Power supply mosfets

[Aqeeb127]

Hi power supply fets heating up with no rectifiers in and drive voltage is around 3v is it too low if it is should i replace the buffer transistors and the 494 or just the buffers thanks in advance

 

[Perry Babin]

You should always check the drive signal with a loading capacitor before installing the FETs.

What does the signal look like at the output of the 494 and at the emitter of the driver transistors?

What make/model amp?

 

[Aqeeb127]

Its a tg 18d targa and i do not have a oscilloscope as yet i only have a multi meter and a current limiter 12v supply

 

[Perry Babin]

What is the DC voltage on the points I listed?

 

[Aqeeb127]

The voltage at the 494 is 4.5approx pin 9 and ten same voltage and voltage at emmiters is 4.19 with out any fets installed

 

[Perry Babin]

You changed 2 things so the numbers don't mean much.

Why did you remove the FETs?

Are you sure that the gate resistors are the right value?

 

[Aqeeb127]

The reason i removed the fets is that i cannot keep it too long on otherwise it heats up. It doesnt heat up extremely fast but does heat up at a quick rate. The gate resistors are 33 ohms and those where in the amp

 

[Perry Babin]

In a situation like this, you clamp them to the heatsink to give you extra time. You only need to power up, measure, power down, move to the next test point and repeat.

 

[Aqeeb127]

Ok will do so tomorrow

 

[Aqeeb127]

The voltage at the gate goes up until 3.14v. The voltage at the emmiter is the same 3.14v. The output at the 494 does not give a steady voltage it grows slowly but i cannot leave it too long on then the fets heats up

 

[Aqeeb127]

The highest voltage i got from the 494 is 1.8v

 

[Perry Babin]

What is the DC voltage on pins 11 and 12?

 

[Aqeeb127]

Pin1=118mv
Pin2=77mv
Pin3=104.1mv
Pin4=starts from 2.2v then drops to 200mv
Pin5=80mv
Pin6=3.7v start and then drops to 3.3v
Pin7=108mv
Pin8=b+
Pin9=1.8v
Pin10=1.8v
Pin11=b+
Pin12=5.6v
Pin13=4.87v
Pin14=78mv

 

[Perry Babin]

The 494 has 16 pins, not 14.

 

[Aqeeb127]

Pin1=118mv
Pin2=77mv
Pin3=104.1mv
Pin4=starts from 2.2v then drops to 200mv
Pin5=80mv
Pin6=3.7v start and then drops to 3.3v
Pin7=108mv
Pin8=b+
Pin9=1.8v
Pin10=1.8v
Pin11=b+
Pin12=5.6v
Pin13=4.8v
Pin14=4.8v
Pin15=4.8v
Pin16=almost 1mv fluctuating

Sorry for the previous one was still busy measuring

 

[Aqeeb127]

Pin one and two seem to change everytime i power up the amp

 

[Perry Babin]

Pin 12 needs 12v. Find why the voltage is low.

 

[Aqeeb127]

It starts at 12v but then drops down rapidly to 5-6v

 

[Aqeeb127]

With the fets out i get the 12v

 

[Aqeeb127]

Ive found out why it doesnt have 12v its pulling 8.9A of current no rectifiers in so i think ive got a faulty transformer