Question about Legacy Audio LA720 Car Audio Amplifier

1 Answer

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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  • ve9jmc Aug 19, 2007

    OK thank you for the reply. i am getting 40 volts to my Audio transistors, and all 8 of the audio output transistor are testing good. all of my Power Supply transistors are testing good as well. as far as testing other components i have been through the amplifier twice over testing transistors as well as the ones i have replaced. now wheni go into briged(mono) mode on this unit all i get is a hum and no audio, i know the Amplification circuit is functioning as when i turn up the levels the hum gets amplified. so it's just an issue of not getting theaudio through to the amplification curcuit.

  • ve9jmc Aug 19, 2007

    ok i can attach an image to the posts here so here's an image of the unit.

  • ve9jmc Aug 19, 2007

    and here's an image of the Audio Header(where the Inputs are for the audio) the 8 IC's are all 353d 8 pin ic's. if you need more info just let me know and i'll get you more info on this unit.

  • ve9jmc Aug 19, 2007

    sorry i'll comment on the first image i uploaded as to the transistors that you can see. on the far left both top and bottom are row's of 4 transisters IRFZ46's next to them top and bottom are F12C20C's then next to them are the D1184 and B863 pairs and there are 4 pairs of them to finish out the Major transistors.

  • ve9jmc Aug 19, 2007

    ok well just to let you know the ground wire wasn't burnt, i think what happened was that someone had tried to repair this unit before, and had not taken the ground wire that runs from the pcb to the top of the Transistors and re soldered it back to the pcb, which in turn when the amp was re-installed that end of the Ground wire came into contact with on of the legs of the IRFZ46 transistors( which they're in rows of 4) and caused a cascade effect on the taking out the entire row of 4.

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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

  • Mike Ryan
    Mike Ryan Aug 18, 2007

    Oh... also ohm between pins#2 and #3 of each audio outpuit transistor... if any check shorted that series of transistors has to be replaced....

  • Mike Ryan
    Mike Ryan Aug 19, 2007

    oh man.. i wonder if these people let a power wire touch the rcas or arced a screwdriver from the power terminal block to the amps casing or something.. finding a problem in the pre-amp section is damn near impossible unless you haver another working amp just like it to compare to..

    I would make sure that the small capacitors that go form the circuit board to the case ground screws are not blown(common if what i suspect happen is true) that might also explain your burnt ground wire...

    Unfortunately I have not had too much personal experience with this amp so thats about all i can tell ya...

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