Vintage Synth Repair

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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RAGE



  
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