You should be able to check the diodes in-circuit; as long as they don't have anything in parallel that would affect readings.

Thanks I will start there

 

I would say the TNY264 chip is probably blown if it's popping the mains fuse.

 

I would say the TNY264 chip is probably blown if it's popping the mains fuse.

Many thanks
I have ordered a few things including that chip and the MOSFETS (IRF840s) as they were short circuit between all pins when off the board, I guess there may well be a few things to fix. The bridge rectifier and the diodes I could test in-circuit are all ok though.
We will see in a few days.

Many thanks again

 

Thanks to everyone for their help
We see in a few days when I get the parts through, I will update then.
Best wishes

 

Update
Swapping out the blown MOSFETS Q7 and Q8 fixed the problem.
Though the supply is still very sensitive to conditions for setting the current limit, I've been using 1A supply fuses to test the unit and any voltage above 3V when you try to set the current limit will blow that fuse. There appears to be some kind of threshold beyond which, shorting the outputs pulls too much current

 

Update
Swapping out the blown MOSFETS Q7 and Q8 fixed the problem.
Though the supply is still very sensitive to conditions for setting the current limit, I've been using 1A supply fuses to test the unit and any voltage above 3V when you try to set the current limit will blow that fuse. There appears to be some kind of threshold beyond which, shorting the outputs pulls too much current

Thanks, everyone for the support and help, If anyone has any suggestions (short of a full redesign or buying a new one) on ways I might think about mitigating this MOSFET weakness I would be keen to hear, I would like to learn more about the field. On a very basic level, it appears that better current control into the MOSFETS or more highly rated ones may help?

Best wishes

 

It's a bad design, it can be improved by just using the TL494 chip and controlling the voltage and current sensing using the feedback pins 1,2 /15,16 .

 

It's a bad design, it can be improved by just using the TL494 chip and controlling the voltage and current sensing using the feedback pins 1,2 /15,16 .

Ah very interesting, I will take a look at that, many thanks
I now have a big bag of Chinese MOSFETS in case things go badly
Karl

 

I now have a big bag of Chinese MOSFETS in case things go badly

Remember, you get what you pay for. Low priced counterfeit parts usually aren't worth the bother...