Vintage Synth Repair

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.
> > 
> >   __________________________________________________________
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> > 
> > 
> > 
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