drive board problem SSL EVO

[yoppi]

Hello

I have this SSL EVO 4K with the output faulty. The amplifier, without the output transistors is in constant protection.
I have removed the driver board and the amplifier turned on well, green light.
I have reviewed the card, I have changed all the transistors MPSA92,56,42. C1027 and A1023.
I put it back on the amplifier and it's back in protection.

I have shorted the LED side A of the octo-coupler U7, it turns green, but I do not get oscillation in the output transistors.

I have rail tension +70 -70. Regulator voltage +15 -15

What can be happening?

Greetings.

 

[Perry Babin]

Post a good quality photo (in focus, no flare from the flash) of the area shown in the image below.

HAve you tried to get a schematic from the manufacturer (SSL/Soundstorm)?

 

[yoppi]

Yes, I tried to locate the scheme, but I have not found anything.

 

[Perry Babin]

Is there any DC voltage measured directly across the speaker terminals with the driver board in the amp?

 

[yoppi]

I have reviewed. It effectively has 60v on the speaker terminals.

Drive board connected and output mosfet removed from the amplifier.

 

[Perry Babin]

DC voltage on all 3 legs of the two output FET locations on each end of the row of outputs? 6 voltage readings. Black probe on speaker negative terminal.

 

[yoppi]

side closest to output terminals:

G: -47,3
D: 69,6
S: -59

side closest to power supply:

G: -66,8
D: -59
S: -68

 

[Perry Babin]

Are you sure that you have the right transistors in the locations, Q35, 36, 48, 49?

Are you sure that you don't have a solder bridge somewhere on the driver board?

 

[yoppi]

Q35,36: MPSA42
Q48,49: MPSA56


Already check welding bridges and this is all correct.

 

[Perry Babin]

Shine a bright light at the back of the board. Looking from the other side, do you see any areas where there is no trace or component but the light is being blocked (by a darkening of the internal fiberglass due to heating)?

 

[yoppi]

I found the fault. a tiny speck of fiberglass burned wedged between the gate resistor of an output transistor. I was bridging a rail with the gate of Q2.
Already works correctly without DC VOLTAGE at the output.