Rockford Fosgate Power 250.2

[seankane]

Hello everyone.

I have a Power 250.2 chrome series board number PC-0767-J.
I recieved the amp in to repair a terminal. I neglected to test the amp before service. I removed one of the negative p/s terminals and replaced it with a new one. Upon powering up the amp it went to full current draw and was shorted. I removed the amp again from the sink thinking the old tape Rockford had used might have let one of the output fets drain to the sink.

I replaced the tape under the fets and re-powered with same result. I replaced all the output fets and still had same result. Upon testing I did find a few fets in one channel shorted. The amp powers up fine out of the sink. I put it back in the sink and tried it again and it powered up fine and played about 4 songs running hard and went to full short again.

I turned the amp off let it sit awhile and same thing. Played fine for a few songs then to short. When you remove the board screws that ground it to the sink by the power supply they are arcing and sparking.

I touched my black probe to p/s ground and red probe to amp sink and bus bar and the amp case and bus bars are charged to 45.50 vdc. Could something be touching the sink somewhere? I dont know why the amp heat sink and rail bus bars have 45.50 vdc on them. I have never seen this before.

Any advice would be great, because I am thinking the amp is fine until something may be touching the sink but not sure.

[Perry Babin]

Can you post a photo of the inside of the amp?

[seankane]

Sure it looks to be the same board as the old DSM Power 250x2 looks like Rockford may have stuffed some left over boards in to some newer sinks, they were famous for that.

[Perry Babin]

250M2

What is the DC voltage between the center legs of the rectifiers on either side of the amp?

[seankane]

Both sides read the same at 85.0 vdc.

[Perry Babin]

It looks like you have one rail shorted to the heatsink. Check the tabs of the transistors for burrs.

If the amp is currently clamped to the sink, measure the resistance from the heatsink to every center leg/tab of every component clamped to the heatsink to see if any show a direct connection.

[seankane]

Q315, Q313, Q311 all IRF540's have 0.006 ohms in one channel everything else reads OL. So those 3 fets are some how making contact with the sink?

[Perry Babin]

It would appear that at least one of the 3 is shorted to the sink. Unclamp them and push them down one at a time to see which one of the 3 causes the short.

Also, there was a problem with the legs shorting to the sink in a few amps, if the tabs aren't shorting, check the sink under the legs of those transistors to see if something could be shorting there.

[seankane]

Upon removing the amp from the sink I found a burn mark underneath one of the MUR1620 rectifiers and a rough spot on the rectifier where it had arced since in a pair they produce around 85 vdc could one shorting to the sink give me the 45 vdc?

[seankane]

Putting the amp back in originally with the same tape probably allowed the rectifier to puncture contacting the sink and making the burn mark on the bottom of the rectifier. Then putting in the new tape with the rough rectifier allowed the rectifier to puncture the new tape?