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Old 23rd January 2011, 05:03 PM   #1
Dr Zeus is offline Dr Zeus  United States
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Default Power Acoustic SPL2600

This amp blew something up before the audio section, and then it looks like someone replaced 2 of the 4 rectifiers with the wrong parts. I pulled all the rectifiers and replaced them with good units, as well as removed all the shorted power supply fets.

They gate resistors all looked good at 99 ohms so I left them in place. Using two IRFZ44 units, I put one on one side of the amp and one on the other. What I usually do with an amp so big is I put ~1 new power supply fet into each phase so that if there are any more problems I dont blow a whole new set.

The problem now is; as soon as I power the amp up, one phase - one fet gets instantly hot. The other side remains completely cool

I pulled all the rectifiers and the problem remains - The one fet gets instantly hot.

I checked the traces under the board and there are no solder bridges.

I removed the rects and fets and measure 5v on Pad 1 of all the fets, ~13v on Pad 2, and 0v on Pad 3. There is no significantly low resistance between any of the pads.

The TL494 seems to be checking out fine, and so are the fet driver transistors; Q8, Q9, Q14, Q19.

I found a similar thread here but with no resolution:
Power Acoustic, unhappy mosfets

At this point I'm questioning the livelyhood of the transfomer coil on the south-west side of this photo, or perhaps one of the A1266 drivers are on the fritz.


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Old 23rd January 2011, 06:18 PM   #2
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If the transformer were defective, it's likely that both FETs would overheat.

Did you check the gate signals with your scope?
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Old 23rd January 2011, 07:52 PM   #3
Dr Zeus is offline Dr Zeus  United States
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The reason i think the transformer is possibly shorted is because; 1 - The wrong rectifiers were put into D6 and D7, and 2 - The transformer is discolored on one of it's sides. I'm in the process of removing both transformer coils to see if they both equate.

The gate signal with my scope is a nice +10v square wave.
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Old 23rd January 2011, 08:20 PM   #4
Dr Zeus is offline Dr Zeus  United States
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After removing both coils I noticed that one is defective and shorted. I went ahead and started unwinding the coil and noticed that it no longer registers shorted. I wound it back up and it is no longer shorted. I dont have a spare transformer coil.

Since it is no longer shorting, I think I can use it.... But I heard something about dipping a transformer into some kind of glue-base so that it potentially will not short out again. Shalac? Urethane? Goop?
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Old 23rd January 2011, 08:32 PM   #5
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You should have found the precise point where the windings were shorted and inserted an insulator.

Blindly applying a fixative isn't a good solution. If the windings laid down the same as they did originally, there may be nothing more than a thin layer of oxide insulating them. If the fixative doesn't get precisely where it needs to be, it could short again. This is why it's important that the short be found before the inductor or transformer is removed.

Did you find any burned/flaking enamel?
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Old 23rd January 2011, 08:42 PM   #6
Dr Zeus is offline Dr Zeus  United States
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Darn I guess I missed a step. There is sign of burning/flaking on the red winding, but thats not the one which was shorted. The gold windings were shorted and all "4 legs" had continutiy between each other; but now they do not. I'm not positive where the short actualy was at.
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