Eros power supply PCB board problem

[Steve Reese]

My wife bought me the Eros phono preamp as a gift for Christmas this year. The build was going well until I was on page 44 to 45 where you do the power supply voltage test. Where the voltage should be close to 375V and mine read only 270V.. then the 100uF 450V cap broke open and began to smoke. I quickly shut it down, but I don't know what the existent the damage may be. I tried getting hold of the home office, but I know being a holiday week, that they may be out of town, etc. I ordered new caps from MCM, and I'm reckoning I'd try to replace the 100uF and see what happens there. Are there any measurements I can take to get an idea how bad the damage is? Many thanks to all who may reply.

[Paul Birkeland]

To make the 100uF cap smoke, it is most likely that it was in backwards.  I'd check the remains to see if this was the case.

How is that 270 Ohm resistor?  It may have turned slightly brown, but it likely survived.  I'd just measure its resistance again to check.

If the cap was in correctly, how about those Cree diodes?

While the parts are making their way to you, double check all the parts on that board to be sure they are in correctly, and e-mail queen (At) bottlehead (dot) com about the 100uF cap, she may be able to get one to you more quickly than Mouser.

Generally the phones will be muted on Fridays when we do demos, but e-mails will still roll through.

-PB

[Steve Reese]

I checked the direction of the cap, in fact, all of the large caps, and they were installed correctly. The only thing I think might have happened was either a cold solder joint on one of the resistors or a bad cap. I retouched most of the joints and installed a new 100uF... worked like a charm. Now for the C4 and C5 on tube socket C was a complete bear to all those parts on there. I'm going to have to look those over again later today...

[Grainger49]

Steve,

As Desert Pete said, "Have faith my friend...."  Except for the extremely rare instance of a bad capacitor the reason a power supply cap gives up the ghost is reverse polarity or reversed diodes sending AC to the cap.  You have ruled out the cap polarity so it almost has to be the diodes.

The power supply caps are rated at a high enough voltage that they can take the full output of the rectifiers with no voltage drop in the circuit.  That is because when the Eros turns on that is exactly what happens.  A cold solder joint is an open circuit.  Mostly benign, but a short after the power supply caps is disaster!  That is another possibility.  If the output is shorted it should blow the fuse, or as Murphy says, blow the capacitor protecting the fuse.  That is tongue-in-cheek! 

Hang in there, this is an exquisite phono stage!