Rockford Fosgate T4000 - Output Whine

[Jat40]

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?

 

[Jat40]

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.

 

[Jat40]

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?

 

[Perry Babin]

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?

 

[Jat40]

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

 

[Perry Babin]

That's insignificant. Set the scope to 2v/div and 2mS.

Does it whine with no RCAs plugged in?

 

[Jat40]

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?

 

[Perry Babin]

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?

 

[Jat40]

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.

 

[Perry Babin]

Double-check the terminals on the output filter capacitors. They may be broken.

 

[Jat40]

I pulled all of the filter caps including the rail caps, checked them all and did not find anything out of the ordinary. I even put them back in, in a random order. The amp is still outputting a squeal.

Going to check all of my grounds throughout to see if there is an open somewhere, like between the top/bottom of the circuit board.

 

[Jat40]

Something else that is odd, T2 makes a squealing noise the same as the output, when the amp initially turns on, just before the output kicks in.

 

[Perry Babin]

The output filter caps are the large brownish mylar capacitors near the output inductor (near the speaker terminals). Did you check those?

 

[Jat40]

Pulled those 3 caps, checked and resoldered back in. Nothing out of the norm.

 

[Perry Babin]

Two of those are directly connected across the speaker terminals.

Confirm that two of the capacitors have terminals that are directly connected to the positive speaker terminals.

For those two capacitors, confirm that their other two terminals are directly connected to the negative speaker terminals.

 

[Jat40]

Two of those are directly connected across the speaker terminals.

Confirm that two of the capacitors have terminals that are directly connected to the positive speaker terminals.

For those two capacitors, confirm that their other two terminals are directly connected to the negative speaker terminals.

Yes they are, C441 and C5.

 

[Perry Babin]

I've never seen those fail. If the 10uf cap across the output reduced the noise to an acceptable level, you could possibly replace those with two 10uf caps. If you do this you should test it thoroughly to make sure that there are no problems associated with the increased capacitance.

 

[Jat40]

I've never seen those fail. If the 10uf cap across the output reduced the noise to an acceptable level, you could possibly replace those with two 10uf caps. If you do this you should test it thoroughly to make sure that there are no problems associated with the increased capacitance.

Thanks for the help, Perry. I am going to just try some additional trouble shooting to see if I can find where this sqealing is coming from.

 

[Jat40]

Been a while on this one, but started going back through, and found Q311 measuring in the kilo ohms. Replaced and the whine went away. The amp is performing great.