Vintage Synth Repair

>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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