My PPI PCX-4125 is different inside, and now makes a humming noise.

[Phoenix]

First of all I need to admit that I am very new to opening up amplifiers.

With that being said, on with my story. I had purchase a PPI PCX-1500 case, so that I could paint it to match my car, then I was going to transfer the insides of the 4125, and have a custom looking piece without ruining my original case. Since I do not plan ahead very well, I did not even thing about the board mounting screws being different, so after taking my 4125 apart, nothing would line up on the 1500 case. I decided to scrap the project and put my 4125 back together.

While pulling the board out, I noticed that the lack square things (MOSFETs I believe) had a white paste on them, and then were placed on a pink tape that was glued to the case. Here is where my ignorance comes in. Not knowing how a MOSFET works, I went out and got some conductive paste to place on them and put it all back together.

When I reinstalled the 4125 in the car, I could hear a "hum" coming from the speakers, in what I believe sounded like a ground loop. The amp never made this noise before my "surgery".

What kind of paste should I use?

Also, while comparing my guts to the ones on ampguts, I noticed that there are a metal basr that holds the MOSFETs in place, where mine does not have them. Is this an issue?

I'm attaching a picute of mine and a picture from ampguts to compare. Please forgive my crude MS Paint skills.

Click to read more...

 

[EriCCirE 10+ year member]

Those "metal bars" are to dissipate heat

 

[Phoenix]

Ok. So the humming sound I'm getting is not from possibly using the wrong paste. Sounds more like I have something still loose inside. But what?

 

[whitedragon551 5,000+ posts]

Why would you use conductive paste? That paste Im certain is thermal paste to keep the mosfets cool, not conductive paste to help them conduce.

 

[ciaonzo 5,000+ posts]

The metal bars are there to provide a degree of clamping force on the devices to ensure good thermal transfer to the heatsink. If you operated it without those bars in place there's a strong possibility that you've done damage to the amplifier. That damage could be responsible for the hum you hear through your speakers.

 

[WashYourWinky 10+ year member]

Any computer store should have thermal paste.

 

[Phoenix]

Why would you use conductive paste? That paste Im certain is thermal paste to keep the mosfets cool, not conductive paste to help them conduce.

Now that I know a little of what is going on I will change the paste to the correct kind.

The metal bars are there to provide a degree of clamping force on the devices to ensure good thermal transfer to the heatsink. If you operated it without those bars in place there's a strong possibility that you've done damage to the amplifier. That damage could be responsible for the hum you hear through your speakers.

Te back cover of the amplifier has some hold down tabs that line up with the little black boxes. I will search for the right paste and report back if that was the issue.

 

[ciaonzo 5,000+ posts]

The back cover of the amplifier has some hold down tabs that line up with the little black boxes. I will search for the right paste and report back if that was the issue.

Okay, you should be alright from that standpoint then. I don't think it would matter if you used thermally AND electrically conductive paste since you have the insulating pads under the devices. I'm not 100% on that one.
Perhaps it suffered some ESD damage while it was being handled/stored?

 

[Phoenix]

Okay, you should be alright from that standpoint then. I don't think it would matter if you used thermally AND electrically conductive paste since you have the insulating pads under the devices. I'm not 100% on that one.
Perhaps it suffered some ESD damage while it was being handled/stored?

Pardon my ignorance, but does ESD stand for "Electro Static Discharge"?

 

[Phoenix]

Figured out what the hum was caused by. I pulled out my head unit to install a back up camera, and I decided to try and isolate what RCA the hum was related to. Guess what, no more hum! I went over the front RCA outputs from the head unit and just unplugged them and plugged them back in, and now it's crystal clear!