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a/d/s/ P840 Repair Help
a/d/s/ P840 Repair Help
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Old 6th June 2006, 06:32 AM   #1
jvr826 is offline jvr826  United States
diyAudio Member
Join Date: Jun 2006
Default a/d/s/ P840 Repair Help

I'm a newly registered user of the site and hope I can gain some knowledge from the experts here.

I have an a/d/s/ P840 circa 1999, pre-DEI. It has been a great amp and installed in three different cars. The most recent install was under the floor of my trunk in the spare tire well along with the crossovers for my speakers. Well, this decision has proven to be a bad one for long trips in 95+ degree heat.

Last Friday I was on a 5 hour trip enjoying the tunes and the amp cut out suddenly. I figured it overheated and shut down, so I let it cool down for a couple of hours and tried it again - nothing. I pulled the on board 40A fuse and it was blown, so I replaced it and tried again. It blew immediately.

I pulled the cover off and inspected the board to find this:

Click the image to open in full size.

It looks fried to me.

I've had the board out of this thing several times. Mostly to repair the +12v connection post. The damn thing has broken off many times since I've had the amp. I just fixed it again a week ago prior to installing in the new car.

Click the image to open in full size.

And, lucky for me, when I pulled the cover off tonight to snap these pictures, one of the 2 position switches broke off, and another is loose.

Click the image to open in full size.

So, three problems:

1) There is an apparent short due to overheating that is blowing the fuse. I believe it's the larger of the two transformers. How can I diagnose? How can I identify the part and get a new one?

2) The power connection post to the board is sketchy yet again. I have a hard time getting solder to stick to it, especially at the top. I was able to do a fairly decent job from underneath.

3) I have two switches that need replacement. Where can I buy them?

Thanks in advance folks. I'm no PCB specialist, but I'm also not afraid to tackle these repairs myself, if they are as simple as desoldering and replacing the said parts.
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Old 6th June 2006, 06:54 AM   #2
Whortless is offline Whortless  Turkey
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Join Date: Nov 2005
Location: Kocaeli,Turkey
Hi JVR826,

The problem probably comes from PSU of your car amp. You should check the switching elements. It is easy to start with switching(SW) mosfet (IRFIZ44? or any other). I do not know the amplifier nor the shematic.But the blowing fuses says you have a short circuit on the PSU. If your SW mosfets are ok then check 494chip or the gate circuit of SW mosfets. Again my interperations on experience could not help more now. Can help on more detailed photos or shematics. You can ask for the other guys if they have your amplifier shematic. The switch can not cause so kind of a problem and at first you solve the blowing fuse problem!

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Old 6th June 2006, 01:49 PM   #3
Perry Babin is offline Perry Babin  United States
diyAudio Member
Join Date: Nov 2003
Location: Louisiana
The windings of the power transformer look like they've overheated. If so, the transformer has likely shorted between windings. 'Generally', when that level of current passes through the windings, the transformer has failed due to shorted windings and not due to problems elsewhere in the amp. The power supply FETs may have survived but if they didn't, they're generally inexpensive and easy to replace.

The big problem is replacing the transformer. It CAN be rewound but you'll need to find the magnet wire to rewind it. It looks like 16 or 17g wire . You can use a dial micrometer to measure the diameter to know for sure. They are using two parallel strands for the primary and one for the secondary. If you decide to rewind it, you need to take several photos of it from different angles so you can be sure to rewind it the same way.

Before rewinding it, it's VERY important to get the exact number of turns for both the primary and the secondary. If they are using 2 strands of wire for the primary, each turn of two strands is a single turn.

The switches seem to be relatively standard and although you may not be able to find exact replacements, you should be able to find suitable replacements.

The terminal block may be difficult to solder if you don't have enough heat. I'd think that you'd need at least a 50 watt iron to get the terminal hot enough. If you have two smaller irons, you can use one to heat from the top and the other to heat from the bottom.
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Old 6th June 2006, 09:07 PM   #4
jvr826 is offline jvr826  United States
diyAudio Member
Join Date: Jun 2006
Thanks to both of you for responding. This work is beyond my comfort level.

I got a recommendation of a repair facility in MA, ampdoctor.com, from a/d/s/ this morning. The guy is very reasonable and will hand wind a new transformer, fix the other problems, and fully test the amp.

Question on cooling. Will a fan moving air around the spare tire well that gets very hot be adequate to prevent a future meltdown? My equipment board looks like this...

Click the image to open in full size.

There is about an inch of space above the amp fins, per a/d/s/ recommendation, but not a large volume of air. The muffler is to the left of the well and gets seriously hot. I touched the metal in the well and had to pull my hand away instantly.

My other option is to put a fan in the trunk floor that can either draw cooler trunk air in to the well space, or pull hot air out, or do both in a push/pull scenario.

The other less risky option is to redo the installation and put the amp in the trunk and not under the floor.

Thanks again guys!
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