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Fan doesn't run
• The fan is defective.
• Check the fan connection to the AC board.
• Check R1 and BR1 on the AC board.
Fan runs always on high speed
• Q3 and/or U2 are shorted.
• Check PTC thermistors R4 and R7.
3.3 Faults with signal present
The customer compains that the amplifier passes a signal but
doesn't run correctly.
Symptoms covered:
• The output signal breaks up or is distorted
• "Ringing" sound in loudspeaker when no audio signal is present
• The output signal collapses when driving a normal speaker load
• Supply rails OK with no signal, but collapse when a signal passes
• The amplifier gets too hot
• One channel clips prematurely
• Excessive hum in loudspeaker when no audio signal is present
Possible situations:
The output signal breaks up or is distorted
• (RMX 850 and RMX 1450) Check the hum-null resistors R118,
R119, R218, and R219.
• Check the ground traces for continuity among speaker ground,
input ground, and AC ground.
"Ringing" sound in loudspeaker when no audio signal is present
• Check C114, C124, C214 and C224.
• Check or replace dual op amp U101 or U201.
The output signal collapses when driving a normal speaker load
• R139, R140, R239, and/or R240 is misadjusted.
• Check R118, R119, R218, and R219.
The supply rails are OK with no signal but collapse with a signal
• C112, C113, C212, and/or C213 is leaky.
• Check C114, C124, C214, and C224.
The amplifier gets too hot with no load
• Bias trimpots R131 or R231 are misadjusted, burned, or open.
• Bias diodes D108, D109, D208 and/or D209 are incorrect (should
be 1N4934).
• If the amplifier is producing high-frequency oscillations, check
C114, C124, C214, and C224.
• Check resistors R136, R137, R236, and R237.
• Op amp U101 or U201 is unstable.
One channel clips prematurely
• R146 or R147 (Channel 1) or R246 or R247 (Channel 2) are open.
• R139 or R140 (Channel 1) or R239 or R240 (Channel 2) are misadjusted.
• Check R157, R158, R198 (RMX 2450 only), D115, and D116
(Channel 1) or R157, R258, R298 (RMX 2450 only), D215, and
D216 (Channel 2).
• (RMX 850 and RMX 1450) Check R118 and R119 (Channel 1) or
R218 and R219 (Channel 2).
RMX Series Technical Service Manual
• R130 or R132 (Channel 1) or R230 or R232 (Channel 2) is open.
Excessive hum in loudspeaker when no signal is present
(RMX 850 and RMX 1450)
Check R118, R119, R218, and R219.
Excessive current draw with signal present (RMX 2450)
• Check triacs Q113 and Q213.
• Check D119, D120, D219 and D220.
3.4 Instability
The customer complains of gain problems, spurious noises, or oscillations.
Symptoms covered:
• General output distortion
• "Ringing" sound in loudspeaker trailing an audio signal
• Excessive crossover distortion
• The output waveform appears fuzzy on an oscilloscope
Distinguish among the different symptoms of fuzziness (instability),
ringing (momentary instability after a transition), crossover distor-
tion (often causing ringing), or general distortion.
Possible situations:
General distortion in the output signal
• Severe distortion, at any load, often with abnormally high current
draw: check the slew rate capacitors C114, C115, and C116
(Channel 1) or C214, C215, and C216 (Channel 2).
• Moderate distortion, especially with light loading: stability
capacitors C124 and C126 (Channel 1) or C224 and C226 is too
high in capacitance; also check the slew rate capacitors C114,
C115, and C116 (Channel 1) or C214, C215, and C216 (Channel
2), and the output filter resistors R161, R162, R154, and R155
(Channel 1) or R261, R262, R254, or R255 (Channel 2).
• Distortion with low gain: check the feedback shunt components
R120, R138, and C125 (Channel 1) or R220, R238, or C225
(Channel 2); also check for broken circuit traces around the
components; U101 (Channel 1) or U201 (Channel 2) is defective
or its socket is contaminated.
Ringing sound trailing the audio signal
• This usually indicates marginal instability and is usually triggered
by the signal passing through zero volts (the crossover point).
Check the stability components and output filters.
• With a sine wave test signal, use an oscilloscope to check for
excessive crossover notch at the output signal's zero crossings.
Excessive crossover distortion (unbalanced, asymmetrical,
or excessively large crossover notch)
• Severe crossover discontinuity: bias diodes D108 or D109
(Channel 1) or D208 or D209 (Channel 2) are shorted.
• Moderate discontinuity: bias diodes D108 or D109 (Channel 1) or
D208 or D209 (Channel 2) are out of spec.
• R131 (Channel 1) or R231 (Channel 2) is defective.
• Base resistors R136 or R137 (Channel 1) or R236 or R237
(Channel 2) are open. Also check the NTC thermistors R134
(Channel 1) or R234 (Channel 2).
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