Vintage Synth Repair

Thanks, I'm going to replace all W02M with W04 (is the ok?...)
Will also replace the regulators and electrolytic caps, and report 
results.




--- In vintagesynthrepair@yahoogroups.com, "synthparts" 
<synthparts@...> wrote:
>
> Have you replaced to 5V bridge rectifier with one of higher amp
> rating? The original W02Ms used in OBs were to wimpy...
> 
> --- In vintagesynthrepair@yahoogroups.com, "gil_we" <gil_we@> wrote:
> >
> > Thanks guys !
> > 
> > News - I replaced the tantalum cap with one that can handle up to 
25v 
> > and disconnected the other boards from the synth, the cap has not 
got 
> > fried at all, but voltages are inaccurate-
> > 
> > -15v read -14.6
> > +5v  read +4.25
> > 
> > the +15v and -5v are adjustable so they're ok...
> > 
> > What could cause the inaccurate voltages ? should I replace the 
7915
> > and 7805 ? or maybe some particular caps ? or do they appear like 
> > this because no board is connected to the PS ?
> > 
> > 
> > Also, does this indicate that some other board shorts the -15v 
rail ?
> > 
> > Thanks again !!!
> > 
> > 
> > 
> > 
> > 
> > 
> > --- In vintagesynthrepair@yahoogroups.com, Joan Martinez 
> > <piniflopa@> wrote:
> > >
> > > I don't know so much about OBXa  just, obiously, that is a 
great 
> > synth. But
> > > I'm agree with Scott. Another possibility is a wrong connection 
> > from primary
> > > winding. Maybe a transformer configurated in diferent country 
as 
> > must be?
> > > Regards.
> > > 
> > > Joan M.
> > > 
> > >   -----Mensaje original-----
> > >   De: vintagesynthrepair@yahoogroups.com
> > > [mailto:vintagesynthrepair@yahoogroups.com]En nombre de Scott 
> > Nordlund
> > >   Enviado el: lunes, 26 de marzo de 2007 17:54
> > >   Para: vintagesynthrepair@yahoogroups.com
> > >   Asunto:RE: [vintagesynthrepair] Power Suppy problem
> > > 
> > > 
> > > 
> > > 
> > >   >Powering my OBXa up immediately blows up the tantalum 
capacitor 
> > placed
> > >   >between the IN and GND legs of the -15v 7915 regulator... I 
also 
> > found
> > >   >two partially fried 1ohm fuse resistor on the +15v rail. Any 
> > idea what
> > >   >can cause it ? Is there any chance that the transformer is 
bad ? 
> > What
> > >   >is your bet as for the responsbile part ?
> > >   >
> > >   >I uploaded the diagram to the Files section.... any help 
would be
> > >   >appreciated !
> > > 
> > >   Tantalum capacitors are particularly sensitive to overvoltage 
> > (and tend to
> > >   fail in a rather spectacular way). Obviously you need to test 
> > what voltage
> > >   is at that point. Disconnect the power supply from the synth 
to 
> > avoid
> > >   causing damage. 78xx/79xx regulators need at least about 2 
volts 
> > of
> > >   overhead to operate properly, and the LM723 needs at least 3, 
so 
> > the input
> > >   needs to be at least 18 V. Specifically, the voltage doesn't 
> > really matter
> > >   (as long as it doesn't exceed the rated voltage of any of the 
> > components),
> > >   but it determines how much power will be dissipated as heat, 
so a 
> > very
> > > high
> > >   voltage is more indicative of a fault condition.
> > > 
> > >   The 1 ohm resistors may have failed for a different reason. 
Their 
> > purpose
> > >   in the circuit is to tell the regulator chip (LM723) how much 
> > current is
> > >   being drawn, and establish a maximum limit. Obviously there 
was 
> > too much
> > >   current. A short-circuit on the output shouldn't blow the 
> > resistors, it
> > >   would just cause the regulator to limit the current. This 
leads 
> > me to
> > >   suspect that maybe the transistor failed, or the regulator.
> > > 
> > >   The transformer may have a shorted primary winding that 
caused the
> > > secondary
> > >   voltages to go too high. In that case, ALL of the secondary 
> > voltages are
> > >   too high, but maybe the +/- 15 V parts of the supply were the 
most
> > >   sensitive.
> > > 
> > >   __________________________________________________________
> > >   The average US Credit Score is 675. The cost to see yours: $0 
by 
> > Experian.
> > > 
> > > http://www.freecreditreport.com/pm/default.aspx?
> > sc=660600&bcd=EMAILFOOTERAVE
> > > RAGE
> > > 
> > > 
> > > 
> > >   
> > >   --------------------------------------------------------------
----
> > --------
> > > -------------------------
> > >   Panda Antivirus + Firewall 2007 ha detectado que este mensaje 
> > puede estar
> > > falseado.
> > > 
> > >   Te recomendamos la maxima precaucisn ya que los mensajes 
> > falseados pueden
> > > indicar la existencia de un intento de estafa.
> > >   --------------------------------------------------------------
----
> > --------
> > > -------------------------
> > >
> >
>