QSC | RMX 1450a | Service Bulletin

Service Bulletin
Title:
RMX Turn-on Delay
Bulletin #:
RMX0002
Issue Date:
September 28, 2000
Models Affected: RMX 850, 1450 & 2450 Bulletin Revision: A
Production Range: 01/2000–08/2000
Description
Some RMX Series amplifiers take longer than 10 seconds to fully turn on. This is caused by the 5.6-volt zener diodes (D107,
D110, D207, and D210) in the bias circuitry causing the op amps to remain in non-symmetrical cutback even after turn-on
muting ends. The channel bias will take perhaps 10 to 30 seconds to set before the amplifier begins working normally.
This bulletin describes a corrective procedure that involves replacing the original zeners with 4.7-volt diodes and 180Ω
resistors to improve op-amp turn-on.
Symptoms
After the amplifier is turned on, both channels' clip LEDs remain lit for more than 10 seconds. This occurs only when speaker
loads are attached to the amplifier outputs.
Instructions
Tools and materials required:
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Soldering iron with fine tip (recommended range 25 to 60 W)
Rosin-core solder (60/40 or 63/37 eutectic type)
Long-nose pliers
#1 and #2 Philips screwdrivers
Small diagonal cutters
Desoldering equipment or solder braid
Four 4.7V zener diodes (QSC part # QD-0004.7-ZT) and four 180Ω ¼W 5% resistors per RMX amplifier
Test equipment required:
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Oscilloscope
2-ohm resistive load (rated for at least 1200 watts)
Shorting connector for amplifier output
Variable AC transformer (e.g., Variac, Powerstat, etc.) rated for 25A (120V) or 12A (230V). Make sure the amplifier and your
AC supply are compatible.
1 kHz audio sine wave generator
Digital multimeter
Digital current meter (e.g., Fluke 30 Clamp Meter)
Trimpot adjustment tool (non-conductive)
Procedure: replacing the zener diodes
The RMX 850 and RMX 1450 use single-sided printed circuit boards, so replacing the zener diodes requires removal of the
two channel module boards from the chassis. The RMX 2450 uses double-sided boards with plated-through holes, so zener
replacement can be done while working from the top side of the board, and removing the channel modules is unnecessary.
Preliminary work
1.
Disconnect the amplifier from the AC service and wait several minutes to let internal voltages bleed down.
2.
Remove the top cover.
RMX 2450
1.
Locate diodes D107 and D110. See Figure 1.
2.
With the diagonal cutters, clip out the body of the diodes.
RMX0002
1
R139
R140
R131
D107
D110
D207
D210
R239
R240
R231
R146
R246
(under
ribbon
cable)
(behind
ribbon
cable)
Figure 1. Components D107, D110, R131, R139, R140, R146, D207, D210, R231, R239, R240, and
R246 in the RMX 2450. Note that in this model, the positions of channel 1 and 2’s channel
modules are the reverse of where they are in the RMX 850 and RMX 1450.
3.
With long-nose pliers, hold one of the lead remnants. Melt the solder with the iron, then pull the lead out of the circuit
board. Use braid or desoldering equipment to remove excess solder from the hole. Repeat for the three other lead remnants.
4.
Twist the anode lead of a new 4.7-volt zener diode with one lead of a 180Ω
resistor to form a “V” and solder them together; trim the excess. Then trim
the cathode lead and the other resistor lead so they fit into place on the
board without short-circuiting anything. Repeat for the other three diodes
and resistors. See Figure 3.
5.
Insert the zener/resistor combinations. Pay attention to their correct orientation:
the cathode (the banded end) of D107 should point toward the front of the amp,
while the cathode on D110 should point to the back. Solder them in place (Figure 3).
6.
Repeat for diodes D207 and D210. D207’s cathode should point away from
the heat sink, while D210’s should point toward it.
7.
Continue with bias and current limiting adjustments.
Fan shroud
RMX 850 and RMX 1450
1.
2.
Figure 2. Removing the fan shroud makes
it easier to remove the channel module
Underneath the amplifier, remove the four screws that secure the heat sinks boards on the RMX 850 and RMX 1450
to the chassis.
At the front panel, pull the two gain control knobs straight off.
3.
Remove the eleven screws that fasten the two channel modules to the
chassis (six screws on channel 1's module, and five on channel 2's).
4.
Detach the input ribbon cable from channel 2's module.
5.
At the front of the channel module boards and in front of the
heat sink tunnel, disconnect one end of the ribbon cable that
connects between the two channel modules.
6.
On the rear panel, loosen the four screws on the fan enough to
allow you to remove the fan shroud (Figure 2). Lift the shroud
straight up to remove.
7.
Slide the two channel module boards straight back to clear the
LEDs and gain control shafts of their chassis front panel
openings. Lift the boards up to gain access to the undersides.
8.
2
Soldering
iron
Solder
Figure 3. Soldering the zener diode and resistor to
the circuit board. On the RMX 2450, the components
can be soldered on the top side of the board as
Locate diodes 107 and 110 (Figure 4). Unsolder and remove them shown; on the RMX 850 and 1450, they are soldered
from the board.
on the underside.
RMX0002
Twist the anode lead of a new 4.7-volt zener diode with one lead
of a 180Ω resistor to form a “V” and solder them together; trim the
excess. Repeat for the other three diodes and resistors. See Figure 3.
9.
Insert the diode/resistor combinations into the circuit board. Make
sure they are correctly oriented: the cathode (the banded end) of D107
should be on the diode end pointing away from the heat sink, while
on D110 the cathode should be on the end closer to the heat sink.
10. Solder the diode and resistors to the circuit board. Trim off the
excess leads.
R146
R131
R140
R139
4.
D107
11. Repeat for diodes D207 and D210.
D110
12. Re-assemble the channel modules into the chassis. Remember to
re-attach the small ground wire from the fan shroud to the nearest
mounting screw on channel 2’s module. Reconnect the input ribbon
cable as well as the short ribbon cable that joins the two modules.
Figure 4. D107, D110, R131, R139, R140, and R146.
The components for channel 2 are similarly located 13. Continue with bias and current limiting adjustments.
on the other channel module. The model shown is
an RMX 1450, but the RMX 850’s components are
located in the same places.
Procedure: bias and current-limiting calibration
Note: During the current-limiting part of this procedure, the amplifier will draw high amounts of AC: approximately 22
amperes at 120V, or 11 amperes at 230V. Be thorough, but work quickly to avoid tripping the amplifier's circuit breaker.
1.
Start with the variable transformer turned all the way down to zero and the amplifier turned off and its top cover still
removed.
2.
On the amplifier's circuit boards, locate resistors R146 and R246, and trimpots R131, R231, R139, R239, R140, and R240
(Figures 1 and 4).
3.
Plug the amplifier into the transformer.
4.
Turn the gain controls all the way down.
5.
Turn the amplifier on and gradually turn up the transformer to the amplifier's normal operating voltage, while monitoring
its AC current draw. Make sure the amplifier is functioning correctly.
Table 1
Calibrations
Adjust
RMX 850
RMX 1450
RMX 2450
Channel 1 bias: DC voltage across R146
R131
0.16 V
0.14 V
0.09 V
Channel 2 bias: DC voltage across R246
R231
0.16 V
0.14 V
0.09 V
Output current into shorted load
Channel 1: R139 & R140
Channel 2: R239 & R240
4–4.5 A
4–5 A
8–9 A
AC current when driving shorted load*
Channel 1: R139 & R140
Channel 2: R239 & R240
3.75–4.5 A
4.5–5.5 A
5–6.5 A
Clipping voltage into 2 ohms (RMS)
Channel 1: Adjust R139
for symmetry
Channel 2: Adjust R239
for symmetry
26–29 V
33.5–37.5 V
44–49 V
Clipping voltage into 2 ohms (peak)
Channel 1: Adjust R139
for symmetry
Channel 2: Adjust R239
for symmetry
36.8–41 V
47.4–53 V
62.2–69.3 V
0.4 A, ±10%
0.4 A, ±10%
0.5 A, ±10%
Idle AC demand* (at ambient temperature;
higher when hot)
*Figures shown are for 120V amplifiers; multiply current by 0.5 for 230V.
RMX0002
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Bias adjustment (perform only with unit at ambient temperature)
Note: This calibration must be done with the amplifier circuitry at ambient temperature. If the amp has begun to warm up,
you must turn it off and wait until it cools down.
6.
Measure the DC voltage across R146. Adjust bias trimpot R131 to reach the correct voltage shown in Table 1.
7.
Next, measure the DC voltage across R246. Adjust bias trimpot R231 to reach the correct voltage shown in Table 1.
Current limit adjustment
8.
Set the audio sine generator to 1 kHz at 1 volt RMS and connect it to channel 1's input. Connect channel 1's output to a
2-ohm load, and connect the oscilloscope probe across channel 1's output.
9.
Turn up channel 1's gain control partway. On the oscilloscope you should see the amplitude of the sine wave increase
accordingly.
10. Turn the gain control back down and apply a short circuit across the output terminals of channel 1. Clamp a current
probe either onto one of the brown wires running to the AC switch or onto the gray output wire from channel 1's module.
11. Turn the gain control all the way up. Adjust trimpots R139 and R140 equally until the current measured falls within the
range shown in Table 1.
12. Turn the gain control all the way down and remove the short circuit so the channel drives the 2-ohm load. Turn the gain
control back up until the output clips. The voltage at which the signal starts to clip should fall within the range shown in
Table 1. If the clipping is asymmetrical, that is, the signal clips on either the positive or negative side first, adjust R139
to make it symmetrical.
13. Turn the gain control down. If the amp has begun to warm up shut it off and let it cool a few minutes before proceeding
with channel 2.
14. Repeat steps 8 through 12 for channel 2. Use trimpots R239 and R240 to adjust the current limiting in steps 11 and 12.
15. Turn both channels’ gain controls all the way down. Clamp the ammeter onto one of the amp’s AC wires and check the
amp’s idle current. If the amplifier is still at about room temperature, the idle current should match the value shown in
Table 1.
Procedure: finishing the repair
1.
Turn the amp off and disconnect all cables.
2.
Re-install the top cover. The amplifier may now be returned to use.
Contact information
If you need any further information regarding this service procedure, please contact QSC Technical Services at the addresses
or numbers below.
Telephone:
800-772-2834 (within USA only)
714-957-7150
Fax:
714-754-6173
E-mail:
tech_support@qscaudio.com
Web site:
www.qscaudio.com (product info and support)
www.qscstore.com (on-line technical services and accessory sales)
Postal and parcel address:
QSC Audio Products, Inc.
Technical Services Group
1665 MacArthur Blvd.
Costa Mesa, CA 92626
4
RMX0002
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