Pioneer A-307R

[andiukas223]

Hi folks,so i got scammed by some people in buying a audio amplifier ,its a good amp,but its not working.I got it today and pluget in for a test,and magic smoke with a flash got out  .I opened it up and saw a burnt R373 resistor,well one of them  :

so i screw it open and lifted the main board up to poke around with my multimeter  ,i turned it on ad guess what, magic smoke again  ,but this time out of smd components:

my friend suggested me to trow this amp trough my window rather the haste,but i think that is half truth    i hope you can help me fix this device  :)schematic <img src=http://www.filedropper.com/download_button.png width=127 height=145 border=0/>
<div style=font-size:9px;font-family:Arial, Helvetica, sans-serif;width:127px;font-color:#44a854;> <a href=http://www.filedropper.com >online backup[/url]</div> . p.s.sorry for my English skills.

[Grapsus]

R375 is burnt which suggests that a high current passed through it. It may have happened if Q323 was shorted. I'd desolder and check Q323 which is the big MOSFET nearby mounted on the big heatsink.

[andiukas223]

I replaced couple of parts: Q323,Q451 ( and 3 160ohm resistors-R469,R469,R465),then tryed to turn it on ,this time R373 in my eyes went hot ant shortened out the fuse,well i dont know ,some progress  ?

[Grapsus]

There's also Q325, the other MOSFET with a heatsink that could provide the sort path for the resistor. What did you use to replace Q323 ? Did you measure the Q323 that you removed ? was it damaged ? If it's not Q323 or Q325, it could be the circuitry that drives their gates which decided to turn both of them on at the same time. A way to troubleshoot that could be the remove Q323, Q325 and power up the device to measure the voltage that comes at their gates.

[andiukas223]

Q323 was replaced with IRF530 and no i didn't check the MOSFET after,that's kinda niewb mistake,and im not saying that i aint one  , ok ill try to mesure the voltages on the gates,i noticed a couple transistors replaced on the bottom , it might be fixed erlier,ok ill post what i get .

[andiukas223]

Today i had progress,but before that i did couple of things ,as you said i removed two MOSFET's IRF530/9530 ant tested them,they were shorted also resistors were blown (R373/375) so replaced those  and the main fuse.Also discovered o cap with dry soldier marks (C307)and fixed the soldier joint ,checked big transistors,they're good.Then i thought about connecting a load in fuse's place,but didn't do that and turned it on  .It turned on,the led's didn't blink,i turned the selector knob and it was working,the relays were clicking.So far for my it's a big step forward,not seeing "magic" smoke coming out  ,but i didn't check voltage in gate so now i feel a bit dum.Will check the voltages next time i visit the amp.

[andiukas223]

Ok got the gate voltages, IRF530 17V and IRF9530 12V

[Grapsus]

Looks like you managed to remove the short. Does the amp do its job now ?

[andiukas223]

I was waiting your opinion on the matter ,ill try to soldier MOSFET's in and check ,fingers crossed 

[andiukas223]

Ok the amp is working  , Grapsus thank you very much for your help  .

[Asmyldof]

Don't forget to do a monitored test-run with your intended loads.
About an hour at a decent volume level and regular checking should do it, see if all stays within limits of heat and such.

Else you might close it up and find it overheats and self destructs because the pre-amp stage has drifted for some reason forcing both MOSTs slightly on at the same time. You never know what caused the initial defect. Could have been a bad load, could have been internal.

[Grapsus]

Well done !

Don't forget to do a monitored test-run with your intended loads.
About an hour at a decent volume level and regular checking should do it, see if all stays within limits of heat and such.

Else you might close it up and find it overheats and self destructs because the pre-amp stage has drifted for some reason forcing both MOSTs slightly on at the same time. You never know what caused the initial defect. Could have been a bad load, could have been internal.

+1

That's the problem every time you replace a component and the thing works again, you can't be sure that the failed part was the first cause of the failure. So test the amp with a load at a high volume for some time.