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Old 14th January 2009, 01:46 AM   #1
diyAudio Member
Join Date: Oct 2007
Default Rockford Fosgate 501s power supply

Hi all,
Need a little help with an older RF 501S amp. I got it with all 4 of the IRF3205 fets cooked. I also found one channel with bad fets so I replaced them all. IRF9540 and IRF540's.

I also thought the TL494 PWM might have gone when the PS fets did so I replaced that too.

All that done and I try to bring it up slowly on my bench PS and the transformer starts to whine. I can see the amp starting to draw a lot of current so I shut it down.

I checked the LM317,LM337 and the 1620 and 1620R also.

Any ideas on how to approach this problem? I can't bring it up to 12v to troubleshoot the PS and I can't find any other blown components. Any chance RF will give me a schematic?

Is there any way to seperate the PS from the amp to see if it will start by itself?

Thanks in advance for the help...
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Old 14th January 2009, 02:30 AM   #2
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Join Date: Nov 2003
Location: Louisiana
When the amp tries to power up, measure the DC voltage across the 0.1 ohm source resistors to see if excessive current is flowing through the output transistors. You should read something very near 0.000v DC. Check the voltage across ALL of the resistors. If one channel reads high, turn the bias down, if that doesn't help, there are probably drive circuit problems. Check the gate resistors for the FETs. Also check the 20 ohm driver resistors and the driver transistors marked 1a and 2a.

If there is no excessive current flowing through the outputs...
On most of the rockford amps, you can connect only the remote and the ground to check the power supply gate waveform (check it on the gate leg of the FET) to determine if the drivers are working properly. If you have a scope, make sure the voltage is going all the way to ground. If you only have a multimeter, you should read ~5v DC (approximately the same as you'd read on pins 9 and 10 of the 494).

You can desolder the center leg of the rectifiers to isolate the audio section.

Rockford will send you the schematic. I have the 551S and the 801S. Email me and I'll send them to you.
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Old 14th January 2009, 08:07 PM   #3
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Join Date: Mar 2007
Did you use a meter and check for shorted output transistors?
If it ain't broke Don't fix it
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Old 15th January 2009, 02:57 AM   #4
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Join Date: Mar 2007

I guess if I had READ your post, I would have noticed you changed the defective outputs.
If it ain't broke Don't fix it
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