Alpine MRP-M500 Mono amp not working

[carlosg815]

I have had this amp since about Feb of 2009. I have never had a problem with it since I got it.

I put my car away for the winter and just yesterday brought it back out.

It would work intermittently. The sub would turn off, and then after a while it would turn back on.

Well last night it shut off and never came back on.

I checked my wiring and it all seems to be ok. I unbolted the ground and took the wire brush to the chassis where it is grounded just to rule out that the ground isn't any good.

The power light on the amp stays lit up blue to show that it is on.

With the multimeter I verified that the remote wire is getting 12v, but when I put the meter on the 12v and ground ports on the amp with it connected, it only reads about 4 volts.

When I disconnect the power wire from the amp, I put the meter on it and the ground and I get 12+ volts. This is strange to me.

I have tried everything I can think of in that area and I can not get the amp to kick out any power. I have even tried another speaker just to rule out the sub.

I want to try hooking up another amp to rule out a problem with the wiring but until I can get my hands on one I want to continue troubleshooting.

I pulled the amp apart and put the ohm meter on the mosfets and I am getting a great amount of resistance from all but 2, but I believe those are supposed to be 0.

The two that read 0 ohms are labeled RF2001 and FRH20A15.

The 4 that read off the charts are RF3415, so I'm assuming those are all good.

Any help is much appreciated.

The amp is powering a dual 4ohm Alpine Type R 12" subwoofer.

[DREW028]

You won't get an accurate reading while the transistors are installed and frh20a15 is not a mosfet.

Check out this page from on of them members here. It will help you understand a lot Basic Car Audio Electronics

[Perry Babin]

That's generally caused by excessive resistance. I'm assuming that the battery voltage (at the battery) remains near 12v when it drops to 4v at the amp.

It sounds like you have a defective fuse at the battery. This is more likely if you have an AGU fuse in a waterproof holder.

It could also be a bad connection somewhere in the power line.

[carlosg815]

Originally Posted by Perry Babin

That's generally caused by excessive resistance. I'm assuming that the battery voltage (at the battery) remains near 12v when it drops to 4v at the amp.

It sounds like you have a defective fuse at the battery. This is more likely if you have an AGU fuse in a waterproof holder.

It could also be a bad connection somewhere in the power line.

I have a spare fuse, I will replace it today and see if that helps.

[carlosg815]

Originally Posted by DREW028

You won't get an accurate reading while the transistors are installed and frh20a15 is not a mosfet.

Check out this page from on of them members here. It will help you understand a lot Basic Car Audio Electronics

Ok. I just learned what a mosfet is today so when I saw the frh20a15 I assumed it was cause it had the three prongs.

[carlosg815]

Originally Posted by Perry Babin

That's generally caused by excessive resistance. I'm assuming that the battery voltage (at the battery) remains near 12v when it drops to 4v at the amp.

It sounds like you have a defective fuse at the battery. This is more likely if you have an AGU fuse in a waterproof holder.

It could also be a bad connection somewhere in the power line.

Perry,

you were correct, it was a bad fuse. I was getting 4-6v at the amp when I connected it. So under the hood I was getting 12v from the battery, but not on the other side of the fuse.

The fuse is in a waterproof holder. When I took it out, I noticed that it was not blown but it was dark on the battery side and there was some "honey" like residue on the glass. I replaced it with a new fuse and the amp sounds great.

Why is it more likely to go bad in a waterproof case? What should I be running to avoid this?

[Perry Babin]

The AGU fuses were never designed (by the original manufacturers (Bussmann and Littelfuse) to go above 60 amps. The design is not suitable for that much current.

The enclosed fuse holders won't allow the fuse to dissipate heat. That with poor quality fuse holders combine to make them unreliable.

This is covered on the 'fuses' page of the site.
http://www.bcae1.com/fuses.htm

[carlosg815]

Originally Posted by Perry Babin

The AGU fuses were never designed (by the original manufacturers (Bussmann and Littelfuse) to go above 60 amps. The design is not suitable for that much current.

The enclosed fuse holders won't allow the fuse to dissipate heat. That with poor quality fuse holders combine to make them unreliable.

This is covered on the 'fuses' page of the site.
http://www.bcae1.com/fuses.htm

I will replace the fuse holder if need be. In looking at the link you provided, I see that you recommend a 125 amp fuse for my wire. I am only running a 60 amp fuse.

Should I get the larger fuse?

[Perry Babin]

No if the 60 amp fuse hasn't been blowing (in the center of the fusible element). The recommendations are for the maximum fuse rating. It's OK to use a smaller fuse if that's all you need.