[sdiy] Korg DW-8000 power supply help?

[Mike Pepper]

> Hold on, we were talking linear power supplies just, not switched mode.
>
> Graham
>

Just realised. Doh!

I think it was the fact that I've been working on them recently, cued by the
statement "I started with the bridge rectifier". I'd have started with the
transformer, diconnecting the bridge, then checking for a reasonable output
before replacing the bridge. Disconnect the load side, check for damaged
regs, then for load side shorts. If any tantalum caps, suspect them at once.

A minor note: some 79xx series regs need a load or they won't work properly.
Forgot that myself a few months back and swapped out a reg in one of the
boards I'd built until I remembered.

||\/||ike



> On 3 Nov 2010, at 20:06, Mike Pepper wrote:
>
> >> 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
> >
> >
> >
> >
> >
> >
> >
> >
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