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Old 22nd June 2006, 04:04 PM   #1
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Question Need Help: RF 500S amp with tonns of DC in output

I have an old Rockford Fosgate Punch 500S 2-channel amp, which I really want to return to life. When I first got it, it had all, except 3 FETS shorted in power suply and outputs. Replaced all of them, now power supply is working but output FETS always open, no matter what I tried to do - around 50V in each channel It's suspicios, that both channels are showing exact same behavior.
Gate resistors are fine, couple of drivers (I am gonna cry, they are SMD's ) were dead, subbed them.
I am desperately need some help here. Thank you!
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Old 22nd June 2006, 06:42 PM   #2
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Default Re: Need Help: RF 500S amp with tonns of DC in output

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Originally posted by valterdaw
I have an old Rockford Fosgate Punch 500S 2-channel amp, which I really want to return to life. When I first got it, it had all, except 3 FETS shorted in power suply and outputs. Replaced all of them, now power supply is working but output FETS always open, no matter what I tried to do - around 50V in each channel It's suspicios, that both channels are showing exact same behavior.
Gate resistors are fine, couple of drivers (I am gonna cry, they are SMD's ) were dead, subbed them.
I am desperately need some help here. Thank you!
Sounds suspicious. If both channels are outputing DC, it may be that a OPAMP in the preamp may be bad. A dual channel OP that is used for both channels might do that. I would start there.
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Old 22nd June 2006, 07:46 PM   #3
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Default Re: Re: Need Help: RF 500S amp with tonns of DC in output

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Originally posted by TO-3


Sounds suspicious. If both channels are outputing DC, it may be that a OPAMP in the preamp may be bad. A dual channel OP that is used for both channels might do that. I would start there.

Thanks for the suggestion, I'll try tracing leads and pulling some OP's legs. They are all SMD's At least, If everything fails, I can rip out power supply an use it somewhere else
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Old 22nd June 2006, 07:49 PM   #4
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Default Re: Re: Re: Need Help: RF 500S amp with tonns of DC in output

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Originally posted by valterdaw



Thanks for the suggestion, I'll try tracing leads and pulling some OP's legs. They are all SMD's At least, If everything fails, I can rip out power supply an use it somewhere else
They are not that bad. Add a little solder and then with a traingular Xacto Knife blade heat the leads up and use the Xacto to carefully pry. I do it all of the time.
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Old 22nd June 2006, 08:55 PM   #5
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If the output has plus OR minus on BOTH channels, it's not likely in the preamp section. Rockford inverts the right channel by feeding the signal to the inverting input of the differential amp of the power amplifier section. If a preamp level glitch was sending the same errant signal (i.e. positive OR negative DC) to the amplifier sections, the output on one channel would swing to the positive rail while the other channel would swing to the negative rail.

Are you sure that you have both positive AND negative rail voltage to the outputs?

Are the 20 ohm driver resistors OK?

If any one of the 20 ohm resistors is open, you likely have one or more blown driver transistors.
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Old 23rd June 2006, 03:35 AM   #6
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Fixed! You guys are the best!!! And I am still learning. I went through all SMD transistors, found 2 bad ones; one 20 ohm resistor was bad too (but it looks like new lol). I'll put it in my car tommorow morning, and test it hard on a way to work and back. Thanks a lot!
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