QSC RMX 2450, Техническое и сервисное руководство

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12 QSC Audio Products, Inc.
3.5 Power supply & rail balancing
problems
Symptoms covered:
• Insufficient or excessive current limiting into a shorted load
• Op amp rails too high with a shorted load
• Uneven voltages on supply rails
A channel’s output current should remain unaffected when driving
resistive loads as low as 2 ohms per channel. When driving a short
circuit, the current limiting circuit should collapse the output to a
lower current. This is done by dropping the op amp’s supply rails
from a normal ±14–15 volts down to about ±5–6 volts. Normally, the
output signal helps replenish the op amp supply rails through D115
and D116 (Channel 1) and D215 and D216 (Channel 2), but a short
circuit or excessively low load impedance prevents the replenish-
ment, and the op amp rails collapse because they can’t supply
enough current to let the op amp drive both the clip LED and the
driver transistors. The current limit trimpots R139, R140, R239, and
R240 permit adjustment of the current limit thresholds. See the
RMX calibration section of this manual for adjustment procedures.
Possible situations:
Excessive current into short (insufficient limiting)
• If the op amp rails are dropping to ±5 to 6 volts as they should:
the 5.6V zener diodes D107 and/or D110 (Channel 1) or D207
and/or D210 (Channel 2) are reversed or shorted.
• If high crossover distortion is present: bias diodes D108 or D109
(Channel 1) or D208 or D209 (Channel 2) are shorted.
Excessive current into short (op amp rails are not dropping)
• The op amp U101 (Channel 1) or U201 (Channel 2) is defective,
with insufficient output current.
• Clip LED LD100 (Channel 1) or LD200 (Channel 2) and/or its
rectifying diodes (Channel 1: D102, D103, D105, D106; Channel 2:
D202, D203, D205, D206) are open.
• When driving a short circuit, the output section’s positive and
negative supply rail voltages should be equal, within 3 volts. If
they aren’t, check D107, D110, R146, and R147 (Channel 1) or
D207, D210, R246, and R247 (Channel 2).
Weak current into 2
Ω Ω
Ω Ω
Ω
or short (excessive or premature limiting)
• Bias resistors R130 and R132 (Channel 1) or R230 and R232
(Channel 2) are too high.
• Driver transistors (Channel 1: Q105, Q106; Channel 2: Q205,
Q206) have very low gain.
• One or more emitter resistors in the output section are open.
Current OK at 2
Ω Ω Ω Ω Ω
, weak into short
• LD100, D102, D103, D105, or D106 (Channel 1), or LD200, D202,
D203, D205, or D206 (Channel 2), are shorted.
• Zener voltage of diodes D107 or D110 (Channel 1), or D207 or
D210 (Channel 2), is too high.
Current limits properly into short, but current is weak at 2
Ω Ω Ω Ω Ω
• If the op amp rails are low (< 14–15 volts) when driving a 2-ohm
load without clipping, check the resistors R157 and R158
(Channel 1, all models), R198 (Channel 1, RMX 2450 only) and
diodes D115 and D116 (Channel 1, all models), or R257 and R258
(Channel 2, all models), R298 (Channel 2, RMX 2450 only) and
diodes D215 and D216 (Channel 2, all models).
• If the op amp rails are normal (14–15 volts) when driving a 2-ohm
load without clipping, usually the output section gain is too low: weak
driver transistors, open output transistors, or open emitter resistors.
Check the value of the driver transistors’ emitter resistors, too:
R146 and R147 (Channel 1) or R246 and R247 (Channel 2).
Rail voltages unequal
The balance between the positive and negative rail voltages is set
by a voltage divider comprising resistors R118 and R119 (Channel 1)
and R218 and R219 (Channel 2). If the amplifier channel passes a
signal but clips unevenly due to unequal rail voltages, this voltage
divider is the likely culprit.
4.1 Setting bias
Always set the bias
• after replacing any output or driver transistor.
• after replacing any diode or resistor in the driver/output circuitry.
• if the amplifier seems to run too hot at idle.
• if the amplifier exhibits crossover distortion.
The bias network sets the quiescent base current in the NPN and
PNP driver transistors, which in turn sets the quiescent current in
the output transistors. The driver transistors should both be slightly
“on” at idle so that the transitions of the signal voltage between
positive and negative are smooth and free of gaps or glitches. Too
4. RMX calibration procedures
much bias current will cause the amplifier to run hotter than it
should, especially at idle, while too little will cause noticeable
crossover distortion, especially at low signal levels.
The amplifier circuitry must be cool, or at least within a couple
degrees of ambient air temperature, and the top cover must be
removed. If the driver and output transistors are significantly
warmer than the ambient air, leave the amplifier off and let it cool
before proceeding.
Before turning the amplifier on to set bias on one or both channels,
familiarize yourself with the locations of the trimpots (R131 and
R231) and the voltage measuring points so you can work quickly but
thoroughly. If the amplifier warms up before you finish setting the