Faults With Signal Present; Symptoms Covered; Possible Situations - QSC ISA280 Technical & Service Manual

Too much or too little muting delay
• Excessive delay at turn-on: check for open LD1, R2, or R5; check
for incorrect R10 or R15; check for shorted D9; also see service
bulletin RMX0002.
• Too short at turn-on (amplifier unmutes before the circuits stabilize,
causing a thump): check for incorrect or bad C7, C8, or R10.
• Amplifier doesn't stay muted at turn-off: see service bulletin
RMX0003
Power LED doesn't light
Check for open or shorted LD1; check R2.
No clip limiting
• U10 is defective.
• DIP switches SW1:1 or SW1:10 is defective.
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
• (ISA280, ISA300Ti, ISA450, and ISA500Ti) 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.
18
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 (ISA750 and ISA800Ti only), D115, and
D116 (Channel 1) or R157, R258, R298 (ISA750 and ISA800Ti
only), D215, and D216 (Channel 2).
• (ISA280, ISA300Ti, ISA450, and ISA500Ti) Check R118 and R119
(Channel 1) or R218 and R219 (Channel 2).
• R130 or R132 (Channel 1) or R230 or R232 (Channel 2) is open.
Excessive hum in loudspeaker when no signal is present
(ISA280, ISA300Ti, ISA450, and ISA500Ti)
Check R118, R119, R218, and R219.
Excessive current draw with signal present (ISA750 and
ISA800Ti)
• Check triacs Q113 and Q213.
• Check D119, D120, D219 and D220.
QSC Audio Products, LLC
TD-000163-00 Rev. B