Rockford Fosgate T4000 - Output Whine - diyAudio
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Old 4th November 2010, 10:00 PM   #1
Jat40 is offline Jat40  United States
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Default Rockford Fosgate T4000 - Output Whine

I just repaired this T4000, replaced all of the power supply FETs, drivers, and 10 FETS in the output. The amp is running, plenty of power but have a whine. Sounds like it is coming from 1 of the coils in the power supply.

On the bench with no input, I can hear the whine and when connecting a speaker, the whine can be heard at the speaker. The whine is constant and does not change even when an adjustment is made to the amp. Regular audio comes through but the whine is still there and does not change with volume adjustment.

Anyone have this problem with an RF in the past?
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Old 5th November 2010, 10:15 PM   #2
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Going to go ahead and replace U2. Seems that the squilling is coming out of the IC. Not getting a clean 25KHZ square wave on PIN 13. Also, when I touch certain pins, the squilling changes in amplitude and frequency.
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Old 20th November 2010, 08:14 PM   #3
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OK. I replaced the PWM controller and that did not make any difference. One note I should point out, is when I got this amp, it was missing the MEHSA on the power supply. So, I soldered the FETs into place and used an insulator pad between them and the amp. I am using small pieces of aluminum to mount them tightly to the heat sink.

The amp is running, voltages are OK, nothing is getting hot out of the ordinary, but there is an obvious squeal on the output. Any suggestion? Would not using the MEHSA have anything to do with this problem?
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Old 20th November 2010, 08:54 PM   #4
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As long as the insulators are doing their job, they shouldn't be causing this problem.

Some of the MEHSA insulators also have small surface mount thermistors. They are connected to the main board via the 3-terminal jumpers. I don't think this could cause the whining but it could mean that there is no thermal protection for the power supply.

Do you have a scope? If so, post a photo of the output waveform with the noise.

Are you sure that the output filter caps don't have broken leads on them?
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Old 21st November 2010, 02:03 PM   #5
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Here is the pic of the amp and the scope output. There is no input being applied and the scope is tied directly to the output terminals
Attached Images
File Type: jpg DSCN0122.JPG (301.9 KB, 81 views)
File Type: jpg DSCN0123.JPG (251.8 KB, 82 views)
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Old 21st November 2010, 06:52 PM   #6
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That's insignificant. Set the scope to 2v/div and 2mS.

Does it whine with no RCAs plugged in?
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Old 22nd November 2010, 03:12 PM   #7
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Here are a couple more pics, showing the tone with the above settings. Can't really see anything with a 2V setting.

Here is a youtube clip showing the tone coming from the speaker.

YouTube - DSCN0141


This is all with no input. When I input a tone, like 50HZ, the amp produces a nice output but the sqealing is still there. Something else, when I press done on the filter caps, the sqealing changes in freq.

Could there be a leaky filter cap that is causing the trouble?
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File Type: jpg DSCN0139.JPG (251.2 KB, 72 views)
File Type: jpg DSCN0140.JPG (253.7 KB, 69 views)
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Old 22nd November 2010, 05:52 PM   #8
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Do any of the controls on the amp make a difference in the sound?

Do you see noise on any power supply rail that corresponds to the noise at the output?

If you parallel a 10uf cap across the speaker terminals (no audio), does that eliminate/reduce the noise?
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Old 22nd November 2010, 07:46 PM   #9
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Do any of the controls on the amp make a difference in the sound?
No. The sqealing remains the same even when the controls are adjusted.

Do you see noise on any power supply rail that corresponds to the noise at the output?
YES

If you parallel a 10uf cap across the speaker terminals (no audio), does that eliminate/reduce the noise?
Yes, it is reduced quite a bit.
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Old 22nd November 2010, 07:55 PM   #10
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Double-check the terminals on the output filter capacitors. They may be broken.
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