[sdiy] Weird Prodigy problem

[David Brown]

In looking at the schematics, it has a two bus keyboard where one bus is at +12 volts to generate the trigger.  The base of Q18 is pulled down by R77 and the switch closure pulls this back up to +12 volts.  I don't see how any obvious part faults might pull so much excess current, but I suppose the keyboard might have developed a fault where it is pulling down on the +12V rail directly. I would measure the voltages on the keyboard, both at P2-1 and P2-5 when you depress a key and see what the voltages do.  P2-5 should switch from 11.3 volts to 12 volts when a key is depressed.  Since the +12 drops to +10 I'm not sure you will be able to see anything useful.  Perhaps you can disconnect these two wires and manually connect the pads together to validate that the circuit operates correctly.  This would eliminate the keyboard as the fault.

Dave


-----Original Message-----
>From: Louis van Dompselaar <louis at dompselaar.org>
>Sent: May 2, 2011 11:57 AM
>To: sdiy diy <synth-diy at dropmix.xs4all.nl>
>Subject: [sdiy] Weird Prodigy problem
>
>I have a weird problem with a Prodigy that I have here to have cv/gate installed.
>I'm not sure if it's anything caused by the mod or something that has occured for unrelated reasons.
>
>The +12V power drops to about 10V when you press a key, jumping back to +10V about half a second after release.
>Strangly enough, it occurs less often once the Prodigy warms up, so it's not a short somewhere in the gate circuit.
>
>I have already replaced the power supply capacitors and the 78M12 for good measure, so it's most likely something in the Prodigy that is causing this.
>
>Any suggestions?
>
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