Output MOSFETs getting hot.

[Ashleyzn]

Yes they are identical

 

[Ashleyzn]

They are reading in mega ohms

 

[Perry Babin]

How much current does the amp draw at idle?

Looking at the waveforms on the terminals of the various outputs, do you see anything different on the ones that are getting hot fast?

 

[Ashleyzn]

Only pin 5 and 6 and 11 and 13 are are not in mega ohms but they like 2000k

 

[Perry Babin]

They should read the value of the pulldown resistor (100k?) divided by the number of pulldown resistor (3).

 

[Ashleyzn]

I will check tonight.

 

[Ashleyzn]

I pulled the card out the main board to test. And also removed ICs out of the card to test to be sure.

 

[Perry Babin]

With the driver board in the circuit, 5-6 and 11-12 are the ones that will read as described in post #25.

2000k is 2M ohms.

These ICs are very easy to damage from heat. It's best if you only install them once.

I don't know if you marked the locations they were previously installed. If you did, don't reinstall them in the same locations. Mix them up. That may make it easier to find a pattern. If they are installed in different locations and the same outputs heat up, that seems to indicate a problem with the IC. If the same outputs heat up, that would likely mean that there is a problem on the driver board, or on the main board.

 

[Ashleyzn]

I removed the two that drives the outputs that get hot. Why would you say if I change the location and if the same outputs get hot it indicates that the ICs is faulty.

 

[Ashleyzn]

I stated 2000k just to give you the exact readings I know it's 2meg

 

[Perry Babin]

That was wrong. If the problem follows the IC, the IC is suspect. If the problem remains the same regardless of the IC, the problem is likely a driver board or main board issue.

 

[Ashleyzn]

Will check it out tonight. Will let you know tomorrow.

 

[Ashleyzn]

Don't want to mislead you. Pin5 and 7 measures 6.8k on the card but removed from the main board.
Ic out of card measures 800k. Pin 11 and 13 measures 2000k on the card and out of the card. The other hi and lo pins measure 15 mega ohms measuring as per diagram left and right.

 

[Ashleyzn]

Hi Perry. I fitted the card back on, left out the two ICs that were initially driving the fets that were getting too hot and same problem the others get just as hot. Is there any other possibilities other than you suspecting the ICs. I tried to check the switching signals one mosfet popped. Then I quit and went to bed.

 

[Perry Babin]

Where were you checking the signal?

Could you have shorted between pads?

 

[Ashleyzn]

I could have shorted the pads. I need to replace the one fet and start again.

 

[Perry Babin]

The driver ICs often fail when an output fails. Re-check the resistance readings for the IC for the blown FET before applying power.

 

[Ashleyzn]

Thanks will do.

 

[Ashleyzn]

Hi Perry. I worked on the amplifier again this weekend. I decided to take a short cut instead of finding the exact cause. I replaced all the driver ICs together with all the small transistors, blown fets not all just the ones that popped. The amplifier switches on the current draw is 3.5amps outputs are nice and cool everything looking good. Now I decided to put a signal in, with one woofer as load. The MOSFETs weren't clamped to the heatsink, I just want to test for sound and make sure it works before assembling as you know these amplifiers are a mission to strip. It worked for 30 seconds and poof another fet blows in my face. Was I wrong in not clamping the fets.

 

[Ashleyzn]

I thought it could survive a little more than not even 30 seconds