JL Audio G-MAX 1200 Replacement part

[Achim1409]

Hello,i got this amp for myself. In the audio section, only one mosfet (NTP35N15G ) is broken. Couldnt find a substitute.

Does somebody know a replacement. I want to change only the damaged fet,

could this work with other mosfets paralleled?

 

[Perry Babin]

If you have no other suggestions, the IRF3415 may work.

I wouldn't mix FETs in a parallel group. I'd suggest that all of the FETs in the parallel group are not only the same part number but also from the same production batch (matched date codes).

 

[Achim1409]

Hello, received the new parts,
but there is a small problem. Idle current about 4 Amps, output mosfets starts heating.

Amplitude is ok, checked the potis, maybe possible to adjust the dead time, but nothing happen.


Any suggestions?

 

[Perry Babin]

What does the drive signal look like at the gates of the FETs?

JL, in the 500/1 and likely in this amp, used a parallel resistor and diode and a capacitor to generate deadtime. It's rare that there is a problem there. I'd expect the problem to be in the drive circuit nearer the outputs.

 

[Achim1409]

I checked the resistor close to the fets, without fets inside. They are all in tolerance. Signal isn't good.

 

[Achim1409]

Without mosfets allready pulling 2.3 amps.. Checked the gate pads, ground to b-...

1: 12v
2:10v
3:12v
4:10v
5:0v
6:10v
7:0v
8:10v


No signal without mosfet.

 

[Perry Babin]

Recheck the waveforms with the black probe on the source leg for the FET gate being checked.

The v/div isn't shown on both images. Did the scope change ranges between tests?

 

[Achim1409]

Voltage hasn't changed. I will check with black probe at source

 

[Achim1409]

Same again..

Checked with black probe at source

1:12v
2:0v
3:12v
4:0v
5:0v
6:0v
7:0v
8:0v

 

[Achim1409]

If I injected a signal,
Then on the first, and each second place I Have a signal,
But I can't trigger it.

 

[Perry Babin]

It's possibly triggering but with the signal, it's being modulated so it looks blurred.

There is a 555 timer and LM319 together. Find the terminal of the LM319 that has the signal from the output of the 555. The other input of that 1/2 of the 319 is driven by an op-amp. Which op-amp is it? It should be in that general area.

 

[Achim1409]

Pin 3 from the 555 goes over a 471 Resitor to pin 11 of the lm319. The signal from the 555 is ok, but after the resistor its only floating.

 

[Perry Babin]

I think the 555 signal comes from pin 6 and drives pin 5 of the 319. Pin 11 of the 319 is supply voltage.

If the 555 drive pin 5 of the 319, audio drives pin 4.

 

[Achim1409]

Yes, you are right. On the first op amp 505 i have the sine wave, and at pin 4 lm319 there is also a signal.
Is it quit normal, that the amp is pulling 1,9Amps, without outputs?


The signal for Pin 4 is comming from pin 1 of an 2068

 

[Perry Babin]

I wouldn't expect it to be drawing that much current. Monitor the temperature of all of the PS FETs.

For the op-amp driving the 319, solder a bridge between terminals 1 and 2.

Does that give you a good drive to the outputs?

 

[Achim1409]

So, continue here.. If i bridge pin 1 and 2, i have a clean signal.

 

[Perry Babin]

Is this at the gate legs of the FETs with the outputs installed but the rectifier out of the circuit?

 

[Achim1409]

No, fets out of circuit, all other parts inside

 

[Perry Babin]

It would be more definitive with the FETs in the circuit (no rectifier) so that the drive circuit had a load.

I don't know if it works with this amp but the 500/1 would give a drive circuit if you pulsed remote for a second. The 500 gives about 15 seconds of driver supply voltage after remote is removed. The main supply has a delay.

 

[Achim1409]

High and low side fets.. At gate and source.. Rectifier out.. the signal stays on, all the time remote is applied