5channel amp help

[jleaman]

Well i got a free infinity 5 channel amplifier model 255A

When powered up works with no problems at all except the 5th chanel doesnt want to power up. the 5th channel being the 250watt part the subwoofer amp.

All the fuses are all ok no burnt parts or any thing like that. Im wondering where to start. i have tested for voltages near the smps and they are all working i looked at the fets and there all ok also.

any sujestions any one...

If pictueres needed please let me know..

 

[jleaman]

TTT

 

[davesaudio]

what "tools" do we have to work with?

 

[jleaman]

i got a o-scope a soldering iron psu and dmm..

need any thing else. ?

 

[fr0st]

try measure punch through ont he fets (gate -> source). If thats good power it up and measure the secondary and primary voltages. Make sure the primary doesn't overlap.
Check secondary dc, if the PWM is getting past the caps one of the caps must be gone.
If that checks out check all transisters for consistancy. (or fets)
most likely a output transister has died without you knowing. Measuring them out of circuit if you suspect their dead, they usually come up with a dead short.

 

[jleaman]

All the fets seem to be ok. There is power at the fets also. Does any one need a picture of a certain area of the amplifier. ?

all 4 channels are working and playing sound.

 

[davesaudio]

best to test the outputs and drivers out of cct
you have a scope - where do you loose the signal?
if you inject a signal at the driver what happens?

 

[jleaman]

Here. Look at this.