Sundown SAZ3500 -DC on output

[brandes.cm]

The amp has the sine wave riding on a negative 5V dc level. Does that sound like a leaky fet, I have pulled them and checked each one and found nothing out of the ordinary.

I have never run across this issue.

 

[Perry Babin]

Post a photo of the board (overall) and one of the driver board that they're using.

 

[brandes.cm]

They are using the dwm3640 with a piece of aluminum used as a heatsink. FQA24n50 output fets, 4 upper 4 lower per bank.

I wonder if a driver is messed up.

 

[Perry Babin]

Pin 9 on the driver board is+5v, 10 is input, 11 is ground, 12 is -5v. With the black probe on 11 and the red probe on 10, what is the DC voltage?

 

[brandes.cm]

pin 10 has -4VDC, hmm.

 

[Perry Babin]

Is there a 6.8k resistor feeding pin 10? If so, is the DC voltage on the driver board side of that resistor greater than the voltage (farther from 0v) on the other side of that resistor.

 

[brandes.cm]

Is there a 6.8k resistor feeding pin 10? If so, is the DC voltage on the driver board side of that resistor greater than the voltage (farther from 0v) on the other side of that resistor.

Yes it ties to a 6.8K resistor. The voltage on the driver side seems more stable, if that makes any sense. If I try to apply a full 12VDC to B+ the amp starts to draw excessive current. I only have one output fet per bank populated.

<Edit>
I removed the output fets and now unable to reproduce the negative voltage on that pin.

 

[Perry Babin]

Was any work done to the amp previously or was the original problem -5v on the speaker terminals?

 

[brandes.cm]

That is how I received it. The output waveform riding on -5V.

 

[Perry Babin]

I would suspect the TL072 on the driver board.

 

[brandes.cm]

Replaced the TL072, all gate signals are pulled to the negative rail on power up.

 

[brandes.cm]

Disabling the protection circuit, the fets in the upper right get warm.
Q38,12,11,10

An externally hosted image should be here but it was not working when we last tested it.

 

[Perry Babin]

Changing the op-amp should not have done that. If the op-amp was not defective, you should have seen the same problem, not a completely new problem. Are you sure that you don't have a bad solder connection or a solder bridge?

 

[brandes.cm]

Back to the original question, voltage on the 6.8K. Reads 0.00 towards the driver board and .25Vdc on the opposite end.

I swapped the driver board with another amp I have here. With protection disabled I see the lower part of the sine clip before the upper. I also noticed U16 (TL072) pin1 has the signal riding on +5Vdc.

The +-15V supplies read +15.06 and -14.67V.

 

[Perry Babin]

For my reference: cr3kwp

What is the DC voltage on pins 2 and 3 of U16 when there is 5v on pin 1?

 

[brandes.cm]

Each pin measures 2.5V and climbs to 2.7V

 

[Perry Babin]

What do you have on U17 pin 7?

 

[brandes.cm]

Pin 7 11.78V
pin 6 roughly the same
pin 5 -.125V

 

[Perry Babin]

Replace it.

 

[brandes.cm]

OK, so what did you reference for this amp? It stays out of protection now.

Much appreciated