[sdiy] Korg DW-8000 power supply help?

[Mike Pepper]

> I'm trying to troubleshoot a failed Korg DW-8000 power supply.
Disconnected from any load it pops fuses as soon as it's powered up. I
started with the bridge rectifier, it's fine; while it was removed I checked
the xformer, it's fine. There doesn't appear to be anything irregular (trace
damage, burns, blown bits, etc.) on the board.
>
> Do you guys have any techniques you can share for where I might start with
this thing, since I can't power it up at all? It's not like there's just one
rail down and I can fire up the rest of it to test.
>
> And yes, I realize I'm dealing with potentially lethal voltages and I
won't hold any of you accountable if I screw up my own handiwork.
>

I'd more likely supect a blown chopper transistor on the primary. At least
if the fuse is blowing, the ground resistor in the chopper is probably OK,
(choppers can be bipolar or MOSFET, and often have a current sensing
resistor in the 'ground side' lead).

Remember to put some kind of dummy load(s) on the output, a lot of switch
modes don't like working into an open circuit.

The Old TV engineer's trick of putting a standard light bulb in series with
the mains live can save a lot of blown parts. If the bulb lights up, you
still have a short, if it just blinks, and the supply fires up, you can
remove it. I still somewhere have a box with a bulb holder, mains lead and
socket, big bright 'Power On' neons and a bypass switch. Harry is right
about shorted electrolytics. Prime suspect is the big high voltage one in
the primary circuit. Make yourself a discharger resistor, or solder some
on - those caps can stay charged for hours, and deliver a nasty bite if you
touch them.

||\/||ike