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Rockford Fosgate 400.4 hums
Rockford Fosgate 400.4 hums
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Old 2nd November 2010, 02:28 PM   #1
Dr Zeus is offline Dr Zeus  United States
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Default Rockford Fosgate 400.4 hums

This amp seems to be in fair condition for the most part. Its a Rochford Punch 400.4, light grey in color built with what looks like a cast/aluminum heat sync.

Click the image to open in full size.

This amp has two notable problems.

1 - one out of the 4 channels was playing very dirty and I found a transistor way out of tollerance - almost shorted with 50ohms from pin1 to pin 3 - so I clipped it off the PCB. The amp now powers on and is a little more stable for finding the remaining faults.

2 - The power supply seems to be supplying what I believe is osilation voltage throughout the rest of the amp. I'm not sure if it is normal, but heres a couple of pictures of the power fets across the OScope:

Scope shield at amp ground

Pin2: Scope set to 10v/div
Click the image to open in full size.

Pin3: Scope set to 10v/div
Click the image to open in full size.

All power fets test good. I tried removing 1/2 of them and then the other 1/2 and the problem still shows which means it likely is not the fets themselves. The image from Pin3 represents itself throughout the rest of the amp, even at speaker terminal outputs.

This is the second most powerfull amp I've had a chance to work with and I have to say I'd like to see it restored. This one is going to be donated to my Brother-in-law's audio project.

Last edited by Dr Zeus; 2nd November 2010 at 02:31 PM.
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Old 2nd November 2010, 03:45 PM   #2
Perry Babin is offline Perry Babin  United States
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It would appear that you have an open ground. With a multimeter, confirm that you have ~0 ohms between the scope ground and the ground terminal on the amp.

Is pin 2 the drain of the power supply FETs and 3 the source of the power supply FETs?
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Old 2nd November 2010, 10:38 PM   #3
Dr Zeus is offline Dr Zeus  United States
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Pin 2 looks like it 'drains' into the transformer coil. How do I tell if pin3 is the source?

Click the image to open in full size.

I'll take a look around for open grounds...
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Old 2nd November 2010, 10:46 PM   #4
Perry Babin is offline Perry Babin  United States
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Gate, drain, source are the names of the terminals of the FET (see example datasheet).

Use your multimeter to confirm that you read ~0 ohms between the scope and amp grounds.
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Old 2nd November 2010, 10:58 PM   #5
Dr Zeus is offline Dr Zeus  United States
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Scope is connected to amp's ground for all these images (0 ohms).

I'm sorry, I was incorrect with a previously posted image. This image represents what is coming out of PIN1 of the fets:

Click the image to open in full size.

And here is what is coming out of the speaker terminals. Setting the crossover cards to low-pass does not change the noise.

Click the image to open in full size.
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Old 2nd November 2010, 11:02 PM   #6
Dr Zeus is offline Dr Zeus  United States
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Grounding the RCA Shields changes this noise just slightly but no better.
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Old 2nd November 2010, 11:06 PM   #7
Perry Babin is offline Perry Babin  United States
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With no RCA cables plugged into the amp, measure the resistance between the shields of the RCA shields on the amp. Push back gently on the RCA jacks while doing this.
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Old 2nd November 2010, 11:13 PM   #8
Dr Zeus is offline Dr Zeus  United States
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This amp has 4 input RCAs, two for front (0 ohms between shields) and two for rear (0 ohms between shields).

If I measure from front to rear RCA shields I see 560 ohms.
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Old 2nd November 2010, 11:17 PM   #9
Dr Zeus is offline Dr Zeus  United States
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I still think you may be on to something ground related. Resistance between amp ground and RCA shields:

Front RCAs : 1.5k Ohms
Rear RCAs : 960 Ohms

Whats also interesting is there is a summation/out RCA. From the rear RCA input shields the sum-out measures 0 Ohms, but from the front RCA shields the summation measures 560 ohms
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Old 2nd November 2010, 11:24 PM   #10
Perry Babin is offline Perry Babin  United States
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Insert one RCA into a front channel and the other to a rear channel. Does that clean it up?

Is your signal source ground connected to the battery ground?
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