This keyboard wouldn't turn on. The external power supply provided the correct voltages (+-24), the power inlet jack was good, there were no blown fuses.. the power switch was good... this was a perplexing mystery. Although I didn't have a schematic, I was able to figure out that there was a regulated 12 volt rail that was always turned on, and when the power switch was closed a relay was connected to this rail, and the relay turned on the rest of the unit.
The output of the 7812 voltage regulator, which powered this rail, showed up as a paltry 6 volts DC, not the 12 it is supposed to supply.. An ample voltage of 24 volts was supplied to it.
I verified that the 12 volt rail was not shorted or drawing too much current, so I replaced the voltage regulator... and still found the output to be 6 volts! This was very perplexing, until I decided to check that 12volt rail with a scope, and found that there was massive oscillation coming from the 12 volt regulator.
In other words, the capacitor charged with filtering that 12volt rail had failed, and this is a necessary component when using the 7800 series regulators.
I was able to replace the capacitor and the rail came instantly back up to 12 volts... and the keyboard turned on. A homemade schematic is shown below... notice the diode, which is in place to filter out the inductive discharge which comes from the relay coil when the relay is turned off! This diode protects the rest of the circuit.
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