Type 4 Class D GMAudio 3K lowside issue

[Chrisjoel]

Hi im new around here !!
Have this Amp , came in protection !!
NOne mosfet burn or Bad .

Following Mute from Output drive card ,
Find a M6 transistor bad , replace it and the amp turn on !!

Testing the amp find out that output lowside mosfets are a little bit warm , putting signal in and speaker they gone more warm than usual , HIGHSIDE not they have normal temp.

Pullout output mosfets and test all drivers , follow waveform from drivecard to mosfets pads , its fine square.

When I load the output fets the waveform on gates of low side get distorted .
The Highside are ok completely square .
Any idea whats going wrong on it
Thanks

Low Side Gates loaded!.

 

[James361]

The gate would look like that if the drive is being limited, If there is a driver chip or a transistor driving that pin then it is weak and it is not supplying the normal rated current. It looks like an RC time constant.

 

[Chrisjoel]

Hi thanks for respondig .

It use a 2X smd and D667 & B647
The Base of the 2x also distort de waveform , the collector waveform much better .

 

[James361]

I can't tell what is happening exactly because I don't have a schematic. However, I think that there is a problem with the npn D667. I see some solder cracks so I would reflow the solder connections of D667 and B647 first. If resoldering doesn't help then you should check those 2 transistors for opens or shorts. It does seem like there is an open transistor and it might be D667. Make sure that the collector voltage is correct on those 2 transistors.

 

[cush man]

The gate driver transistors might look fine and read fine but when energized go completely wack. I would at least change them both if everthing else is fine.

 

[Chrisjoel]

Thanks !!
I reflow them , still the same .
Changed them with C2073 / A940 nothing change .
Replace the 2X , nothing.
Got 2 mosfets on each side , right now they are cold with that lowside drive deform .

The collectors have 12V from input of the 5v regulator and the other one -B .

I noticed theres +5V on speaker terminal .
Also Change this two Resistors , from rails to GND .suposed to be 100K and only measure 10K .

 

[James361]

You should trace the circuit of the troubled area and make a schematic for us so we can rationalize what is happening.

 

[Chrisjoel]

Hii
Okk i try to make a schematic but i noticed that the SLA1500 Schematic is almost almost identical the only thing that changes its the driver output.

It use a 2X smd transistor after opto 6n137 .
After that change from sla1500 instead of the 2X and 2T it use the D667 and B647.
Trying resolder lowside smd's components . Nothing happen.

Use the C1023 and A940 same thing .
Use the D718 and B668 worst .

Resolder feedBack circuit , samething

Right now at idle have loaded with 2 mosfet on each side , now at idle are cold , load input signal and a speaker and they getting warm , but the waveform like a shar fin

 

[cush man]

I think your issue is further upstream, check the PWM chips and check if they're getting signal from the op-amps.

 

[James361]

The soldering on the opto-coupler looks terrible. Try re-soldering the 6n137 pins.

 

[Chrisjoel]

I Also thinking about that , if the problem is in the driverboard , the LM319 is getting signal , i saw the triangle wave in the 4560 opamp.

Pin 9 of the lm319 show this , is that normal ?.

 

[James361]

I think that pin 9 of the LM319 looks normal. The drive out of the opto-coupler or into the opto-coupler is insufficient. This can happen with cold solder joints or a component that has changed value. I still suspect a cold solder joint because some solder connections look suspicious. Years of thermal cycling can make a cold solder joint reveal itself. Put some pressure with a pencil eraser or your finger on the components in the bad area to see if there is a pressure sensivitity.

 

[Chrisjoel]

Thanks , just did it , no changes on the bad area .
Recently tried with another output driveboard , still the same .

 

[James361]

Someone reported that there was a bad surface mount resistor on a similar amplifier. I suggest that you check all the resistors in the problem area for the correct value. A resistor driving the input side of the opto-coupler may have changed its value.