SKAR SK-2500 - Randomly protects on start-up

[Perry Babin]

Those look OK. With pin 1 that low, Q8 should be off and the output to pin 8 should be near 0v.

Post the DC voltage on all pins of Q4, Q8 and Q10. Place the black probe on one of the negative rail pins of the driver board.

 

[Perry Babin]

Pin 5 is the reference. The voltage you posted is about right.

 

[Troy Dionne]

Q4
PIN1 = 87.4V
PIN2 = 278mV (Climbs as im measuring)
PIN3 = 88.1V


Q8
PIN1 = .516V
PIN2 = 67.9V
PIN3 = 0.00V


Q10
PIN1 = 107.8V
PIN2 = 300mV (climbs as Im reading)
PIN3 = 108.4V

 

[Perry Babin]

When in protect mode, Q10's collector drives pin 8 of the driver board high through R31 and D6. The collector of Q10 is low so that's not driving pin 8 high. Is it possible that something else on the main board is driving pin 8 of the audio driver board high?

 

[Troy Dionne]

The 13V I am seeing on PIN8 of the board is referenced to main ground of the amplifier.

When referencing to main ground I get these readings on Q10

PIN1 = 14.20V
PIN2 = 12.64V (Same as PIN8 of driver board)
PIN3 = 14.82V

 

[Troy Dionne]

There is absolutely no other connections to PIN 8 of the driver board on the main board other than c201

 

[Perry Babin]

OK.

The voltages on Q8 don't look quite right. Can you easily remove it and re-connect pin 8 to see if the amp will go into protect?

 

[Troy Dionne]

With Q8 removed the amp protects every time

 

[Perry Babin]

Check it while it's out to see if it's leaky.

Have the voltages on Q10 changed?

 

[Troy Dionne]

Ok Hopefully these numbers make sense to you

driver board re-connected to PS driver board, Q8 removed

referenced to -rail Q10 reads

PIN1 = .811V
PIN2 = .760V
PIN3 = .634V

PIN8 of driver board = 3.2V

--------------------------------------------------------

Referenced to main ground Q10 reads

PIN1 = -32mV
PIN2 = 0.0V
PIN3 = 32mV

PIN 8 of audio driver board = 3.2V

 

[Troy Dionne]

Q8 measures normally out of board

 

[Perry Babin]

It looks like the amp has shut down. Referenced to ground, the emitter of Q10 should have about 15v.

 

[Troy Dionne]

It was in protect mode

 

[Perry Babin]

You'll have to leave the connection to the PS board open. How are you breaking the connection?

 

[Troy Dionne]

Oh ok. I made a small razor cut in the trace. Easy to jump with solder

 

[Troy Dionne]

ok with the connection to PS board open

Referenced to ground

PIN1 = 14.17V
PIN2 = 14.77V
PIN3 = 14.82V

PIN 8 of driver board = 14.48V

--------------------------------------------

Referenced to -rail

PIN1 = 108.5V
PIN2= 108.9V
PIN3 = 108.8V

PIN8 of driver board = 108.1V

 

[Troy Dionne]

I replaced Q8 with a new one and so far I have not been able to get it to go into protect. The trace is re-connected to the PS board. So it may have been leaky after all, maybe intermittently, or possibly a solder crack? I will continue to test it and keep trying.

 

[Troy Dionne]

It has not gone into protect since I replaced Q8. Does that seem likely that it was leaky?

I think it's all set. Just want to say thanks again Perry! Always a huge help.

 

[Perry Babin]

The only way to know definitively if it was the problem is to reinstall it. That's a bit much for most people but I've done it quite a few times when I wanted to be 100% sure I'd found the problem.

I'm still puzzled as to why it would go into protect without it. I guess I need to study the circuit a bit more.