I am trying to repair a powered sub-woofer's power supply that keeps blowing fuses. I have attached picture and part of the schematic where I think the problem resides.

What I have done:
- Removed the isolation transformer and the fuse doesn't blow.
- Checked the diodes, transistors and capacitors and all seem to be good.
Not sure if the isolation XFMR is good. How do I check?
I don't know how to check the MOSFET - IRF740 either.

Any help would be appreciated.

Regards

 

Farther down the posting page is a thread titled, "Troubleshooting a TV power supply". It is the same kind of supply you have. The "standard" advice is that you should learn from that thread. For example, what I learned from that thread is that several months and hundreds of responses later, it still isn't fixed.

 

Hello,

The powersuppl is probably a SMPS type.
See this FAQ for more info:
http://www.repairfaq.org/sam/smpsfaq.htm

Bertus

 

Can you not upload the circuit in more conventional format. I can't read whatever you have used.

In particular which fuse is blowing?

This looks like an SMPS as Bertus said.

If the ouput stages are blown they will simply load the oscillator so it does not run.

If the input stage is blown it may well operate an input fuse.

What do you mean you removed the isolation transformer?

 

Be extremely careful with this supply. The input side is not isolated from the line, so it presents a lethal shock hazard.

That transformer is "the" transformer.

A power FET shows no conduction drain to source with the negative lead on the source. In the other direction, you should read a body diode PN junction. The gate should have no conduction to either drain or source. They tend to fail shorted.

 

I found two IRF740's to be bad. All leads shorted. I have ordered them and they should be here this week.

The input of this board is DC from an EMI filter board (not shown) 120V.