Ensoniq
Diagnosing an Ensoniq Power Supply
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If your Ensoniq keyboard is dead - no display, no LED's, no apparent activity whatsoever when turned on - then this indicates a likely problem with the power supply. The earlier Ensoniq units used an external fuse mounted on the back panel, so first open this fuse holder, and make sure that the fuse is still good.
If all is well there, it's time to open up the keyboard and check whether any fuses on the power supply are blown; this is most easily done with a continuity tester (found on most multimeters). Set your meter to beep when it detects continuity, then with the keyboard powered off and unplugged, touch the two meter leads to the opposite ends of each fuse on the board. Beeping means the fuse is fine; no beeping means it's time to replace the fuse. Be sure you use a new fuse of the same rating! The fuse ratings are sometimes specified on the power supply circuit board; if not, check the Service Manual.
If fuses are blown, does this mean the power supply is bad? Well, it depends... Replace the blown fuses, then unplug all of the plugs from the power supply except those directly from the transformer. Then turn on the power and see if the fuses hold. If any fuses blow now, then you will need to replace the power supply or have it repaired. If not, then it's a good idea to test to make sure that the voltages are correct - both those from the transformer, as well as the output voltages from the power supply; refer to the Service Manual for the specifics.
But if you don't have the equipment or skills to do this voltage testing, you can proceed anyway. If the power supply works okay "unloaded" (i.e. with no other boards plugged into it), then something farther down the line is probably the source of the trouble. So you can now start to plug in the other cables that run to the power supply, one at a time - those that feed the main board, the display board, etc. Plug in one cable, turn on the keyboard, and see if fuses blow. If not, plug in the next cable and try again. When and if fuses do blow, you know that the trouble is with whatever board you just plugged in.