[sdiy] Prophet 5 -15v rail is shorting... Need advice

[harrybissell at wowway.com]

If you are using the 79xx series I hope that you noticed the
pinouts are very different from the 78xx series. You might have
a problem there. 

Tantalums ~will~ blow out easily on a reverse voltage supply, electrolytics
will die MUCH later from the same reason. Maybe there is ripple on that
supply that reverses the polarity ???

H^) harry




On Wed, 7 Nov 2007 09:46:19 -0800 (PST), Gil W. wrote
> Barry,
> 
> the first time I powered it all up I used small heat
> sinks for the PS regulator (which I build, as I said).
> 
> Could the -15 regulator partially go defective, in a
> way that it blows tantalums but works fine using
> electrolytes ?
> 
> Thanks all !
> 
> ======================================================
> 
> 	I would wonder what would create high ripple on the
> supply.
> Tantalums should be fine.  Heck, we use em on disk
> drives... tell me
>  about
> ripple...
> Feel the electrolytic cap itself in use to see if it
> is getting hot.
> I just got one of those pc fan controllers that had
> electrolytic caps
>  all
> over and they were blowing up and failing/getting hot.
>  Poor design
>  (extreme
> ripple if you put it across the PWM output!), under
> rated caps.
> Could be there is a bad ground in the supply circuit
> or a bad
>  regulator.  It
> may be oscillating etc.  A more tolerant cap may just
> be covering up
>  the
> problem.
> 
> Barry
> 
> --- "Gil W." <gil_we at yahoo.com> wrote:
> 
> > Hi, thanks .
> > 
> > This particular v3.2 I got without its bottom/rear
> > panel. I build the power supply myself, following
> > the
> > schematics, so you can imagine yourself that the
> > powering system is not completely compatible to the
> > designed one (I used a polysix transformer).
> > 
> > One thing for sure - with regular electrolytes used
> > as
> > main filter caps, I get perfect voltages. Could it
> > be
> > that the tantalum is too sensitive for heavy
> > filtering
> > applications ? I might consider taking it to a
> > friend
> > once I complete restoring it, and scope it. What
> > should I ask him to look at ?
> > 
> > Thanks very much.
> > 
> > 
> > --- GRAHAM ATKINS <gatkins at blueyonder.co.uk> wrote:
> > 
> > > Mmm,
> > > 
> > > I always think if the "designed" component is
> > > failing it
> > > suggests as you say an underlying problem before
> > > this
> > > stage. It does not pay to cover up problems which
> > > could
> > > lead to more serious damage to the keyboard later.
> > > If it
> > > was my Prophet 5 I would want to ensure it was
> > > safe.....
> > > PCB's not readily available !!!!!!.
> > > 
> > > I'd like to get a scope across the rails.
> > > 
> > > Graham
> > > On Nov 7, 2007, at 16:23, Gil W. wrote:
> > > 
> > > > I asked myself the same question - does any
> > > problem
> > > > actually *exist* or is the tantalum just not
> > good
> > > > enough for this task ? I think the original cap
> > > was a
> > > > 10uf/10v tantalum...
> > > >
> > > >
> > > >
> > > > --- GRAHAM ATKINS <gatkins at blueyonder.co.uk>
> > > wrote:
> > > >
> > > >> David,
> > > >>
> > > >> The electrolytic may be more tolerant than the
> > > >> tantalum but
> > > >> the underlying problem with high ripple off the
> > > >> rectifier may still
> > > >> be there, are you sure it is OK to connect to
> > the
> > > >> rest of the circuits.
> > > >> Are you just covering up the problem ?
> > > >>
> > > >> Graham
> > > >> On Nov 7, 2007, at 9:46, Gil W. wrote:
> > > >>
> > > >>> Hi,
> > > >>>
> > > >>> The problem has been solved by replacing the
> > > tant.
> > > >>> with regular 10uf/50v cap...
> > > >>>
> > > >>> Thanks all !
> > > >>>
> > > >>>
> > > >>> --- "Cornutt, David K"
> > > >> <david.k.cornutt at boeing.com>
> > > >>> wrote:
> > > >>>
> > > >>>> Possibly either the rectifier has an open
> > > diode,
> > > >>>> or something else is causing the supply rail
> > to
> > > >>>> oscillate.
> > > >>>> Tantalums don't like a lot of ripple current.
> > > >>>>
> > > >>>>
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Harry Bissell & Nora Abdullah 4eva