Repair Car audio amp, Distorted Output, Squealling

[denimdragon]

Hi everyone,
Ok so this is my first project. I have a Rockford Fosgate Bd500.1 car amp that I bought dirt cheap. It does work when I hook it up in my little ford ranger so I decided it would be a good project. Here are the details about it:

1. As I said, when connect in the vehicle @ 12.6V it in comes on with the protection light on and of course no output.

2. When connect to my DC power supply, it turns on without the protection light at 8.4V and .24-27a and outputs the music, but it's distorted badly. When I turn the voltage up it jumps to .48-.70ish amps and then the protection light comes on, music stops. Then it emits a squeal

The squeal is definitely coming from the power area and it looks like it could be one of the inductors. I know mosfets go bad on amps, but after doing the DMM on off testing, they "seemed" ok. (I still want to check them another way. Right now I'm testing them on the board. Any imput on a solid way to test them without taking them out would be great). I have taken some pictures of the amps board so you guys can have a look. I'm also posting a youtube video so that you can see what's going on. Once again thanks for helping the rookie 

I hope I'm doing this right but here are the youtube videos. You might turn the volume down, the squeal is rough:

https://youtu.be/JpGJ53NwCPw


https://youtu.be/EILWGfiFEP8

[theatrus]

The 500.1 looks like it's a subwoofer monoblock amp? It will probably sound pretty rubbish driving speakers anyway . It claims it's "class BD" which is some sort of hybrid. The FETs connected to the large toroid will make up the class D stage (the toroid is a big filter to remove the switching noise).

I'm not an amplifier repair person - have you probed all the power rails? What tools do you have at your disposal?

[denimdragon]

Hi theatrus, Yes it's a subwoofer amp. So that's what that does (toroid). Now it makes sense why people complain about this specific line of Rockford amp making squealing noises and you have to wiggle and push it to get it to stop LOL. I wish that was the only problem with it.

I have a DMM, DC adj. power supply, o-scope, and 2 function generators. I'm not opposed to building a test circuit if need be. It's helps me learn.

[Trooper_Dan]

Your amp needs to be repaired 8.4v it's to low voltage that y ur protection circuit is being bypass. Protection means u have a bad output so it's to protect the speakers and the amp frm damage. To test your outputs and power supply desolder a pin frm each mosfet any check and see if u get between 0.5 - 0.8 disconnect the output  transistors first the squealing should stop that's pressure on the pwr supply

Sent from my SM-N920T using Tapatalk

[denimdragon]

Hi Trooper_Dan,

Yes I'm aware of the required voltage to drive the amp. The 8.4V was initiated using a my benchtop DC power supply for testing at what point the protection light comes on, on the amp. That's at anything above .28a

Now, can you tell me where the output transistor is? I don't have a schematic for this amp so I have to fiddle around to make myself familiar with it. Thanks for the help and the advice my friend. It is greatly appreciated.

[theatrus]

The output devices would be on the top and bottom rails of the amp. The switcher section also has a few diodes.

[denimdragon]

The output devices would be on the top and bottom rails of the amp. The switcher section also has a few diodes.

I managed to get it going! It was the tl072 OPamps, specifically the one by the Torrid and power rails. I changed them all out and now no issues! Thanks again for the help.