Audio pipe GM354 RF ch kick in protection

[ian33]

This amp came in with 3 burnt power supply fet and one driver transistor Q20.I replace them , amp powers up and plays audio fine on 3 of the channels.The right front ch plays audio fine when the gain is all the way down.when i turn it up a little it goes in protect.Amp has both + and - rail voltage.The voltage across the emmiter resistors is 0.07mv,the other ch have 0.05mv across the emmiter resistors.This test was done with the gain all the way down while the amp was idleing.Oh and it only goes into protect if a speaker is connected.Any idea where i should start looking for what is causeing that?

[ian33]

gm 354

[Perry Babin]

If you drive a strong enough signal into the amp with the gain at the minimum, can you drive the 4th channel all of the way to clipping with a speaker connected?

Clamp transistors before trying this.

Do you have a scope?

[ian33]

I try it with a strong signal and the gain all the way down.It goes into protect after the first word of the song.I dont have a scope

[AKHeathen]

i take it q20 was an output driver by the 2nd pic? can't read it. if so, i would suspect one of the outputs had a changed value or short to over-draw that driver, and is still doing the same thing. have you measured the resistance between the legs compared to the other channels? also, i would look at the caps/resistors by the output terminal. my .02

[ian33]

The driver was for the PS fet.The next pic is just to show the output layout.After my last post i install two new outputs but no change.I will check the caps and resistors u mention

[Perry Babin]

Are the emitter resistors within tolerance?

Can you determine which transistor is being used for over-current protection. It's likely to be an NPN transistor with one leg directly connected to one of the emitter resistors and a diode connected to its collector. Find it and remove it. Does the amp still shut down?

[ian33]

both emitter resistor seem to be within tolerance in circuit .I found and remove the over current transistor,amp plays good audio and does not go into protect.so is the problem directly connected to this transistor?

[Perry Babin]

Check the transistor carefully, looking for leakage to see if it is defective. If it appears to be OK, swap it with the corresponding transistor in another channel to see if the problem follows the transistor that you removed.

[ian33]

I went ahead and replace the transistor with a new one and amp is shuting down again.