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Old 30th October 2007, 06:00 AM   #1
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Default Amp trouble - infinity Kappa 202a

Have a problem with my newest investment
I bought a Infinity 202a amp from internet.

I opened it up and saw that the big electrolyts was
a bit swollen on top so i replaced them.

I powerd the amp upp and noticed that the last pair
of output FETs got quite hot after just a few seconds
so i disconnected. (the pair to the very left on pic.)

None of the other FETs was not even near hot.

Something must be wrong here? What should i look for.

I also noticed that someone has marked one of the smaller
i think power supply fets with black and a mark on the side.
(the one to the very left on the pic.)

What does this usualy means?
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Old 30th October 2007, 03:04 PM   #2
ppia600 is offline ppia600  United States
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If you're talking about the white square things, those are resistors. The fets are under the long metal bars, strapped to the heatsink. I've seen many amps with marks like that, I think the people that build them do that for some reason. You'll probably need to test the power supply transistors and output transistors, using a volt/ohm meter. There are a few people who can make suggestions on the best way to do it.
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Old 30th October 2007, 03:27 PM   #3
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At the lower left edge of the board one can also see power transistors. Are you talking about those ?
Did you test without heatsink - like shown on the photograph ? If yes, then sometransistors may for sure get hot even under normal conditions. I wouldn't run the amp like that !

Regards

Charles
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Old 30th October 2007, 06:54 PM   #4
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If only two of the 4 FETs for that channel are getting hot, the first thing I would do is to pull all 4 of them and check them out of the board.

If none are defective, power up the amp and measure the DC voltage on the gate pads of the FETs that you removed. The voltage should be ~1/2 of B+ and the same voltage for all 4 gates.

If/when you get the amp working, you're going to need to replace the insulators. You'll need to clean all of the transistors and the sink. 100% of the old insulator material must be removed. This is very time consuming but must be done if the amp is to be reliable. The easiest replacement insulator I've found is Kapton tape or Kapton insulating film.
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Old 30th October 2007, 08:25 PM   #5
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Okay yes i know that the white "squares" are resistors
Im talking about the capacitor thats marked with a pen
on the top, just below the white square thats also marked.

And you are right it must be the power supply transistors
that gets hot and not the outputs. Its the lower transistors
to the very left. The pair of them gets hot.

And yes i know that i cant runt the amp without the heatsinks.

Just did it for testing to se if anything got abnormal hot,
and it did so i shut it down.

The power supply transistors should heat upp quite
equal or am i completly lost?

Perry its matter of "when" haha, took me 2 weeks to finish
my last project, my alpine mrv-1505. Thanks for all the help!

I will remove them and check them out of the board.

By the way how do you know that it should be around half
the "battery" voltage on these? is it similar to all amps?

Just want to learn something =)
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Old 30th October 2007, 08:41 PM   #6
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The power supply FETs should operate at the same temperature as the others in that power supply. Typically, power supply FETs run cool at idle (no load on amp). If they do run slightly warm, all should have had the same rise in temperature.

For virtually all amps, the duty cycle will go to ~50% when the FETs have been removed. Most meters will see this 50% duty cycle square wave as 1/2 of the peak voltage. The peak voltage depends on several things but it's generally equal to (or slightly less than) B+ voltage. This means you'll read ~1/2 of B+. If one bank of FETs has ~6v and the other bank has significanly more or less (near B+ or near ground), you know that the side that's not near 1/2 of B+ has a problem.

This only works reliably when the FETs have been removed. For amps with regulated power supplies, the duty cycle will be reduced as the target voltage is reached and the DC readings on the gates may be very low.
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Old 31st October 2007, 08:02 PM   #7
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Tonight i desolder two of what i taught was the power supply
transistors. But in fact it was the rectifyer, correct me if
im wrong. The power supplu transistors are placed in the same
row and i 3 pairs of totaly 6 transistors.

And it was the rectifyer that got so damn hot i noticed.


You can se it on the picture below. I measured it.

From center to pin 1 no connection.

From center to pin 3 - 2Mohm, this must be bad!?


From pin 1 to center 253 ohms.

From pin 3 to center 37Kohm.

What could cause the rectifyer to crash?
Maybe this caused the swollen electrolyts that i replaced?
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Old 31st October 2007, 08:04 PM   #8
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By the way whats the other thing on the picture? (SF163A)
Is it even a transistor? Can i measure it in some way?
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Old 31st October 2007, 08:12 PM   #9
ppia600 is offline ppia600  United States
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They are both rectifiers. The left is a negative rectifier, the right is a positive one. The one you measured is bad. Make sure to check the other rectifier and the output transistors as well.
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Old 31st October 2007, 08:24 PM   #10
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Ok can i measure it the same way as i did with the other one?
It doesnt have any schematic symbol that shows how its connected internally.
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