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Mote 5th September 2012 06:42 AM

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:

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.

Perry Babin 5th September 2012 05:09 PM

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.

Mote 9th September 2012 05:43 PM

Either way I get nice clean squares.

Perry Babin 10th September 2012 12:21 AM

That would seem to eliminate the drive circuit as the fault if you checked the signal on the gate pad for the FETs.

Mote 10th September 2012 04:02 AM

I checked signals on the 1k resistors.

Perry Babin 11th September 2012 10:05 AM

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?

Mote 11th September 2012 06:14 PM

2 Attachment(s)
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.

Perry Babin 12th September 2012 07:22 AM

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.

Mote 16th September 2012 06:38 AM

3 Attachment(s)
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.

Perry Babin 16th September 2012 08:25 AM

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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