Figure 3.9: Switching Signal With Dead Time At Nodes N397 And 398 (Pins 11 And 14 Of U19) - QSC PowerLight3 Series Technical & Service Manual

3.2 PL325 and PL340: Symptoms, causes, and remedies (continued)
Figure 3.9: Switching signal with dead time at nodes N397 and 398
(pins 11 and 14 of U19).
If the previous steps are okay, check U18, L6, and C144. L6 and C144
rarely go bad.
Protection, Fan, and Muting Problems
The customer complains that the amplifier stays in protect—power
supply alternates turning on and turning off; both clip indicators
remains lit; unit stuck in mute protect; or no mute delay.
Symptoms may include:
• Power supply alternately turns on and off; power LED and relay
repeatedly cycle on and off.
• Amplifier stays muted and in protect; both clip LED indicators
remain lit.
• Fan doesn't run or stays on high speed.
• No or brief mute delay.
Possible situations:
Power supply repeatedly turns on and off.
Usually this indicates shorted outputs or drivers, or bad or defective
parts in the current limit circuit. Follow the below steps to trouble-
shoot the unit:
• Check the output transistors and drivers. If any are damaged, see
Troubleshooting and Replacing Blown Output Transistors.
• Check for leaking capacitors C179, C180, C189, or C190.
40
• If the outputs and drivers are okay, check the current limit
circuitry. See Troubleshooting Current Limit (Audio Power Stage).
Hint: shorting across C86 will put the unit in mute protect. This
might stop the repeadted switching on and off and allow you to
troubleshoot for faulty part(s).
• Check for a faulty in DC protection circuit. See Troubleshooting
DC Fault Shutdown .
Amplifier stays in mute protection.
• If fan runs at high speed, C86 is leaking or the thermal sensor's
lead has shorted to the heat sink. See Troubleshooting Thermal
Tracking .
• Defective U10.
No mute delay
• Defective U10 or shorted D56, Q13, or Q58.
• Channel 1: check Q16 and Q17; Channel 2: check Q61 and Q62.
Fan doesn't run or stays on high speed
See Troubleshooting Fan Speed for in-depth troubleshooting.
Instability
The customer may complain of gain problems, spurious noise, or
oscillations, or running warm at idle.
Symptoms may include:
• Gain problems
• Oscillations
Possible situations:
Intermittent output
• Check the gain pot.
Gain problems and oscillations
See Troubleshooting Stability Feedback .
Runs warm in idle
• Check the step FETs. If any is shorted, it will cause the amplifier
to run inefficiently. See Troubleshooting Step Problems for in-
depth troubleshooting.
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