Fuse keeps blowing!

[soundofvoid]

One of my trusty SE100's has a problem.
Keeps blowing the fuse at turning on.
I have taken all tubes out but the fuse is still blowing.
So i suspect the rectifier bridge,a diode or a cap at the PS somewhere.
Taking the PS out is a days work and i am probably facing a week without music...
Sniff...

[amirm]

Bummer. Sounds like a power supply problem. Just in case though, I am assuming if the unit came with slow-blow fuses, you didn't put a normal fast blow fuse. If so, that could be the reason it is blowing.

[kach22i]

Ioannis I don't know what to say, you are the guy I turn to for problem solving of this nature.

When my pre-amp did something similar it was a capacitor.

Could it be one of your solder joints? My Rotel amp had that problem years ago when I first bought it.

[Wardsweb]

Does the fuse just split or does it vaporize? It is just explodes and turns all black, it is a dead short in the power line and should be easy to find. If it just opens the fuse, you do have a current rush but is a little tricker to isolate. Check your bridge rectfier and regulator(s). Breaking the circuit will allow you to deternine if you need to work forward or backwards.

[soundofvoid]

Thanks for the input guys!I suspect a short somewhere in the PS board-probably a cap!The rectifier bridge measures ok and so do all the other diodes that i've measured.
The fuse was a 10A fast blow for the 115VAC and replaced it with a 7.2A for 230VAC.It was working like this for the last 5 years so i don't think it a fuse related problem.
With the tubes pulled off it explodes/evaporates instantly.I have added thermistors in the power line to keep the inrush current in check.Most likely it will be a PS cap.
I will try to empty them with a big resistor and take some measurements today.Will see...
Good thing is that i am making a total parts upgrade in three other SE-100s as well.I have now about 1/100th of the total factory production!
Part No: 01015/016/018/049/095!I am keeping the first three of course...
John

[soundofvoid]

Bummer!Rechecked the rectifier and the all the accessible diodes.All measure ok.I have access to 5 PS caps without taking the board out.
Emptied them with a big resistor and checked them too:all is good-no shorts and the capacitance value is where it should be.
For the remaining 9 big caps to be measured,the PS board has to come out...tonight!

[amirm]

That is the problem with electronic troubleshooting: you hope for easy, but half the time it is not .

You know your stuff but just in case, here is a little trick that might save time: look at the parts on the PCB (PC board) side. And see if any areas seem heated up, discolored. 8 out of 10 times, that is the problem area. this kind of examination also shows blown traces as a result of the short.

Also, the electrolytic caps tend to bulge up most of the time when they dry up and go bad. Feel the top of them and see if it is flat.

[soundofvoid]

Thanks Amir,but that's a sleeper!There is nothing -but the fuse- that looks brown, burned,or discolorated.Traces look ok!First thing i did was to check for a bloated cap - no such luck.
I took the board out last night but i was too tired to troubleshoot.I hope i will find the time to do it today.Will keep you posted!

[soundofvoid]

Got it!A pesky little bypass cap to one of the big 560uf /450V was shorted.I will change all these (6) to save me from future troubles.
The order to Farnell is already made.All the rest is intact.

[Wardsweb]

Great, glad you found it.