Driver board repair help

[imraneesa]

Hi, attaching the class d driver board. All the lower supply voltages exist without outputs I have sinewave going to the driver board and pin 15 of driver board has squar wave. But no oscillations on outputs when outputs installed. Once scope probe touches audio opamp pins amp goes into protect.

 

[imraneesa]

Sorry forgot to attach picture.

 

[Perry Babin]

Did you replace Q7 and 11?

Did you measure the voltage across the collectors of the drivers to confirm that voltage was there.

 

[imraneesa]

Did you replace Q7 and 11?

Did you measure the voltage across the collectors of the drivers to confirm that voltage was there.

Yes transistors were good. But R24 and one if 470ohm were bad. But now I am having another problem. It working good with 5 pfets and 4 nfets. Any time I include 5th nfet it is pulling current.

 

[Perry Babin]

How does the drive signal vary with or without the last FET?

Does it matter which FET location is the 5th N-channel FET?

 

[imraneesa]

How does the drive signal vary with or without the last FET?

Does it matter which FET location is the 5th N-channel FET?

Place doesn't matter. I tried to change Q11 and Q7 now it is working normal with all fets. These old transistors still showing like they are good with diode test with multimeter. So it is funny I dont understand.

 

[twilliar]

Place doesn't matter. I tried to change Q11 and Q7 now it is working normal with all fets. These old transistors still showing like they are good with diode test with multimeter. So it is funny I dont understand.

I have been seeing this a lot lately. Transistors will check fine but the scope will show otherwise. It is a learning curve to observe, understand and analyze the characteristics of a transistor during operation under load using a scope.

 

[Perry Babin]

This is like the two 1D transistors on the 11 pin driver boards. Here it's Q7 and 11 (and 19 and 22 for the larger boards). You replace them as the first course of action. They run too hot and while they may show as OK with the DC of a multimeter, they will not perform to spec at higher frequencies.