low speed.
3. Turn the power switch off and on a few times to verify the 3 second power-up muting
delay.
l CHANNEL OUTPUT
I. Look for amplified signal on the scope for channel 1. Switch the input signal and scope
/
to channel 2 and repeat output test. Check for noisy
contaminated gain pots by
observing general instability on the distortion waveform while adjusting the gain control
levels.
2. Select an 8 ohm load and confirm that this amplifier is producing 200 watts at 1 kHz just
below the point of clipping. Check both channels.
l BRIDGE MODE
1. Turn the power switch off.
#7
2. Set Mode Switch
on CH2 are disabled with Mode Switch
3.
Set load to both red output binding posts
4. Apply a 1.30
in the on position. The gain control, limiter, and filter switch positions
#7
on.
(CHI
positive and CH2 negative).
VRMS,
1
kHz
sinewave input to channel I of the amplifier. Check the power
and verify that the output does not immediately collapse. Check for 700 watts at 8 ohms.
9OVAC
5.
Turn power off and place the amplifier back into the Stereo mode with output
loads connected to each channel.
. BIAS
1. Let the amplifier cool down to room temperature.
2. With an input amplitude of
1.3OVrms
input signal 20dB (80%) from full output. Adjust the crossover
VR166 (CH2) for about a
4OOmVpk-pk
increase the input frequency to
trimpot
2OkHz.
VR43
crossover spike protruding from the noise trace
on the oscilloscope. It will be necessary to have the oscilloscope measure unfiltered
distortion from the amplifier in order to see the crossover spike. It is necessary to
disable the 80kHz
lowpass
filter on the analyzer for this test.
Further trim so that
the total distortion for that channel is less than 0.1% THD+N.
3. With the trim settings achieved, and with no signal plugged into the amplifier and with an
8 ohm load, verify that the AC idle current from the AC service is no more than 1 .O
amperes.
4. Let the amplifier cool down and check channel 2.
. SHORT CIRCUIT CURRENT
1.
Select a 2 ohm load and apply a
of the amplifier.
Ensure that power is on and that the gain controls are fully up.
1.3Vrms
sinewave (1
kHz)
input signal to both channels
2. While the amplifier is producing power into the loads, apply a short to the output binding
posts of each channel. In other words, apply a jumper between the red and black
binding posts of each channel. Once this is done, combined AC line current draw for
both channels should be no greater than
13A ac.
This is with a 120 volt AC service to
the amplifier. Current may be lower if AC line voltage is lower.
3. Remove the short from each channel and verify that the channels recover in to 2 ohm
loads. The output should not experience any hang up and a full
sinewave
should be
present just as it was before a short was applied for this test.
4. If the amplifier does not pass any of the above steps, troubleshoot the current limit
section of the amplifier. If steps 2, 3, and 4 above pass, continue to the next test
FREQUENCY RESPONSE.
Reduce the
(CHl)
and
l FREQUENCY RESPONSE
1,
Set load to 8 ohms and scale the input generator to gain 1 watt of power from the
amplifier on each channel. Gain controls on the amplifier should be fully up.
2.
Check frequency response from
2OHz
to 20kHz
(+I- 0.2OdB)
by sweeping random
frequencies between these extremes. This is done by verifying the same voltage
amplitude at each of the frequencies selected (within
2OHz
to
2OkHz).
Check both
channels.
l POWER vs. DISTORTION TEST
I. Check to ensure that both channels will produce rated power at
2OHz, 2KHz,
into an 8 ohm load.
2. While verifying rated power, check that at all frequencies the distortion measurement is
less than or equal to 0.03% THD.
l THERMAL TEST
1. Set input frequency to 1 KHz and short both channels while they are producing power into
a load.
2. Apply a short to the output of each channel.
3. AC line current draw should be about 11 - 13.5 amperes for both channels. As the
amplifier gets hot, there will be some current drift upwards and the fan speed will
increase. This is not a problem as long as the case temperature on the output
transistors does not exceed
105
degrees C.
and 20kHz.
4
4.
Verify that the NTC circuit causes thermal shutdown after an extended period.
5. When thermal shutdown occurs, verify AC idle current of less that 0.90 amperes
l CM TEST
1. Select an 8 ohm load and confirm that this amplifier is producing rated power.
2. Check the Common Mode of the amplifier by inserting a
channel and observe about 6
dB
of output voltage reduction. There will also be a 180
1/4”
input jack halfway into each
degree phase inversion at the output of the channel under test.
l OUTPUT NOISE
1. Set the amplifier gain controls all the way up, with a 1
kHz 1.3OVrms sinewave
signal. Note the output level at full power just below clipping. Adjust gain if needed.
2. Remove the input signal connector from the amplifier and measure the residual noise
level produced into the load by the amplifier. The noise signal should be 107
from the full output power point measured. A signal to noise ratio should be better than
or equal to
l FINAL CHECK
107dB.
Check both channels.
This completes the amplifier test procedure for this model.Inspect the amplifier for
mechanical defects. Inspect the solder connections. Reassemble the amplifier and verify
the amplifier’s operation before returning the product to service.
input
dB
down
PLX 1602 Test Procedure
.
SET-UP
1.
Connect a test load to the output terminals of the amplifier.
2.
Make sure Mode Switches 1
Connect a distortion analyzer with a resolution of
3.
output terminals of the amplifier.Enable the 80kHz low pass filter.
4.
Connect a dual-channel oscilloscope to the following test points:
Chl -
a 1 OX (vertical sensitivity -
Ch2
-
a IX scope probe (vertical sensitivity -
output.
5.
Set amp gain pots fully clockwise and turn on power switch.
Connect the output of the signal generator to the input terminals of the amplifier and
6.
select an output of 1.30
Plug the amplifier into a variac and set up an AC line current monitor.
7.
l POWER UP 8 MUTE DELAY TEST
CAUTION: To avoid damage to the main printed circuit board, place a 50 ohm 225W resistor (100 ohm,
240VAC) in series with the high
supply has a shorted device at initial power up, this AC resistor pad will help prevent undue damage. After
the amplifier has been fully powered up via the variac, confirm that the amplifier has achieved
stable operation during idle. Remove AC power from the amplifier and disconnect the series
resistor for normal operation. Continue with the test
Slowly raise the variac voltage and watch for excessive current draw (line current
greater than
(2OOVAC
European) for three seconds until the mute I protect circuit disengages.
Continue to
0.5A a.c
12OVAC (24OV
at 60 Volts).
Verify that the fan is operating at low speed.
-
10 are in the default position (1 on, 10 on, all others off)
VRMS,
1
(+)
lead on the AC cable during variac ramp up. If the switching power
European).
O.Ol%, 20-20kHz
2V/cm)
scope probe to the channel speaker output.
O.lV/cm)
kHz
sine wave.
&
calibration process.
T/I;s is s/;ghf/y less
to the distortion analyzer
for 240V. Pause at
(or better) to the
9OVAC
3. Turn the power switch off and on a few times to verify the 3 second power-up muting
delay.
. CHANNEL OUTPUT
1. Look for amplified signal on the scope for channel 1. Switch the input signal and scope
to channel 2 and repeat output test. Check for noisy
/
contaminated gain pots by
observing general instability on the distortion waveform while adjusting the gain control
levels.
2. Select an 8 ohm load and confirm that this amplifier is producing 300 watts at 1 kHz just
below the point of clipping. Check both channels.
. BRIDGE MODE
1. Turn the power switch off.
2. Set Mode Switch
on CH2 are disabled with Mode Switch
3. Set load to both red output binding posts
4. Apply a 1.30
#7
in the on position. The gain control, limiter, and filter switch positions
#7
on.
(CHI
positive and CH2 negative).
VRMS,
1
kHz
sinewave input to channel 1 of the amplifier. Check the power
and verify that the output does not immediately collapse. Check for 1000 watts at 8
ohms.
5.
Turn power off and place the amplifier under test back into the Stereo mode with output
loads connected to each channel.
l BIAS
I. Let the amplifier cool down to room temperature.
2. With an input amplitude of
input signal 20dB (80%) from full output. Adjust the crossover
VR166 (CH2) for about a
1.3OVrms
increase the input frequency to 20kHz. Reduce the
4OOmVpk-pk
trimpot
VR43
crossover spike protruding from the noise trace
on the oscilloscope. It will be necessary to have the oscilloscope measure unfiltered
distortion from the amplifier in order to see the crossover spike.
disable the 80kHz
lowpass
filter on the analyzer for this test. Further trim so that
It is necessary to
the total distortion for that channel is less than 0.1% THD+N.
3. With the trim settings achieved, and with no signal plugged into the amplifier and with an
8 ohm load, verify that the AC idle current from the AC service is no more than 1 .O
amperes.
4. Let the amplifier cool down and check channel 2.
(CHl)
and
l SHORT CIRCUIT CURRENT
1.
Select a 2 ohm load and apply a
of the amplifier.Ensure that power is on and that the gain controls are fully up.
2. While the amplifier is producing power into the loads, apply a short to the output binding
posts of each channel.
binding posts of each channel. Once this is done, combined AC line current draw for
both channels should be no greater than
the amplifier. Current may be lower if AC line voltage is lower.
3. Remove the short from each channel and verify that the channels recover in to 2 ohm
loads. The output should not experience any hang up and a full
present just as it was before a short was applied for this test.
4. If the amplifier does not pass any of the above steps, troubleshoot the current limit
section of the amplifier. If steps 2, 3, and 4 above pass, continue to the next test
FREQUENCY RESPONSE.
6
1.3Vrms
sinewave (1
kHz)
input signal to both channels
In other words, apply a jumper between the red and black
13A
ac. This is with a 120 volt AC service to
sinewave
should be
l FREQUENCY RESPONSE
I.
Set load to 8 ohms and scale the input generator to gain 1 watt of power from the
amplifier on each channel. Gain controls on the amplifier should be fully up.
2. Check frequency response from
2OHz
to 20kHz
(+I- 0.2OdB)
by sweeping random
frequencies between these extremes. This is done by verifying the same voltage
2OHz
amplitude at each of the frequencies selected (within
to 20kHz). Check both
channels.
l POWER vs. DISTORTION TEST
1. Check to ensure that both channels will produce rated power at
2OHz, 2KHz,
into an 8 ohm load.
2.
While verifying rated power, check that at all frequencies the distortion measurement is
less than or equal to 0.03% THD.
l THERMAL TEST
1.
Set input frequency to 1 KHz and short both channels while they are producing power into
a load.
2. Apply a short to the output of each channel.
3. AC line current draw should be about
II -
13.5 amperes for both channels. As the
amplifier gets hot, there will be some current drift upwards and the fan speed will
increase. This is not a problem as long as the case temperature on the output
transistors does not exceed 105 degrees
C.
4. Verify that the NTC circuit causes thermal shutdown after an extended period.
5. When thermal shutdown occurs, verify AC idle current of less that 0.90 amperes.
and 20kHz.
l CM TEST
1. Select an 8 ohm load and confirm that this amplifier is producing rated power.
2. Check the Common Mode of the amplifier by inserting a
dB
channel and observe about 6
of output voltage reduction. There will also be a 180
1/4”
input jack halfway into each
degree phase inversion at the output of the channel under test.
l OUTPUT NOISE
I. Set the amplifier gain controls all the way up, with a 1
signal.
Note the output level at full power just below clipping. Adjust gain if needed.
kHz 1.3OVrms sinewave
2. Remove the input signal connector from the amplifier and measure the residual noise
level produced into the load by the amplifier. The noise signal should be 107
from the full output power point measured. A signal to noise ratio should be better than
107dB.
or equal to
l FINAL CHECK
This completes the amplifier test procedure for this model.
Check both channels.
Inspect the amplifier for
mechanical defects. inspect the solder connections. Reassemble the amplifier and
the amplifier’s operation before returning the product to service.
PLX 2402 Test Procedure
l SET-UP
1. Connect a test load to the output terminals of the amplifier.
2. Make sure Mode Switches 1
- 10
are in the default position (1 on, 10 on, all others off).
input
dB
down
veri@
3. Connect a distortion analyzer with a resolution of
O.Ol%, 20-20kHz
(or better) to the
output terminals of the amplifier. Enable the 80kHz low pass filter.
4. Connect a dual-channel oscilloscope to the following test points:
Chl
-
a
1OX
(vertical sensitivity - 2V/cm) scope probe to the channel speaker output.
Ch2
-
a IX scope probe (vertical sensitivity - 0.
IV/cm)
to the distortion analyzer
output.
5. Set amp gain pots fully clockwise and turn on power switch.
6. Connect the output of the signal generator to the input terminals of the amplifier and
select an output of 1.50
VRMS
1
kHz
sine wave.
7. Plug the amplifier into a variac and set up an AC line current monitor.
l POWER UP
&
MUTE DELAY TEST
CAUTION: To avoid damage to the main printed circuit board, place a 50 ohm
240VAC) in series with the high
supply has a shorted device at initial power up, this AC resistor pad will help prevent undue damage. After
the amplifier has been fully powered up via the variac, confirm that the amplifier has achieved
stable operation during idle. Remove AC power from the amplifier and disconnect the series
resistor for normal operation. Continue with the test
(+)
lead on the AC cable during variac ramp up. if the switching power
&
calibration process.
225W
resistor (100 ohm,
1. Slowly raise the variac voltage and watch for excessive current draw (line current
greater than
(2OOVAC
European) for three seconds until the mute / protect circuit disengages.
Continue to
0.5A a.c
at 60 Volts).
12OVAC (24OV
This
European).
is
slight/y /ess fof
24OV.
Pause at
2. Verify that the fan is operating at low speed.
3.
Turn the power switch off and on a few times to verify the 3 second power-up muting
delay.
l CHANNEL OUTPUT
I. Look for amplified signal on the scope for channel
to channel 2 and repeat output test. Check for noisy
1.
Switch the input signal and scope
/
contaminated gain pots by
observing general instability on the distortion waveform while adjusting the gain control
levels.
2. Select an 8 ohm load and confirm that this amplifier is producing 425 watts at 1 kHz just
below the point of clipping. Check both channels.
l BRIDGE MODE
1. Turn the power switch off.
2. Set Mode Switch
on CH2 are disabled with Mode Switch
3. Set load to both red output binding posts
4. Apply a 1.30
#7
in the on position. The gain control, limiter, and filter switch positions
#7
on.
(CHI
positive and CH2 negative).
VRMS,
1
kHz sinewave
input to channel 1 of the amplifier. Check the power
and verify that the output does not immediately collapse. Check for 1500 watts at 8
ohms.
5. Turn power off and place the amplifier under test back into the Stereo mode with output
loads connected to each channel.
l BIAS
1.
Let the amplifier cool down to room temperature.
2. With an input amplitude of
input signal 20dB (80%) from full output. Adjust the crossover
VR166 (CH2) for about a
1.5OVrms,
increase the input frequency to
4OOmVpk-pk
2OkHz.
trimpot
VR43
crossover spike protruding from the noise trace
on the oscilloscope. It will be necessary to have the oscilloscope measure unfiltered
distortion from the amplifier in order to see the crossover spike.
It is necessary to
9OVAC
Reduce the
(CHl)
and
disable the 80kHz
the total distortion for that channel is less than 0.1%
lowpass
filter on the analyzer for this test.Further trim so that
THD+N,
3. With the trim settings achieved, and with no signal plugged into the amplifier and with an
8 ohm load, verify that the AC idle current from the AC service is no more than 1 .O
amperes.
4. Let the amplifier cool down and check channel
l SHORT CIRCUIT CURRENT
1.
Select a 2 ohm load and apply a
1.3Vrms
2.
sinewave (1
kHz)
input signal to both channels
of the amplifier.Ensure that power is on and that the gain controls are fully up.
2. While the amplifier is producing power into the loads, apply a short to the output binding
posts of each channel. In other words, apply a jumper between the red and black
binding posts of each channel. Once this is done, combined AC line current draw for
both channels should be no greater than
13A
ac. This is with a 120 volt AC service to
the amplifier. Current may be lower if AC line voltage is lower.
3. Remove the short from each channel and verify that the channels recover in to 2 ohm
loads. The output should not experience any hang up and a full
sinewave
should be
present just as it was before a short was applied for this test,
4. If the amplifier does not pass any of the above steps, troubleshoot the current limit
section of the amplifier. If steps 2, 3, and 4 above pass, continue to the next test
FREQUENCY RESPONSE.
l FREQUENCY RESPONSE
1. Set load to 8 ohms and scale the input generator to gain 1 watt of power from the
amplifier on each channel. Gain controls on the amplifier should be fully up.
2. Check frequency response from
2OHz
to 20kHz
(+/- 0.2OdB)
by sweeping random
frequencies between these extremes. This is done by verifying the same voltage
amplitude at each of the frequencies selected (within
2OHz
to 20kHz). Check both
channels.
l POWER vs. DISTORTION TEST
1. Check to ensure that both channels will produce rated power at
2OHz, 2KHz,
into an 8 ohm load.
2. While verifying rated power, check that at all frequencies the distortion measurement is
less than or equal to 0.03%
l THERMAL TEST
THD.
1. Set input frequency to 1 KHz and short both channels while they are producing power into
a load.
2. Apply a short to the output of each channel.
11
-
3. AC line current draw should be about
13.5 amperes for both channels. As the
amplifier gets hot, there will be some current drift upwards and the fan speed will
increase, This is not a problem as long as the case temperature on the output
transistors does not exceed 105 degrees C.
4. Verify that the NTC circuit causes thermal shutdown after an extended period.
5,
When thermal shutdown occurs, verify AC idle current of less that 0.90 amperes.
l CM TEST
I. Select an 8 ohm load and confirm that this amplifier is producing rated power.
2. Check the Common Mode of the amplifier by inserting a
114”
input jack halfway into each
channel and observe about 6 dB of output voltage reduction. There will also be a 180
degree phase inversion at the output of the channel under test.
and 20kHz.
9
l OUTPUT NOISE
1. Set the amplifier gain controls all the way up, with a I
signal.
Note the output level at full power just below clipping. Adjust gain if needed.
kHz 1.5OVrms sinewave
2. Remove the input signal connector from the amplifier and measure the residual noise
level produced into the load by the amplifier. The noise signal should be 107
from the full output power point measured. A signal to noise ratio should be better than
or equal to
l FINAL CHECK
107dB.
Check both channels.
This completes the amplifier test procedure for this model.Inspect the amplifier for
mechanical defects. Inspect the solder connections. Reassemble the amplifier and verify
the amplifier’s operation before returning the product to service,
PLX 3002 Test Procedure
l SET-UP
1. Connect a test load to the output terminals of the amplifier.
2. Make sure Mode Switches 1 - 10 are in the default position (1 on, 10 on, all others off).
3.
Connect a distortion analyzer with a resolution of
output terminals of the amplifier. Enable the 80kHz low pass filter.
4. Connect a dual-channel oscilloscope to the following test points:
-
a
1OX
Chl
Ch2
(vertical sensitivity -
-
a 1X scope probe (vertical sensitivity -
2V/cm)
scope probe to the channel speaker output.
output.
5.
Set amp gain pots fully clockwise and turn on power switch.
6. Connect the output of the signal generator to the input terminals of the amplifier and
select an output of 1.70
VRMS,
1
kHz
sine wave.
7. Plug the amplifier into a variac and set up an AC line current monitor.
O.Ol%,
O.lV/cm)
20-20kHz
(or better) to the
to the distortion analyzer
input
dB
down
. POWER UP 8 MUTE DELAY TEST
CAUTION: To avoid damage to the main printed circuit board, place a 50 ohm 225W resistor (100 ohm,
240VAC) in series with the high
supply has a shorted device at initial power up, this AC resistor pad will help prevent undue damage. After
the amplifier has been fully powered up via the variac, confirm that the amplifier has achieved
stable operation during idle. Remove AC power from the amplifier and disconnect the series
resistor for normal operation. Continue with the test
1.
Slowly raise the variac voltage and watch for excessive current draw (line current
greater than
(2UOVAC
Continue to
0.5A
a.c. at 60 Volts).
European) for three seconds until the mute / protect circuit disengages.
12OVAC
(+)
lead on the AC cable during variac ramp up. If the switching power
TIG.s is s/ight/y
(240V European).,
&
calibration process.
less for
24UV.
Pause at
2. Verify that the fan is operating at low speed.
3. Turn the power switch off and on a few times to verify the 3 second power-up muting
delay.
l CHANNEL OUTPUT
1.
Look for amplified signal on the scope for channel 1. Switch the input signal and scope
to channel 2 and repeat output test. Check for noisy I contaminated gain pots by
observing general instability on the distortion waveform while adjusting the gain control
levels.
9OVAC
10
2. Select an 8 ohm load and confirm that this amplifier is producing
550
watts at 1 kHz just
below the point of clipping. Check both channels.
l BRIDGE MODE
1. Turn the power switch off.
2. Set Mode Switch
on CH2 are disabled with Mode Switch
3. Set load to both red output binding posts
4. Apply a 1.30
#7
in the on position. The gain control, limiter, and filter switch positions
#
7 on.
(CHI
positive and CH2 negative).
VRMS,
1
kHz
sinewave input to channel 1 of the amplifier. Check the power
and verify that the output does not immediately collapse. Check for 2000 watts at 8
ohms.
5. Turn power off and place the amplifier under test back into the Stereo mode with output
loads connected to each channel.
l BIAS
1.
Let the amplifier cool down to room temperature.
2. With an input amplitude of
input signal 20dB
VRl66
(CH2) for about a
(80°h)
1.7OVrms
increase the input frequency to 20kHz. Reduce the
from full output. Adjust the crossover
4OOmVpk-pk
crossover spike protruding from the noise trace
trimpot
VR43
on the oscilloscope. It will be necessary to have the oscilloscope measure unfiltered
distortion from the amplifier in order to see the crossover spike. It is necessary to
disable the 80kHz
lowpass
filter on the analyzer for this test.Further trim so that
the total distortion for that channel is less than 0.1% THD+N.
3,
With the trim settings achieved, and with no signal plugged into the amplifier and with an
8 ohm load, verify that the AC idle current from the AC service is no more than 1 .O
amperes.
4. Let the amplifier cool down and check channel
2.
(CHI)
and
l SHORT CIRCUIT CURRENT
1.
Select a 2 ohm load and apply a
1.3Vrms
sinewave (1
kHz)
input signal to both channels
of the amplifier.Ensure that power is on and that the gain controls are fully up.
2,
While the amplifier is producing power into the loads, apply a short to the output binding
posts of each channel. In other words, apply a jumper between the red and black
binding posts of each channel. Once this is done, combined AC line current draw for
both channels should be no greater than
13A
ac. This is with a 120 volt AC service to
the amplifier. Current may be lower if AC line voltage is lower.
3. Remove the short from each channel and verify that the channels recover in to 2 ohm
loads. The output should not experience any hang up and a full
sinewave
present just as it was before a short was applied for this test.
4. If the amplifier does not pass any of the above steps, troubleshoot the current limit
section of the amplifier. If steps 2, 3, and 4 above pass, continue to the next test
FREQUENCY RESPONSE.
l FREQUENCY RESPONSE
1. Set load to 8 ohms and scale the input generator to gain I watt of power from the
amplifier on each channel. Gain controlson the amplifier should be fully up.
2. Check frequency response from
to 20kHz
(+I- 0.2OdB)
by sweeping random
2OHz
frequencies between these extremes. This is done by verifying the same voltage
amplitude at each of the frequencies selected (within
2OHz
to 20kHz). Check both
channels.
should be
11
l POWER vs. DISTORTION TEST
1.
Check to ensure that both channels will produce rated power at
2OH2, 2KHz,
into an 8 ohm load.
2. While verifying rated power, check that at all frequencies the distortion measurement is
less than or equal to 0.03% THD.
l THERMAL TEST
I. Set input frequency to
1 KHz and short both channels while they are producing power into
a load.
2. Apply a short to the output of each channel.
3. AC line current draw should be about 11
-
13.5 amperes for both channels. As the
amplifier gets hot, there will be some current drift upwards and the fan speed will
increase. This is not a problem as long as the case temperature on the output
transistors does not exceed 105 degrees
C.
4. Verify that the NTC circuit causes thermal shutdown after an extended period.
5. When thermal shutdown occurs, verify AC idle current of less that 0.90 amperes.
l CM TEST
1. Select an 8 ohm load and confirm that this amplifier is producing rated power.
2. Check the Common Mode of the amplifier by inserting a
channel and observe about 6 dB of output voltage reduction. There
114”
input jack halfway into each
will
also be a 180
degree phase inversion at the output of the channel under test.
l OUTPUT NOISE
1. Set the amplifier gain controls all the way up, with a 1
signal.
Note the output level at full power just below clipping. Adjust gain if needed.
kHz 1.7OVrms sinewave
2. Remove the input signal connector from the amplifier and measure the residual noise
level produced into the load by the amplifier. The noise signal should be 107
from the full output power point measured. A signal to noise ratio should be better than
or equal to
107dB.
Check both channels.
and 20kHz.
input
dB
down
l FINAL CHECK
This completes the amplifier test procedure for this model.
Inspect the amplifier for
mechanical defects. Inspect the solder connections. Reassemble the amplifier and verify
the amplifier’s operation before returning the product to service.
12
T r o ubleshooting
PLX 1202 l PLX 1602 l PLX 2402 l PLX 3002
Power Supply -
In order to improve
are driven by an active, direct coupled integrated circuit, rather than a gate drive transformer.
driver failure should be rare (when correctly assembled) but when the
following parts:
CHECKLIST AFTER BLOWN
Q96, Q97,
D78, D79,
Ul8, lR2110
also typically damage the gate coupling parts noted above.
Ul9,
SOMETIMES
RARELY
PROBABLE CAUSES OF MASSIVE
SHORTS IN CONTROL CIRCUIT.
The parts operate well within their ratings and should hold up well in the field. The usual cause of failure is
when both
signal to one part to remain on when the other part is supposed to turn on. Shorts from solder or debris are
one obvious cause.
SHORTS IN THE LOAD.
Although there is peak current shutdown, shorts in the power amplifier transistors or secondary-side supply
components can cause currents to increase too quickly to prevent damage.
(IGBT’s
R358, R359,
high-side gate driver, Fault current when low-side
3525 controller, Blows from currents shorted
Ul3,
IGBT’s
EMI
performance, reduce cost, and increase current capacity, the PLX
generally fail in pairs)
gate drive coupling
Ul4, 556,
which has fairly high supply voltage ratings.
powered from 5V output of 3525, which may fail high when 3525 fails.
turn on at once, shorting Pri-Hi to Pri-Lo. This occurs when something causes the drive
Replacing Blown
-
IGBT’S
IGBT
comonents,
FAULTS
check after removing blown
thru Ul8,
IGBTs.
IGBTs
IGBT
or
IGBT’s
blow, it usually damages the
IGBT’s.
IGBT
shorts to upper rail. Such currents
or possibly by overvoltage on the supply rail
OVERVOLTAGE ON THE BIAS SUPPLY.
If the TOP-210 bias supply fails to operate, no harm occurs, the unit simply does not operate.
However, open circuit (missing part) in several key components can cause the Bias supply voltage to be
much too high, This blows the 2110 and thus the
QUICK TEST OF BIAS SUPPLY.
Ramp the AC voltage up slowly to 25% of regular voltage (30V for
normally, the green “power” LED should come on between 30 and
start-up level. If the LED comes on at
until you have measured the bias voltage. The switching will not start until you reach
the
IGBT’s
from blowing.
Confirm that bias voltage at Cl38 is
feedback to U16 and cause overvoltage.
2OV,
18-19V.
IGBT’s.
or not until
Open or missing
5OV,
12OV
unit). If the bias supply is working
35Vz
with its usual, steady
or blinks, DO NOT RAISE VOLTAGE PAST 60V
9OV,
D63,
64, 65, 66, 67 or R349 will break the
“half-brighr
so you can save
13
Troubleshooting “TOP-210” Bias Supply.
QUICK TEST OF BIAS SUPPLY.
Ramp the AC voltage up slowly to 25% of regular voltage (30V for
normally, the green “power” LED should come on between 30 and
start-up level.
CAUTION: if the LED comes on at
until you have measured the bias voltage. The switching will not start until you reach
the
IGBT’s
from blowing.
Confirm that bias voltage at Cl38 is
BIAS SUPPLY VOLTAGE MUCH TOO HIGH
D63,64,65,66,67
NO BIAS SUPPLY VOLTAGE
U16 missing or blown.
Tl missing, reversed, or open primary
D62 open or missing.
BIAS VOLTAGE ERRORS
The exact voltage is controlled by the feedback through
Cl38 is the
D63,
Cl39 , is the
D66,
R349 subtracts about
Ul6
uses this feedback to adjust the “on” time at pin 5, in order
charging Cl38 and thus maintain regulation of the
C142,
“hunting”.
Q99
and associated R374 reduce the voltage of the Bias supply by 33% when the AC voltage is turned off.
This prevents the Power LED from showing at half brightness after turn-off, since U16 continues to run
from the main filters for some time after shut down R375 and 376 sense the output of
AC” comparator, and cause
AC is turned on.
“+18V’
64, 65 each subtract a diode drop
a
IOV zener
R356,
or R349 open or missing -- breaks feedback to U16
rail with about
“+16V”
rail with about
diode , plus diode
0.5V,
bringing the net voltage at
and R349 form a closed-loop stability circuit which prevents the regulated voltage from
18.8V
Q99
to turn on. If
2OV,
or not until
18-19V.
typical.
(0.7V)
16.6V typcial.
D67,
subtract about 1 IV from
5OV,
from Cl38.
+16V
Q99
is shorted, the bias voltages will remain 33% low when
or blinks, DO NOT RAISE VOLTAGE PAST 60V
D63, 64_
Ul6,
feedback pin 4, to about
and
12OV
unit). If the bias supply is working
35V,
with its usual, steady
9OV,
65, 66, 67 and R349 as follows:
+16.6V.
5.lV.
toraise
+18V
or lower the
supplies
flyback
Ul3:3,
“half-brighf’
so you can save
voltage
the “Loss of
14
REPLACING BLOWN TOP-210.
If U16 has blown, check T-l for continuity after removing
measure about 15 ohms
Ul6.
Its primary may be open. Pins I-2 It should
Replacing Blown Output Transistors
OUTPUT TRANSISTOR SHORTED
Stmrk
in one device tend to cause the opposing device to blow as
If an output transistor shorts:
Drive transistor will be shorted
CXNne
transistors will short in pairs (Q39 &
The rest will short in fours
IGBT’s
8 their associated components may
(Q26, Q27, Q71,
(Q28, Q29,
Q34 &
Q72)
Q40,
Q36 &
Q35; Q73, Q74, Q79 8, Q80)
fail
well,
Q37, Q84
8
Q85, Q81 8, Q82)
CHECK EMITTER AND BASE RESISTORS WHILE DEVICES ARE REMOVED,
Each output transistor has an associatated pair of 0.47 ohm resistors in parallel.
Each BANK of output transistors has a 15 ohm resistor from base to rail (emitter bus).
Audio Outputs, Troubleshooting Current Limit
WEAK CURRENT LIMIT -- PREMATURE CLIPPING.
The usual symptom of weak output current is premature clipping of one or more peaks of the audio voltage.
This could be caused by missing step, weak current limit, or dead output section.
PREMATURE CLIPPING at 60% VOLTAGE, SIMILAR AT ALL IMPEDANCES:
This points to a step problem (2402, 3002 only). See Step Troubleshooting.
If the amplifier reaches full voltage at 8 ohms, but prematurely clips at 4 ohms or 2 ohms, we can assume
the step is OK but the output current is too low (see below).
NO OUTPUT AT ALL ON ONE POLARITY.
This indicates complete failure (open circuit) in the circuit leading to the dead output polarity. Check the
series components in the current splitter for missing or open:
Ch 1:
Ch 2:
CONSTANT, PREMATURE CLIPPING, WORSE AT LOW IMPEDANCES.
First, check the clamping voltages on
table below. At idle, all four voltages should all be similar. If one is out, check parts according to the
following table
Positive,
Positive,
Ql9, R381, Q20, R70, DIO,
Q64, R383, Q65, Rl93, D35,
C21
(Ch
D14. Negative:
D39. Negative:
I+),
C22 (Ch I-),
Q22, R382, Q21, R71,
Q67, R384, Q66, Rl94, D36,
C56
(Ch
2+), C57
(Ch
Dl I, D12
2-),
as shown in
D37
CLAMPING VOLTAGES ARE WRONG AT IDLE
I+
CHANNEL-POLARITY
Measure voltage on:
3002 and 2402, about
1602, about
1202 about
Voltage too high: missing resistor:
Voltage
Voltage
Voltage wrong: wrong value
The exact voltage varies with temperature. Look for the mismatching value on the weak cell.
A too-low voltage causes early clamping of that output section, as explained in the previous several pages.
If the voltage is correct and current is still low, also check for missing - unsoldered output device, or emitter
resistors.
4,6V,
4.9V.
or missing transistor:
0-0.3V:
or missing resistor
0.7V,
missing resistor:
6V.
shorted transistor
CH
c21
R60
Q14
Ql4,24
R
51,
72
R59
R59,60
CH
l-
c22
R61
Ql5
Ql5,
25
R53,
75
R62
R62,61
2+
CH
C56
RI83
Q59
Q59,69
Rl74,
RI82
Rl82,
195
183
2-
CH
c57
RI84
Q60
Q60,70
Rl76,
198
RI85
Rl85,184
15
Audio Power Stage, Current Limit Troubleshooting
CURRENT LIMITS WHICH COLLAPSE PREMATURELY.
An immediate collapse of ALL current limits at full power could be premature triggering of “Power Supply
Cutback”, which is described in the section below. Cutback after several seconds of full power operation at
two ohms is normal.
Cutback of one or more output sections, at full temperature, while approaching full power into two ohms is
also normal. However, cutbacks at 4 ohms, or when cold into two ohms, indicate problems with the
transistor power measuring circuitry.
CLAMPING VOLTAGES COLLAPSE TOO SOON.
CH
CHANNEL-POLARITY
Measure voltage on:
Cuts back too easily: low value,
or high value: R 51,
or missing diode:
CLAMPING VOLTAGES ARE CORRECT, CURRENT STILL WEAK.
Shorted diode
Missing-unsoldered output device or emitter resistor.
TROUBLESHOOTING “POWER SUPPLY CUTBACK”.
As noted in the section on Power Supply, the amplifier’s current limit cuts back when necessary to protect
the power supply. Because the Observed effect is a reduced output voltage, in response to prolonged
operation above the long-term current limit, we commonly refer to this behavior as “power supply cutback”,
but we must remember that it is actually
from the power supply. Full power operation into 2 ohms (both channels) should produce a 50% cutback of
current after several seconds.
If both channels of the amplifier fail to cut back after about 3 seconds, 2-ohms, both channels driven, the
cutback signal is probably missing. CAUTION: Prolonged operation under these conditions could blow
IGBT
or burn out
Check the output (secondary side) pins of U17 (sh 4). Confirm presence of
normally be at about
If
Ul7-pin
4 does not go high, check U17 itself. If it appears OK, trace the circuitry driving U17 (PRIMARY
SIDE, CAUTION). Check for continuity through
R347,
all of which drive optocoupler U17. A short in R346 or Cl31 will also prevent drive to U17.
If
Ul7-pin
4 goes high on schedule, and BOTH channels fail to cut back, trace voltage on
R273 (sh
(sh 2). If only ONE channel fails to cut back, look for missing
(sh 2).
Cl31 controls the speed of cutback.. If missing, the amplifier current limits will enter cutback almost
immediately at or above full power, 4 ohms.
Cl44.
Test for 6-10 seconds maximum.
OV,
and go high (I-5V) after 3 seconds at full power,
3),
which connects to “MUTE+” bus. Continue tracing voltage on MUTE+ to
I+
c21
R67,
73
72
D9
DIO, 14
amp/Tier
CH
l-
c22
R68,
74
R53,
75
D8
Dll,
12
current
L6:2
hmifhg
in response to an overload signal sent
to Pri-Lo, check missing or open
Ql6,
CH
2+
C56
Rl90,
Rl74,
D34
D35,
39
+6V
R 65,
Ql7
CH
2-
c57
196
195
on pin 5. Pin 4 should
(sh I) or
Rl91,
197
Rl76,
198
D33
D36,
37
R343, D61, Q95,
“PS_OL”
Ql6
(sh I) and
Q61, Rl88, Q62
bus to
Q61
16
SHORT CIRCUIT CURRENT DOESN’T CUT BACK.
CAUTION: DO NOT MAINTAIN A SHORTED LOAD IF CUTBACK FAILS TO OCCUR WITHIN 1 SECOND.
It will be necessary to measure the output current with a DC current probe, or by noting the voltage across
a low value resistance with a DC scope, in order to determine which output cell is failing to cut back.
Failure to cut back could indicate either lack of clamping, or lack of voltage cutback. Measure the voltage
on the respective clamp capacitor. If the voltage decreases, but current limiting does not cut back, check
the clamping transistor.
:
CLAMPING VOLTAGE DECREASES, BUT NO CURRENT CUTBACK
CHANNEL-POLARITY CH
Measure voltage on:
Check clamp transistor
If the measured voltage on the clamp capacitor is not decreasing to about 50% during the short, check the
circuitry which measures the current during short circuit,
CLAMPING VOLTAGE DOES NOT DECREASE TO 50% DURING SHORT.
CHANNEL-POLARITY
Measure voltage on:
Missing cutback transistor, resistor
Missing voltage sense resistor,
Shorted or low value shunt resistor R72
I+
c21
Q18
CH
I+
c21
Q24,
R67
R 73 R 74
CH
c22
Q23
CH
c22
Q25,
R75
l-
l-
R68
CH
2+
C56
Q63 Q68
CH
2+
C56
Q69,
RI90
RI96 RI97
RI95
CH
C5?
CH
c57
Q70,
RI98
2-
2-
RI91
Troubleshooting Thermal Tracking
MOUNTING PROBLEMS WITH IOK SENSING NTC.
The thermal sensing for fan and bias tracking depends on a 1 OK NTC which is mounted in a hole in the
heat sink. The hole is filled with thermal grease to improve coupling. If the NTC is not straight while
mounting the heat sink, it may short out against the side of its hole. It is mounted on a standoff which
protrudes into the hole, so this should not occur if care is taken while installing the heatsink.
If shorted to the heat sink, the amplifier output voltage is coupled to the NTC. If the short is to the grounded
lead of the
which will probably damage it.
SHORT FROM “LIVE” END OF NTC TO HEAT SINK:
Replace affected NTC
BE AWARE! This can short (relatively) quietly and then appear to be operating normally. Poor bias
tracking can indicate this problem. Sometimes this short will not happen until the amplifier is driven past IV
input.
NTC BENT OVER AND SHORTED TO DRIVER TRANSISTORS.
May touch
This causes severe overcurrent to the affected output cell, possibly damaging the parts in series with the
shorted transistor. It may also blow the power supply.
Replace affected
NTC,
it may not damage anything. If to the other end, a large voltage is put across the NTC
Ql9, Q26,
Ch 1:
Ch 2:
or
Q64, Q71.
NTC,
drive transistors, check components in series with drive transistor
Ql9
shorted, check,
Q26 shorted, check ALL outputs and opposing driver transistor on this channel.
Q64 shorted, check
Q71
shorted, check ALL outputs and opposing driver transistor on this channel.
R381, Q20, R70, DIO,
R383, Q65, Rl93, D35, D39.
D14.
17
Audio Output, Troubleshooting Stability Feedback
HIGH FREQUENCY OSCILLATIONS
SEVERE-DRAWS CURRENT-GROSS DISTORTION
C27 (62) missing or wrong, or series R367 (368)
C 25, 26 (60, 61) missing
Secondary filter capacitors missing or open (unlikely that ALL are defective).
SEVERE, BUT DOES NOT DRAW LARGE CURRENT
R22 (146) open.
MARGINAL -- MAY APPEAR ONLY AS EXCESS DISTORTION
Cl4 (49) missing
Cl 6 (50) missing or too large
C28 (63) missing
C 25 or 26 (60 or 61) missing or too large
Cl95 or 196 missing (input board).
EXCESSIVE OSCILLATION JUST BELOW CLIPPING, 2-4 ohms
Cl 7 (52) missing
NOTE: about 0.1% oscillation right below clipping at 2 ohms is normal.
EXCESSIVE SWITCHING INTERFERENCE.
Switching interference may LOOK like an instability, however it is at a much lower frequency
most instabilities. It will be more visible at low frequencies
Missing jumper at
Missing
Grounds not connected to chassis at output board and front chassis mounting screw.
Cl29,
R224.
134 on output board.
(2OOHz)
and at lower impedances.
(1lOkH.z)
than
FEEDBACK PROBLEMS: GAIN INCORRECT
Gain of output stage set by
Gain of Ch 1 volume control buffer stage set by RI 1, 16.
Gain of Ch 2 volume control buffer stage set by
at Q48 should not affect gain. Check RI 32 (drives Q48).
Gain of balanced input is set by 4 matched resistors
of balanced input are working. Check
R23,
31 (147, 153)
R5,
6 (123, 124).
Rl37,
139. Make sure
R9,
8, 12, 13 (129, 130, 135, 136). Confirm both sides
Q48
is turned on - grounding
Audio Output, Troubleshooting Clipping, Limiting
EXCESS STICKING (TOO MUCH DISTORTION DURING CLIP LIMITING)
Cl4 (49) much too large (also causes increased high frequency distortion).
R38 (161) missing.
R38-39
(161-162) have wrong values.
Q9,
10 (54, 55) missing
R34 (157) or
Q8
(53) missing
R35
(158) missing
RI37
18
CLIP LIMITING DOESN’T WORK:
BOTH CHANNELS:
Check U3 missing,
Check U3 supply voltages, +I
ONLY ONE CHANNEL BAD:
Probe output of U2
R38 (161) missing.
R38-39 (161-162)
Q9,
10 (54, 55) missing.
R34 (157) or
Q8 (53) missing
(7),
pin 7 while clipping. If output exceeds 4V during clipping, check
have wrong values.
R35
(158) missing
3VCL, -13VCL,
on
C73*
74
:
If output at pin 7 clamps at
R32 (154) missing
Q7 (52) missing
R28 (151) missing
Q6 (51) missing
RI8
(141) missing
RI9 (142) missing
SW I:1 (1:lO) not making contact
Check each pin on U3
CLIP LIMITING OSCILLATES:
Cl 3 (48) missing.
R21,
27 (144, 150) missing
3.5.4V
as expected, check parts surrounding U3:
Troubleshooting Step Problems
EXCESSIVE STEP DISTORTION (STEP GLITCH)
Close scrutiny of the distortion trace, and scope probing of the switched waveform, will help determine the
cause of excess step distortion, The step should switch when the output voltage is within IO-12 volts of its
respective rail, This switching margin should be fairly constant from
should be a fairly uniform up and down ramp, moving at about 25 volts/us, therefore taking about
complete its transition.
Step Switching Too Close to
This will cause increased step glitch, especially at low impedances. If present at all frequencies,
check the reference voltages:
Negref: 17.5 volts above its intermediate rail:
PosRef:
20V below its intermediate rails:
the
Rail:
D88, R276-7-8.
D87, R256,
257,
20-
20kHz. The switching event itself
D53.
2us
to
Confirm correct values in output voltage divider: R48 loaded by
only at high frequencies, check the value of the speed up capacitor
or look for slow switching (see below).
R49-50 (Rl71,
C20 (C55)
and
Rl72-173).
in the output voltage divider,
If present
19
Step Chattering.
If the step repeatedly switches on and off, usually at a low frequency, it creates an oscillation burst
which increases step glitch at low frequency. The tendency is usually worse at low impedances
and low frequencies. 2-3 “false trials” at very low frequency, 2 ohms, is normal, but prolonged
bursts of maximum frequency chattering may cause FET failure.
Positive step: check hysteresis resistor R66 (RI 89)
Negative step: check hysteresis resistor R69 (RI 92) and capacitor Cl 87 (Cl 93).
Slow or Fast Switching.
Slow switching reduces step glitch but puts more strain on the FET. Fast switching increases step
glitch. The usable limit is
If both slopes are equally off speed, check the slope capacitors:
Positive,
If only one slope is slow, check the resistors and buffer transistors:
Positive step:
Negative step:
Step FET Oscillation.
Certain FET types oscillate at extremely high frequency while ramping up and down. This injects
interference into the amplifier which increases the step glitch. Such problems are supposed to be
found while
know.
STEP WON’T TURN ON (Premature Clipping)
If the step refuses to switch high, the amp will clip prematurely, at the intermediate rail, at any load. Make
sure the clipping is not
and intermediate rail voltages to confirm clip point and lack of step action. Trace the circuit from the step
FET back via gate drive to drive circuit to locate cracks, missing part etc. Check DC power on step driver
(14V
on EACH positive step drivers,
+65V
rail) or Negref
ladders.
C30 (C65)
R78,
R83,
(dis)qualifying
acually
(17.5V
above
17-27
volts/us,
and Negative, C29 (C64).
79,
Dl5, Q30-31 (R201,
84,
Dl7, Q32-33 (R206,
specific FET types. If they crop up in production, Engineering needs to
current cutback, usually evident only at 2 ohms. Probe the output voltage
12V
on BOTH negative drivers). Check voltage of
-65V
rail). Look for severe mismatches of the comparator resistor
202,
207,
D40, Q7576)
D42, Q77-78).
PosRef
(20V below
STEP STUCK ON (Switched Rail Voltage Stuck On Full)
If the positive step is stuck on, (evidenced by permanent high voltage on switched rail) the FET is probably
bad, since the positive gate drive cannot sustain DC turn-on due to the bootstrapping. If the negative step
is stuck on, it could be a bad
confirmed by measuring the gate voltage. Malfunctioning gate drive circuitry should be checked as noted
above under “Won’t Turn On”.
REPEATED FET FAILURE.
Repeated failure of step
region). The actual failure usually occurs at 2 ohms, where the dissipation is highest. After replacing the
FET,
the step waveform should be monitored, starting at light load to avoid repeated failure, and advancing
briefly to heavier loads while closely watching the waveform. You will need to use an isolated scope probe
which allows voltage readings to be taken with respect to the intermediate rails, or to FET sources.
FET Does Not Fully Turn On:
Generally causes problems at low frequency, 2 ohms.
Confirm that the step FET remains fully on for the entire cycle
drive and determine cause.
Weak positive drive: check voltage on C32
missing D18
Weak negative drive: check voltage on
FET,
or the gate drive circuit could be holding the FET on, which will easily be
FET’s
is usually caused by failure to fully switch ON or OFF (lingering in the linear
(2OHz).
If not, confirm weak gate
(C51)
for
14V.
(Dl48).
Check
R78, Dl5, Q30 (R201, D40,
C67, 12V.
Check
Check
Q75).
R83, Dl7,
C31 (C66),
Q32
(R206, D42,
low RIO4
(R227),
Q77).
20
FET Turns On or Off Very Slowly:
Generally causes problems at high frequency, 2 ohms.
If both slopes are equally slow, check the slope capacitors:
Positive,
If only one slope is slow, check the resistors and buffer transistors:
Positive step:
Negative step:
Severe Step Oscillation.
Generally observed at low frequency, low impedance, right at threshold.
Positive step: check hysteresis resistor
Negative step: check hysteresis resistor
C30 (035)
R78,
R83,
and Negative, C29 (034).
79,
Dl5, Q30-31 (R201,
84,
Dl7, Q32-33 (R206,
R66
R69
202,
(RI 89)
D40, Q75-76)
207,
D42, Q77-78).
(RI 92) and capacitor Cl 87 (Cl 93)
Troubleshooting DC Fault Shutdown
NORMAL BEHAVIOR OF THE CIRCUIT.
Any amplifier fault which causes a non-symmetrical output, such as premature clipping of one polarity, a
missing step,
TRACING THE CAUSE OF FALSE TRIGGERS.
If amplifier is shutting down for no apparent cause, the source of the false signal must be found. Be sure the
output is checked with a DC coupled scope in order to confirm absence of an actual DC offset. The circuit
will trip on DC offsets exceeding about
etc,
may trigger DC fault shutdown. This indicates normal operation of the circuit
4V.
The optocoupler’s input can be safely disabled by shorted
triggering is before or after
The output of
signal.
SHUTDOWN OCCURS AS SOON AS SWITCHING STARTS.
Disable
series with AC line to limit fault current in case of shorted outputs.
If amplifier output looks OK, check UlO:l output. If low, check voltage on pins 2 and 3
UlO:l,
UlO:l,
Check R348 at Ul.5.
SHUTDOWN OCCURS ABOVE ABOUT 4V OUTPUT:
Q87, C7, R240
Confirm D48 is pulled low
NOTE: this control voltage responds to the Br Mono switch, pole
Check R348 at
Bad connection at step diodes
UlO:l,
Ul5
as noted above, determine if there is a DC fault condition. CAUTION: use
pin 2: should be zero (no signal)
pin 31 should be about
or D48 missing.
Ul5.
Ul5.
pin 1 should be monitored. If it goes low during DC shutdown, it is sending the false
2V!
set by
(-13V),
holding Q87 on. If not, check RI 17, 118, Q42
(D21, D22, D46, D47)
R243,
244, 245.
Ul5,
pins I-2 together. This will indicate if false
7.
50-ohm
resistor in
21
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