Same outputs shorting each time

[RJohns777]

Hi, working on a Sound Stream PIC 1.1700D amp. Can someone shed some light on why a class D amp will short the same outputs each time? Gate resistors and Diodes check fine. Driver board checks fine. It shorts the same two outputs. One on the positive side and one on the negative. Thanks.

 

[nigelwright7557]

I have used the irs2092 quite a few times in class d designs.
I usually find if the mosfets go then the 2092 is taken out too.
Not sure how much this relates to your amp.

 

[Perry Babin]

Photo of the main board and another of the audio output section?

 

[RJohns777]

Here's the pics.

 

[Perry Babin]

The inductor looks like it may be shorted. If not, it soon will be (right arrow).

What's the light-colored spot indicated by the left arrow?

Have you desoldered the diodes that are in parallel with the gate resistors to confirm that they read similar forward voltage?

 

[RJohns777]

Yes I looked at that. They are not touching. I can shine a light between them. I'll insulate it anyway to make sure it doesn't short in the future.

The bright spot looked like white paint. Cleaned off easily with solvent and brush. No burned area around it. Doesn't appear to be rubbing away the insulation on the wire.

Checked the diodes out of the circuit. They read equally with all the other diodes.

Curious why the same two outputs short.

 

[RJohns777]

By the way, thanks for the response Nigel.

 

[Perry Babin]

Try moving one that has not blown to the bad location. Does that same location blow again?

Are the photos current or from a previous time? Don't know what you can check without power supply FETs.

 

[nigelwright7557]

Has the output filter capacitor failed shorted ?

 

[RJohns777]

"Has the output filter capacitor failed shorted ?" No, output filter caps check fine.

I was trying to get an idea before rebuilding the power supply again. Decided to go ahead and put the power supply FETs in anyway. Now what I am getting is low voltage DC on the speaker positive and a loud high pitch squeal in the output. I haven't put the two shorted outputs back in yet.

 

[RJohns777]

Gonna pull all the outputs so I can check the preamp.

 

[Perry Babin]

Disable the output stage. That will allow you to confirm that there are no other problems.

 

[RJohns777]

I don't think the driver board is causing the squeal. Must be something in the preamp.

 

[Perry Babin]

If you disable the driver board, you can check the preamp, regulators... Virtually everything with no chance of damaging the outputs.

 

[nigelwright7557]

The irs2092 is a bit odd if there is no speaker connected.
It powers up in one of two states, either oscillated or the output sits at +17VDC.

Also the output filter inductor will ring with high volts if no speaker is connected.
This can sometimes take output filter capacitor.

 

[RJohns777]

How do you disable the driver board? Pull it?

 

[RJohns777]

"The irs2092 is a bit odd if there is no speaker connected.
It powers up in one of two states, either oscillated or the output sits at +17VDC"

There aren't any IRS2092s in this amp. The outputs are IRFB31N20Ds.

 

[Perry Babin]

I don't know if your board differs fro other boards so you need to confirm a few things.

For IC304, confirm that pin 5 goes to the negative speaker terminal. Confirm that pin 6 goes to pin of the driver board (SW pin). If those are correct, solder a bridge between pins 3 and 4.

 

[RJohns777]

Thanks Perry, I will give that a look. Right now I have dinner plans with the wife and kids. I will continue on this tomorrow.

 

[RJohns777]

I haven't had time to get back into this amp. Trying to repair my scope. Perry have you worked on the high voltage power supply board on a sencore sc61?