Rockford Fosgate Power 851x output voltage problem

[urosb82]

Hello everyone,

I have recently bought a RF 851X which had one little problem (or big, we'll see). When I got the amp, on one speaker output voltage was 6V with or without input signal. I located two blown resistors and two output fets (as shown in attachment). After performed replacement, voltage on that same output channel shows 30 plus volts (rail voltage). Can anyone suspect what can be the problem that also needs to be resolved?

Thank you, Cheers!

 

[urosb82]

Don't know why but attachment didn't happen for some reason. Here it goes again...

 

[Perry Babin]

Are you sure that the adjacent channel isn't defective? The source resistors look burned.


For the channel with the arrows, Did you check the driver transistors and the 20 ohm resistors that drive the gate resistors?

 

[urosb82]

Those two resistors look burned but they are actually ok. Two that were replaced (marked by red arrows) were burned and one of them actually exploded so the nearby board and resistors got that black color.
Must source out what's causing rail voltage to pass directly to speaker out...

 

[urosb82]

As far as I can see now, surrounding resistors (two groups of five resistors) are at 20 Ohms. Their markings are so small I cannot even read them with my magnifying glass ...

 

[Perry Babin]

Can you post a good quality, high resolution of the area shown in the attached image?

 

[Old'n'Cranky]

yeah what i see, or rather dont see, is tracks under/around those resistors.

did you repair the burnt tracks ?

 

[urosb82]

Yes, one of the burnt tracks needed to be repaired and it measured ok now. Here is a close up photo of the problematic area in DSLR quality.

 

[Perry Babin]

I needed it to go further up.

I can't believe that the two resistors on the left are good. The board around them is pristinely clean. Desolder them and check them out of the circuit.

Are R226, R237 and R279 within tolerance?

When the 0.1 ohm source resistors open, it virtually always damages the protection circuit. Even if it's not currently causing a problem, the resistors in the protection circuit need to be checked to make sure it's working properly before the amp is put back in service.