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Does that APU have a 'De-Oil Solenoid'....?
Its a long long time since I worked on B727s and I honestly can't even remember what kind of APU they have fitted, but from years of experience on the old 737s I always reckon those old style 'steam driven APU's were a lot easier to troubleshoot than many of the modern FADEC controlled equivalents as the APU start sequence was done in a series of methodical steps so it was easy to determine the faulty component by analysing exactly how far into the sequence it got. For example you would have something like, switch on, fuel SOV open, inlet door open, starter engagement, oil px rise, ignition on, fuel in, timed acceleration to 50%, starter cut out....etc etc.... It was relatively easy to confirm each step and I'm sure the 727 must be somewhat similar.
What rotation speed are you getting..? Does the APU have a feature where if it hasn't completed the sequence in a certain time it abandons the start..?
Have you tried rotating the APU by hand? Does it feel stiff to turn when compared to another? Does it have a mechanical (or electronic) speed switch for controlling the start sequence?
What components have you already changed? Back in the old days we cured most APU problems by changing the FCU, load control valve, surge bleed valve, speed switch and occasionally a weak starter motor.
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