Question about Legacy Audio LA720 Car Audio Amplifier

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No Audio From Amplifier

Ok i'm having a little problem with this amplifier. it started when a ground wire that travels from the chasis of the amplifier to the PCB came off the PCB and shorted 4 IRFZ46 transistors causing those 4 transistors to smoke. so i have replaced those transistors and have also replaced 5 2200uF 25v Capacitors in this unit as the tops were bulged. now the unit powers up as the power LED lights up. now here's where the fun begins, if i leave the Amplifier in Stereo mode i can't get any sound through the unit and the protection light comes on. now if i bridge the unit and go to Mono the protection light shuts off, but still get no audio through it(exept for a loud hum) and this happens on all 4 channels. if you can think of anything that would help my cause it would be greatly appreciated.

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Re: No Audio From Amplifier

I would first check for "rail votage" at the rectifiers... If all of your audio transistors are not getting around 40 volts then check the transistors you replaced. They should be getting a 5 volt (10 volt square wave is checked with a scope) "drive" signal. If half of your power supply transistors are not getting a drive signal then replace the kia494/tl494/kia594/tl594---whatever it is. thats the ic chip that supplies the "drive" signal to the power supply transistors. Also follow the board closely out of the power supply transistor to the 494 chip.. there should be some small driver transistors in between.. make sure they are not shorted or fried as well... let me knoe if you need more help

Posted on Aug 18, 2007

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