Rockford Power 550x bias issue - diyAudio
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Old 24th November 2009, 02:17 AM   #1
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Default Rockford Power 550x bias issue

Background on this amp, both output fets were blown Q310, Q311, one gate resistor and the .1 ohm. Replaced those parts and got my output signal on the terminals. I then try to adjust the bias, measuring across my .005ohm sense resistor in the power supply line. Watch for the mV's to change and then turn the bias pot back some. This channel the bias pot won't change the current draw of the amp unless I lift D301. That leads to the protection circuit, just for giggles replaced the 3 transistors in there. That didn't help.
R351 (20ohm) then smokes, maybe had the bias pot to far one way.

I get a nice waveform out, but can't move the bias setting. Any insight would be helpful.

thanks
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Old 24th November 2009, 04:26 AM   #2
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Are R349, R350, R351, R365 and R366 within tolerance? You'll have to desolder R349 and R366 to get accurate readings on the other resistors.

If R351 smokes, it would seem that R349 is well out of tolerance.

According to the schematic, R351 is a 39 ohm resistor.
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Old 24th November 2009, 11:42 AM   #3
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R349 was the open, I checked it in circuit and it read fine.

I'm using schematic H400a4, this seems to follow along with the values I see on the board. According to rockford this is the one I want. I will gladly pass it along your way if you want it.

Two more problems to go with this amp, I will be back with more questions.

Thanks again.
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Old 24th November 2009, 12:00 PM   #4
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I have a schematic that's labeled as 550x (by Rockford) and it's significantly different. The model on the schematic is 600a4.

I have the H400a4.

Did you confirm that you have a good connection to the traces on the board for R349. If R351 is overheating, it means that there's excessive voltage across it. If you have too much voltage across R351, it would mean that there's too much resistance between the rail supply and the FET. The only ways I can see that you can have that is a bad connection between the rail and the FET or R349 being out of tolerance. Since you didn't mention it, I'm assuming that there is no excessive draw.

Measure the voltage with one probe on the rail and the other on the source of the FET (not on the resistor's terminals) to see if there is excessive voltage drop across those points.
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Old 24th November 2009, 01:55 PM   #5
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Is there a Punch and Power version of this amp?

Perry, R349 was bad, reading open. I know it measured fine before, either way I replaced it and the bias is now adjusting fine.

This amp I believe was wet at some point. All signs on the PCB seem to show that. D3 in the power supply was fried and the trace leading to it was burned off the board. Wet failures can be a bear to fix, I have plenty of time though.

Any thoughts on why D3 would go bad?
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Old 24th November 2009, 02:13 PM   #6
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D3 likely failed because a power source (12v) made contact with the RCA shields.
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