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In article <740@tekirl.TEK.COM>, donch@tekirl.TEK.COM (Don Chitwood) writes:
> 
> My personal Mac at home has developed a rhythmic ticking, roughly a 1 second
> cycle.  I opened the case yesterday and used a plastic stethoscope to home
> in on the sound.  It was coming from the transformer on the power supply
> board (the one mounted fairly low, and probably the main supply transformer.
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> 
> Don Chitwood
> Tektronix, Inc.


Some time ago I found the same problem.
Since my home mac is slightly different from the standard mac (It has a series
stabilized PSU i.s.o. a concept with flyback transformer) the problem could
be heard a bit better.
After some  investigation I found out that there were "dips" in the 12 volt
Power Suply Line. When expirimenting with internal and external drives, 
Standard Sony and 3rd Party Drives the following conclusion came up:
After Reset the power dips occured when a disk drive (Sony Only) was attached
to the Mac, no matter if it was internal or external.
With a 3rd Party drive (don't remember the brand anymore) these problems did 
not  occur.
This is because the mac is looking for a drive to use, and tries to select
the internal or external drive, thus making the Enable line active.

When the drive is running/spinning (Sony drive, further refferenced as
 "the drive") , no dips in the 12 V. line. (because the drive is selected
continiously)
When the system has started upp (i.e. system loaded) the problem still occurs,
but may dissapear after some twiddling with the drive lines (CA0, CA1, 
CA2, Select, Enable ). This was done using a disk-drive diagnostic utility.

The problem lies in the fact that every time the line Enable is activated
a dip on the 12 volt line occurs. It seems that the 12v line is shortcutted
for a very short time (+/- 1 ms). This results in  a high current floating
through the 12 volt line and has impact on the rest of the mac, wherefrom
you hear the "click tik ticking". The frequency you described is the same
as the one I noticed.
This points probably to a faulty hardware dessign (interaction) of the IWM 
and Sony disk drives.
Or the Sony drive takes to much current on being enabled.

The ticking and power dips / high current peaks were verified
by a friend who's working as a repairsman with a Apple 
dealer, and he found the same things, but not as sever as with my
Mac 'cause the Mac's PSU can give a higher peak current on their 12 v

The dips seem to have no further impact on a proper working mac.
The extra power dissipation in the PSU due to this dips (high current)
is probably minimal.

Hope this helps (?) a bit.

Piter Jonker,
Hilversum, 	(A center Town of ) Holland.

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