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Old 6th September 2008, 05:56 PM   #1
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Default La 4705

OK .....a pair of sony speakers come in to my shop amplifier included in the right box and left was only speaker..... Amplifier was LA4705 .....

Replaced the chip since it wasnt working ( i can not imagine why since its a fully protected chip ) and here is what happens :

The trafo inside is a 12 V ac trafo meaning that it will bring out unstabilized about 16 V ...expect this to drop at 15 or less at "full" power .....Well the amp doesnt work at all at this voltage you can only hear a scratcy distorted sound .

So i think something must be wrong with the psu ( 4 diodes and one cap ) so i supply the chip externally .Before that i check the specs and say 12 volt normal operation ( says so also on the sony board ) max 18 volt and then also 24 volt but only for 2 seconds .

Well if you supply the chip with voltage 18-22 everything works perfect but and also runs quiet cold ....but anything lower than 18 doesnt work !!!!!!!
any ideas ?????
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Old 6th September 2008, 06:54 PM   #2
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Default hooked up

one transformer that supply the all thing with 20 volts dc and all seem to work very fine for the latest one hour

am i missing something ?????
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Old 6th September 2008, 10:32 PM   #3
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Did you measure the voltage at the IC pins? Maybe there is a bad connection somewhere, where the voltage drops.

How about the standby pin? Voltage okay? Resistor okay?
Capacitors at pins 3 and 6 okay?
Pin 8 not shorted to anything?
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Old 7th September 2008, 08:12 AM   #4
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Default nothing

i checked all of it many many times ...... to be on the safe side i actually replace all electrolytics on the board ( only 3)

Hell i ve been even checking the double zobels of every out since i thought that there might was some short circuit in a capacitor or so but nothing .

all voltage is fine , main cap is fine and i got it working from last night with 20V power sup problem free ....

it is something we all miss here .....
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Old 7th September 2008, 08:18 AM   #5
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Default Bias Offset?

Perhaps there is leakage on the AC coupling caps on this input, causing an offset. Essentailly the outputs are mostly clipped with no audio. Then increasing the supply voltage reduces the effect, allowing more audio?

Voltages should be, (best guess)

Pins 2, 3 and 7 = VCC/2

Pins 12, 14, 15 and 17 shoul d be VCC/2 as well
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Old 7th September 2008, 08:25 AM   #6
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Default good point

but caps are replaced anyway
and also mute pin and stand by pin voltage is also ok and also when operating at 15-16V there is no offset in the out just a very scratchy sound ( not especially distoreted ) but mostly like is not enough voltage to make the amp operate .....

i start to think that the LA 4705N ihave in my hands even though is totally identical ( not only same number but also look the same brand same typing on it and so on ) is actually something else .....

speakers now working very well for more than 12 hours and also the heat sink is warm but not hot .....

i am puzled all over this
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