Broken Amplifier Audio System F4-380

[menime]

Hi @all,

i work the last days on a broken amp. One fet in channel 4 was broken.

3 channels run fine. If i replace the broken, the new one gets in a few seconds really hot and the amp goes in protect.

I see that between gate and source was a difference of 6V DC. None of the other channel has it.

I replaced all the green marked fets, transistors, diodes, OP amp too.

but it will not work.

Anyone any idea?

Dropbox - IMG_20200630_211903.jpg - Simplify your life

Thanks

 

[Perry Babin]

Did you set the bias pot to full CCW (or whatever the minimum is for this amp)?

 

[menime]

Then it runs in protect. without fets.

All other gates have +-28V. The broken one has 24V.
If i set the pot the voltage of the NPN fet decreases too

 

[menime]

when i set the pot the NPN decreases, too.

And i think the amp goes to protect. but i will try this tomorrow.

+VCC side runs, every voltage is correct.

 

[Achim1409]

Hello, change the Mosfet with a new one. Turn the Bias Pot fully conterclockwise. From my expirience, the SMD transitors in front of the Fet are leaking, so change them, even if they measure ok.
Then it should work. After that, increase the bias, until the current starts raising a little bit.

 

[menime]

Hi, i changed everything. All is new.

Pot CCW i have 25.5 volt to 28.5 volt.

 

[menime]

The amp works now. After try it again with mounted mosfets and Pot full CCW, it starts with no problem. Then i rised the bias current that they all the same.

Thanks for your help. Now i have the next that wouldn´t run