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Crown PSA-2 power amp. Right after turning power on with volume controls all the way down, the amp continuosly thumps the woofers (alternating from the right to the left. This just started to happen. No prior problems. When the thumping is happening, I can see the cones retracting hard and the thumping is extremely loud. When the right side thumps, the right side standby light blinks and then goes off. Same for the left side. The IOC lights don't come on. This amp is part of a tri-amped system. It drives a pair of Klipsch KI-115"s. Thank you for any assistance you can provide. William.

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Hi! The thumping motion is usually caused by faulty cross-over voltage biasing. What this is is simple; Power amps of high wattage are usually constructed of two similar but opposite polarity amplifiers ;one drives the top (positive) part of the sinusoidal and the other drives the bottom.

The simple causes that need to be eliminated are

1) Earthing :-check earthing to ensure no earth loops
2) Check the Power fuses:- usually, the supply of power to only one part of the amplifier causes thumping motion. The power section in these designs have a positive and negative polarity eg.=80V-0V-(-) 80V
3) Disconnect the speakers and check the power supply Voltages across the output stage sumettry for more details visit
http://www.howtodothings.com/electronics/how-to-fix-your-guitar-amplifier

Posted on Dec 04, 2008

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Hi,
I'd love to have more information to go on, but I can give you a few things to check:
If there is a traditional volume control that is motorized, you will see the volume knob start moving on its own after you start it up. If that is the case, you'll need to have someone look at the motor control circuit. Luckily these are almost so generic that parts would be as common as replacing fuses.
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