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Got some water on the pass. side floorboard, water got into the PA300 amp down there and as I was making a turn out of the driveway, the radio bit the dust and I started hearing awful scratching noises from the speakers.

If you get water in the amp, first thing to do:
1. Open drivers side fusebox and pull the 30A "AMPLIFIER" fuse immediately. The amp is an always-on circuit so you do not want an a electrical fire starting because of some wayward moisture.
2. Pull back the floor and take care of all the water before it causes more damage. Remove the amp.


Now you need to open it and inspect the damage. It opens with a Torx bit.



You should remove the PCB and fully clean it to prevent corrosion. To do this:
3. Using a flat blade screwdriver, pry up on the heatsink clips. Careful about the thermal paste, it gets everywhere.
4. Inspect the power supply.

The PA300 (and probalby PA200) uses a dual-rail flyback SMPS to generate somewhere around +/-35V (I didn't measure). There are two switching mosfets and two rectifiers.
In the picture below you can see that one of the mosfets had completely blown out, releasing the wonderful odor of magic smoke in the case:





I'm not sure if the other mosfet survived, but it might've also been toasted.
5. You now need to remove all four TO-220 devices. The amp uses a 4layer PCB with 2oz copper so you will need a rather beefy iron to do this. I used a rework station.
Clean out the pin holes so you can insert a new part, use wick or heat the joint and blow a strong puff of air down the molten joint (Lol)




6. Now you should clean the pcb of all water residue. I used eyeglass cleaner, seemed to work better than 91% iso. Get in there with an old toothbrush and scrub it down. It should look brand new when you're done. Dry off with compressed air and set outside.

I also replaced the SG2525 SMPS controller just in case.


Ordering replacements.
The mosfets are originally Toshiba 2SK2312 parts. These are long EOL so I picked a suitable replacement after reversing the power supply (there is a similar schematic on the net).

Digikey part numbers:

497-2039-1-ND - smps controller (1)
568-4976-5-ND - mosfet (2)
568-3440-ND - rectifier diode (2)

Bend the package leads to match the old ones and install. Enjoy your music again.
 

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Thank you very much for this post, the advice and for the insights.
As someone that works on a different manufacturer's vehicles daily, I become more impressed with my Volvo the more I learn of it.
My car has a similar amplifier and the repairability and general sturdiness is better than what I'm accustomed to.
 
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