Rockford Fosgate Power 300 MOSFET with excessive current draw. - diyAudio
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Old 30th May 2013, 02:50 PM   #1
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Default Rockford Fosgate Power 300 MOSFET with excessive current draw.

Rockford Fosgate Power 300 MOSFET with excessive current draw.

I have an old Mosfet 300 that I am trying to figure out. I have a two ohm resister in series with my power supply. I installed this because it was drawing about 45 Amps so I immediately unhooked the remote wire it draws no current without the remote wire connected.
It looks as if maybe someone before me tried to fix the amp because the SMPS output mosfets look as if they have all been replaced the # on them is BUZ10
I have removed all the fuses it still is drawing excessive current about 7 amps with the 2 ohm resister in place.
I have tested every diode and transistors for shorts with my Simpson analog meter. The test were done while the part was in circuit and turned up nothing.
Anyone have any ideas of what to do next?
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Old 31st May 2013, 12:39 AM   #2
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Will someone at least let me know what the origonal power supply mosfets were so I can see if these BUZ10 are part of the problem. Is IRFZ44N a good replacement?
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Old 31st May 2013, 12:47 AM   #3
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Also I am wandering how many there are supposed to be. There are 6 on the board but there are two open slots where two more could go. Are there supposed to be 8 power supply mosfets?
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Old 31st May 2013, 12:57 AM   #4
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There are 8 p/s mosfets.

The 4 300's I have use BUZ11 in the power supply.

Sounds like the power supply is not working correctly, especially with the wrong parts and missing parts.

If you are sure there are no shorted outputs or drivers then you need to focus on getting the supply up and running.

Is the amp making rail and regulated voltage?

Is there any DC on the output bridging wires?

Have you tested the various op-amps especially in the feedback circuit to see if they are getting supply voltage or if they have any DC on their outputs?

Were any of the fuses blown when you removed them?

Are the bias pots turned fully counter clock wise?
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Old 1st June 2013, 05:52 PM   #5
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There are no shorted diodes or transistors anywhere on the amp.
There may be a leaky part somewhere I am overlooking.
There is no voltage when I measure between the negative rail fuse
And the positive rail fuses. Which normally would read about 45 or 50 volts?
With no Rail Voltage there is no DC on the output bridging wires.
There were no blown fuses when I got the amp but someone else has attempted repairing this amp they could have replaced one or two of them.
I am going to remove the Rectifier diodes that will totally isolate the SMPS and tell us if we are looking at a Leaky part in one of the four channels.
With the four rectifiers removed it is still drawing the same amount of current as it was with them in the circuit.
I am not sure how to check the ICs. I found a little info in Perry Babbins website about how to check the TL494. The comparator IC is a LM339 I pulled the data sheets but didn’t find a circuit that looked like the one in the amp. So I do not really know how to check the LM339.
Once we get the SMPS working I will set the bias pots
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Old 2nd June 2013, 07:42 PM   #6
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Do you think my problem may be that the gate resistors were never changed out. Do you normally have to lower the value of the gate resistors when you change the origonal SMPS outputs to the BUZ11s
If so what is the Ideal value to go with? I have some new 68 ohm resistors I could replace the 400 ohm resistors with.
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Old 4th June 2013, 02:43 PM   #7
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Check the SMPS switching frequency
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Old 6th June 2013, 06:16 PM   #8
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My scope went down it fell when moving it, I need another. I am working on checking the ICs now I will get back later,Thanks
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