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Old 5th September 2012, 06:42 AM   #1
Mote is offline Mote  Estonia
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Default Amp blowing PS FETs

I have another Auna ALP404CH amplifier. Interesting amps, the other cheap amplifiers usually blow only outputs and sometimes PS FETs and thats it but these have some weird issues, every one of them
One of my first attempts to fix an amplifier is here, also Auna ALP404CH: Auna ALP404CH two channels drawing exessive current when speaker connected

When powering up amp is in protection, then it tries to switch on but goes back to protection, after few seconds tries to power up again, draws current, something crackles and one of the PS FETs on one side fails, I have current limiting headlamp inline with B+.
Gate resistors and drivers are already replaced. I also replaced 494.
It does this with and without rectifiers installed.

When I do not install PS FETs on the problematic side of the amp and disable protection via lifting few jumpers near 494 all channels produce clean audio.

If protection is disabled and all PS FETs installed amp still draws too much current and blows one of the FETs.

If I remove the blown FET but leave others in, amp draws too much current but does not blow any more FETs, if I leave only one FET in the problematic side then amp does not draw too much current.

With 3 FETs installed and transformer from the problematic side removed amp still draws too much current. When the other transformer is removed then amp does not draw current but does not come out of protection either.
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Tools: Perrys tutorial, C1-76 o-scope, Weller WHS40, Mastech MAS830L multimeter, Mastech MY6243 LC meter, 16A supply
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Old 5th September 2012, 05:09 PM   #2
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Remove the FETs. For every FET, install a 1k resistor from gate to source. Does the drive signal remain clean and square?

What if you connect the resistor from the gate to the drain?

Do this for one FET at a time.
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Old 9th September 2012, 05:43 PM   #3
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Either way I get nice clean squares.
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Old 10th September 2012, 12:21 AM   #4
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That would seem to eliminate the drive circuit as the fault if you checked the signal on the gate pad for the FETs.
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Old 10th September 2012, 04:02 AM   #5
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I checked signals on the 1k resistors.
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Old 11th September 2012, 10:05 AM   #6
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For the FET that repeatedly blows, double-check the drive signal while pushing on/flexing the circuit board. Does the waveform ever change from a perfect drive signal?
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Old 11th September 2012, 06:14 PM   #7
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I pulled all Fets, tried the same thing with other side of the board, also all nice squares.
Then i put in new FETs in the problematic side. Two of them blew. Left good ones in, installed 1k resistors from gate to drain in place of blown FETs on one I get clean square, on the not so clean, see first attached photo. Square is clean on the 494 side of the gate resistor.

Also there is a waveform on 22 ohm resistors coming from 494 and going to transformers, I have never checked if there is just +12V or some waveform, anyway, what I saw on those resistors is on second photo.
Attached Images
File Type: jpg DSC00523.JPG (37.0 KB, 54 views)
File Type: jpg DSC00524.JPG (22.4 KB, 52 views)
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Tools: Perrys tutorial, C1-76 o-scope, Weller WHS40, Mastech MAS830L multimeter, Mastech MY6243 LC meter, 16A supply
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Old 12th September 2012, 07:22 AM   #8
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When you post screen caps from your scope, you need to include the timebase and vertical amp settings. When displaying this type of waveform (drive signal for an FET), you should set the trace on the reference line of the display and set the coupling to DC. This is covered on page 67 of the car audio site.
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Old 16th September 2012, 06:38 AM   #9
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Some new screens of waveforms I found from the amp.
Timebase for all photos: 5us, vertical: 2V
First photo is gate signal on a FET.
Second and third photo are gate signal on gate pad with 1k resistor connected from gate to drain, both are driven by the same driver transistor.
Attached Images
File Type: jpg DSC00529.JPG (138.7 KB, 35 views)
File Type: jpg DSC00530.JPG (131.4 KB, 36 views)
File Type: jpg DSC00531.JPG (125.5 KB, 35 views)
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Tools: Perrys tutorial, C1-76 o-scope, Weller WHS40, Mastech MAS830L multimeter, Mastech MY6243 LC meter, 16A supply
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Old 16th September 2012, 08:25 AM   #10
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If the scope is set to 2v/div, it appears that the NPN driver transistors don't have sufficient DC voltage on their collectors.
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