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Old 1st August 2012, 03:39 AM   #1
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Default Rockford Fosgate T20001BD

With no outputs populated I see the signal below on pin 2 of U300. I have a nice sine wave on R300 (C355 side). This is only with the gain pot turned up some. Is this correct, signal doesn't change if I remove D311.
Click the image to open in full size.

One other note, U401 doesn't output anything unless I have a signal being driven in. I assume this is because of the protection circuit.

What should the gate signals look like?
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Last edited by brandes.cm; 1st August 2012 at 03:49 AM.
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Old 1st August 2012, 07:54 AM   #2
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Look for the terminals marked 'carrier' on the schematic diagram. You should see a triangle waveform there. If the other terminal of the same op-amp is near 0v DC, you should see a square wave on the output of the op-amp. If the second terminal of the op-amp isn't near 0v, try shorting the positive speaker terminal to the negative speaker terminal to see if that brings it to near 0v. This is for pin 7 or U301 and U302.

On pin 1 of U300, you should have clean audio. If you don't, you need to follow the signal through toe preamp circuit to see where it becomes distorted.
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Old 1st August 2012, 11:23 AM   #3
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Quote:
Originally Posted by Perry Babin View Post
Look for the terminals marked 'carrier' on the schematic diagram. You should see a triangle waveform there. If the other terminal of the same op-amp is near 0v DC, you should see a square wave on the output of the op-amp. If the second terminal of the op-amp isn't near 0v, try shorting the positive speaker terminal to the negative speaker terminal to see if that brings it to near 0v. This is for pin 7 or U301 and U302.

On pin 1 of U300, you should have clean audio. If you don't, you need to follow the signal through toe preamp circuit to see where it becomes distorted.
The carrier is there, the other pin isn't near zero. I will try shorting the speaker terminals.

The audio fails when it gets to pin 2 of U300, good up to that point. Maybe the opamp is bad. I will lift the input pin.
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Old 1st August 2012, 10:56 PM   #4
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Lifted pin 1 of U300 and the signal looks good on pin 2. I had R303 removed at that point as well. I replaced U300 expecting it to fix the issue, wrong same garbage.

I do have square waves on the outputs of U301 and U302, the pulse width varies with gain applied.
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Old 2nd August 2012, 04:11 AM   #5
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Click the image to open in full size.

That is pin 5 of U300, is this OK. It's ~7vp-p with 40hz input.
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Old 2nd August 2012, 05:15 AM   #6
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Everything looks good to this point. Pin 7 should have the same signal. This is the last point in the circuit that you can trace audio until you get beyond the output filter inductor.
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Old 2nd August 2012, 07:01 AM   #7
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I have the scope set for differential measurement to check the gate signal, since gnd is nonexistent.
Click the image to open in full size.

That is what I see on the one pair and invert it for the other pair. I only have 4 fets populated right now. All driven from U401. Once gain is increased the power supply starts to hum. Do I need to populate all the outputs at this point or remove what I have placed?
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Old 2nd August 2012, 08:44 AM   #8
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It can't produce audio with out at least one transistor in each section of the output circuit. There are 4 sections.

I don't know what you're expecting to see with only some of the outputs in the circuit.
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Old 2nd August 2012, 09:08 AM   #9
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Originally Posted by Perry Babin View Post
It can't produce audio with out at least one transistor in each section of the output circuit. There are 4 sections.

I don't know what you're expecting to see with only some of the outputs in the circuit.
My main concern is the hum from the supply and wondering if the gate signal I'm showing seems OK. I wouldn't expect the supply to hum without a load on the output.
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Old 3rd August 2012, 05:30 AM   #10
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It's unusual for a switching power supply to hum. Humming is generally a low frequency and switching power supplies operate at much higher frequencies. It could be that the hum is in some way related to the input frequency.
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