[sdiy] sem repair

[harry]

Mostly agreed...

sometimes it is VOLTAGE that kills... not current (especially in the
case of FET or CMOS components).

Also... if the short circuit causes a reverse bias on a tantalum cap...
it can be damaged in pretty short order (teehee)  and fail shorted.

that said... I agree with Doug... semiconductors are the first suspects

(you guys just have this terrible phobia about caps, dont'cha?  I mean
they
are almost never the culprits... YOU just hate them because #1: You
checked the
semiconductors first, and didn't find any bad... and #2: A bad cap is
the last thing you look for and can be very hard to find !!!)

Another technique (I don't usually use but...) is to cut the power
traces on the board
in half.... isolating half the power. If the problem persists, its on
the side nearer the
supply. If not... it is on the other half. Repair the trace and cut
again until you find the
problem. My Rev3 Prophet V was like this from the factory. Would I do
this to a vintage SEM... frankly... no!

H^) harry

epeasant at telusplanet.net wrote:

> Well, when a short is accidently applied to a circuit, it
> is excess current flow that causes the damage. It is
> virtually impossible to draw enough current through a
> capacitor in this transient manner to cause damage.
> Semiconductors and resistors are a different story, as they
> can conduct large sustained dc currents. But resistors can
> be seen (and smelled) when they are overloaded, so that
> leaves semiconductors as the primary suspect. If the fault
> occurred spontaneously without apparent cause, I would
> certainly check capacitors (especially tantalum) early in
> the troubleshooting process.
>
> Take care,
> Doug
>
> ______________________
> The Electronic Peasant
>
> www.electronicpeasant.com
>
> > yes, definitely caps of some variety..
> >
> > I have seen things screw up a synth that defied logic..
> > But, 9/10,
> > there is a cap out there dragging down a rail somewhere.
> > ;)
> >
> > Rob
> >
> > ----- Original Message -----
> > From: "Gene Stopp" <gene at ixiacom.com>
> > To: "synth DIY" <synth-diy at dropmix.xs4all.nl>
> > Sent: Friday, July 19, 2002 5:50 PM
> > Subject: RE: [sdiy] sem repair
> >
> >
> > > Tantalum caps?
> > >
> > > Best Regards,
> > >
> > > - Gene
> > >
> > >
> > > -----Original Message-----
> > > From: mark verbos [mailto:a0284520 at addcom.de]
> > > Sent: Friday, July 19, 2002 2:42 PM
> > > To: synth DIY
> > > Subject: [sdiy] sem repair
> > >
> > >
> > > Hi,
> > >
> > > I am fixing an Oberheim SEM and need some guidance. The
> > owner tried
> > to
> > > trim the V/Oct response and sharted something that
> > caused the power
> > > supply to blow up. I have replaced the 723 and got the
> > power working
> > > again, with the VCO's cut off from the power. However,
> > when they are
> > > connected, the positive rail drops from 15 volts down
> > to 8 volts. I
> > > can't check what's wrong with the VCO's if the power
> > isn't working?
> > What
> > > could cause this? I have never seen a dead OP amp mess
> > with the
> > power
> > > rail. could it? Anyone know this problem?
> > >
> > > please help!
> > >
> > > mark
> >
> >

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