Editing Macintosh HDD Maintenance (section)

== Voltage and power supply checks ==

A drive that spins up and then drops out, or that will not spin at all while the machine is
otherwise healthy, is often a supply problem rather than a drive problem — the +12 V rail
sags under the spin-up surge.

'''Two pieces of advice circulate for Macintosh hard-drive supply problems. Both are wrong,
and it matters, because one of them will damage a working machine.'''

'''The rail table.''' "+5 V 4.85–5.15 V / +12 V 11.9–12.7 V" is Apple's '''Macintosh Plus'''
adjustment procedure. It is genuine ''there'': the ''Macintosh Plus Service Source'' has you
fit voltage test cable '''077-0135''' to the external floppy port and read 4.85–5.15 V on the
red lead and '''11.90–12.75 V''' on the orange one — note 12.75, not 12.7. It is not a
general Macintosh specification, and Apple publishes no rail tolerance at all for the SE,
SE/30, Classic, Classic II, Color Classic, the Quadra family, the Power Macintosh family or
any other machine checked so far.

'''The R56 adjustment.''' R56 is the trimmer named in that Macintosh Plus procedure. It lives
on the '''128K/512K/Plus power/sweep board''' and it regulates '''+12 V, not +5 V'''. Telling
a reader to "adjust R56" to fix a hard-drive supply problem is wrong twice over: on most
machines the component does not exist, and on the machines where it does, turning it to chase
a +5 V reading pushes '''+12 V''' out of specification — straight into the drive's spindle
motor supply.

What to do instead:
* Measure at the drive's own power connector, with the drive connected and the machine booted, not at the supply with no load.
* Compare '''+5 V''' against the specification of the 5 V logic it feeds — 5 V ±5 per cent, so '''4.75 V to 5.25 V'''.
* Watch the rails on an oscilloscope while the drive spins up. A rail that dips and recovers points at tired output capacitors in the supply; steady DC with visible ripple points the same way.
* If the supply is out, recap or replace it. There is no adjustment to make on these machines.