It's been a LONG time since I've signed on, let me tell you! It's been years, hehe. Anyway, I've been wondering about something.
On the Macintosh LC/Performa 5xx/6xx series, I've read that as long as the power button on the back of the unit is turned on, there is trickle voltage supplied to the motherboard (I assume a trivial amount). How is this voltage supplied? I am trying to figure out whether the power is supplied directly through the designated power wires (yellow, red) or rather supplied separately through a different wire on the wiring harness? Also, how much is supplied, 5v, 12v? Is the trickle voltage also related ot the "Soft Power On" rather than just maintaining the PRAM?
When talking about the LC 500 series and the CC respectively (built-in screen), I know that power is supplied by the analogue board of the system via the analogue board connector. The analogue board connector also provides additional wiring going from the logic board up to the analogue board for different reasons, such as video, audio, etc. but some pins also seem to carry current (+5v, -12v). What are these used for?
Also, when using the "Soft Power On" key to crank up the system, what exactly happens in the system (how does the PSU know that it needs to cut on, how does this information reach the PSU from the logic board). Is there a specific pin on the logic board that controls this? I've been Googling on the web for ages for information but it's rather scarce.
I know I have a lot of questions and I'm not expecting all these to be answered, but I'd love to hear from you guys 
I was discussing this with Trash80toHP recently, and we both found that most of this information is now gone from the net.
He's doing a project that involved finding this information, so if he doesn't chime in here, you might find him at the 68Kmla.
From my somewhat foggy memory, the Performa/Centris (including CC) boards were much like ATX power supplies, in that one pin supplied a trickle voltage. There was also a sense line that uses the trickle to tell the power supply to go fully on. Where my memory fails is how that process worked: I can't remember if those Macs use sense line to power or sense line to ground to go on. Trickle should be +5V.
Disregard if that was just a typo or I'm reading it wrong, but all power goes from the analog board to the logic board. No power is supplied back the other way.
Hey eeun!
Thanks for the update. I couldn't wait much longer today so I had to test a few things. I finally
brought out my rig after a long period in stasis in order to see if I can figure the whole "Soft
Power" deal out. My CC which has been modded with a 6500 logic board has an additional ATX PSU
attached to the harness in order to supply the much needed +3.3v/+5v. It is attached internally to
the power switch on the analogue board and the PS_ON pin on the ATX supply has been bridged in order
for the PSU to work.
The issue with this setup is that the "Soft Power On" does not work this way, since there is no way
of jumpstarting the ATX PSU. I have to use the actual power switch on the back of the unit to get
both the analogue board and ATX PSU going. Sometimes when I do this, the logic board gets powered up
before the analogue board can do its magic, since the ATX supply is the main power source. When that
happens, I have to do the CTRL-Flower-Power sequence to do a warm restart, which at that point it
will work and power up correctly.
Also, neither the analogue board nor the ATX PSU will receive any power used for e.g. tricke voltage
while the power switch is turned off. Since the ATX supply is providing +5v power to the logic board,
and since trickle charge is a +5v current I figured that I couldn't use the keyboard's Soft Power
button.
Having said all that, I went ahead and tried something out. I disassembled my Mac, unplugged the ATX
PSU, left the logic board attached, and connected my apple keyboard to the system. I flipped the
power switch on, and nothing happenend (since the CC at this stage is expecting Soft Power input to
start). I pressed the power button on the keyboard, lo and behold, I heared the familiar clicking
sound of the PSU trying to fire up the system (which it couldn't since the analogue board's +5v lines
were not connected).
This made me suspicious, therefore I went back to the correlation table and wiring diagram and
noticed that PIN 5 on the analogue board connector (S3 on the 6xx harness) supplies "ADB Power
supply" and PIN 7 (S5 respectively) supply the "Power Off" command. The diagrams also show voltages
for several lines, therefore I think it is safe to assume that the analogue board connector *does*
indeed supply power to the system for this specific purpose using the harness rather than the regular
power lines?
----
Now, specifically regarding the trickle power. From what I've read so far, tricke power is used to
maintain the PRAM information as long as this low current available. When it is not available, it
switches over to the battery which will do the same thing until power returns (or the battery is
used/recharged while trickle is available, no clue). The dilemma in my situation is, when I power off
the system, there won't be *any* trickle power for the system to use due to the missing +5v lines
from the analogue board (since they're supplied by the ATX PSU), unless of course, it is also
supplied through the harness. That though, I have not been able to test yet.
All in all, I do think that using a relay to switch on the ATX PSU is the most elegant solution in
this case, the only downfall would be that fact that if trickle power is not supplied via the harness, I would have to replace the PRAM battery more often, especially when the system is not
in use for an extended amount of time.
Anyhow, talk about being verbose eh *laughs* ... woah.
Yep, there is power supplied to the logic board through the smaller edge connector pins, since soft power couldn't be supplied on the main 5V and 12V rails, which do the heavy lifting once the power supply is officially on.
Such a long time since I built my Takky...wish I'd kept better documentation on how I did it!
I used a relay for mine. My second power supply was a low-wattage PSU from Best Electronics that supplied 3.3V and powered the case fan. I had problems with the Mac's soft power not working unless I put more load on the analogue board so it powered everything else. There were also some ground noise problems that were resolved by limiting use of the second supply.
The relay was activated by the 12V rail from the analogue board, and switched on 120V to the second PSU. That way the soft-power, trickle, etc. to the logic board isn't an issue. Something like this, as an example of using a relay.
In your case, you could use a relay to short the ATX pins you currently (ha!) have connected to the CC power switch. The analogue board would always power on first, and using the ATX power-on trick that you're already using would mean the relay doesn't have to involve any 120V jiggery-pokery. You could salvage a relay from old electronics without worrying about power rating, and that would solve your soft-power and pram concerns.