Vintage Synth Repair

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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