mtx 1501d sawtooth waveform

[Speeed3]

When I first got this amplifier it had blown output transistors,
I leaved just 2 working in place and tested the amplifier shortly, it worked. But when I put the new output transistor in place they blew again,now I again put 2 working transistors in place, and found out that at least lower side mosfet was getting some kind of sawtooth waveform.

Now I am afraid of powering it up to measure so I would hope that you can point me to right direction. I have a basic car audio repair tutorial, but I can't find help from it.

[Perry Babin]

Try the following file in the 81000D folder. The waveforms should be very similar.
repair_tutorial\miscellaneousstuff\mtx\mtx81000d\m tx81000dwaveforms04.swf

The mtx81000dwaveformsandnotes.htm file in the 81000D file should also be helpful.

Have you checked the gate resistors for each of the outputs?

Did LED 238 light up before they blew?

Did you use the exact replacement parts for the outputs?

Where the 2 that you left in place the originals or were they the same as the ones you used as replacements?

Some of these amplifiers will not work properly with the new replacements. If you cannot find any defective components and the amp continues to blow the outputs, you may have to increase the resistance of the 2 large resistors near the opto-couplers.

[Speeed3]

One gate resistor is open, but in there is no fet. Im not sure was it ok before.
It took long time for transistors to arive.

Im not sure what led do you mean, there is led 463 and 399 but i think they bouth ligh up, at some point.

exact IRFB42N20D

I leaved 2 in place so they are the new ones.

But what I really doesnt understand is how can gate singnal to lower side mosfet in waveforms be less than 5vpp

Waveform from my amps lower side mosfets gate AC about 5Vdiv

[Perry Babin]

LED238 is near the board mounting screw that's closest to the opto-couplers. If it's not coming on, and you have a drive signal, it may be defective.

LED463 is the power LED and should be on when remote voltage is applied. LED399 (in-between large caps?) tells you that the caps are charged (a warning t be careful).

If your scope was set to 5v/div, it appears that you have a 10v-peak signal (not including the spike below the rail).

Did you have an FET in the location where there is an open gate resistor? If so, that could have caused the new FETs to blow.

[Speeed3]

Yeh, that open resistor may have caused fets to fail, if it was open before.

But that waveform of mine cant be richt?

Fets cost so much that I dont want to just try, if they work, unless I have to.

(I did mean that repairtuttorials gate waveform is less than 5Vpp)

[Perry Babin]

The amplitude of the waveform isn't significant at idle. The amp's servo/feedback circuit changes the pulse width to maintain 0v DC on the output at idle. Since the pulse width determines the amplitude on the gate (due to the charging of the gate capacitance), the actual peak gate voltage will vary.

[Speeed3]

So you mean that I just put new fets and gate resistors in and try?

[Perry Babin]

From the information you provided, I'd feel confident in replacing the output and resistor and applying power. If the over-current protection is working properly and the outputs are clamped to the sink, the amp should go into protection if there is a problem.

If it powers up normally and the audio section oscillates properly (as it seems to be doing now), drive a signal into it and drive the output to clipping with no load. If LED238 doesn't blink off, connect a load and run it at increasingly high output. If LED238 remains on at full power, the amp will probably be OK.

[Speeed3]

Ok. Thank you for help. I will order outputs and instal them when they arive (about three weeks )

[Speeed3]

New outputs + resisitor are in.
Amp powers up, but it makes buzing noise, and when I try to drive amp into clipping with no load it shuts down course of current limiting resistor. Without resistor noise is louder.