Re: //e power supply

[Rubywand <rubywand@swbell.net>]

Adalbert Goertz writes ...
> 
> re
> I had problems about a year and a half ago when I first installed a
> Focus
> Drive.  I upgraded to high power, 80 watt, power supply which worked for
> a
> while, then peterred out.
> >>
> Adalbert Goertz responds >>>>>>>>>>>>
> >>
> I am wondering whether I would have insufficient power in my //GS
> if I installed a Focus drive.
> Anyone had a power problem with a GS-Focus?
> 

     The Focus will want its share of power; but, it does not seem to be a hog.

     The reason most IIgs power supplies fail appears to relate to a design flaw--
the stupid leads from the power supply to the motherboard are too skimpy.

    "Skimpiness" is relative. At low current demand levels-- i.e. when you have a
pretty bare bones IIgs system with no power grommer plug-ins-- the leads are not
skimpy. The wires are heavy enough to get the job done.

     The +5V output is the most important one on Apple II power supplies. It
powers nearly all of the IC's. Unlike the +12V and other outputs, it needs to
deliver a voltage fairly close to the claimed value of 5V-- something between 4.8V
and 5.1V-- under current pulls ranging from about an Amp up to the rated capacity
of about 4 Amps. If everything is okay on the +5V circuit, you are most of the way
along to being free of power supply problems.


     The standard, garden variety power supply that comes with the IIgs can
produce plenty of juice. Unfortunately, the under-size +5V and Ground leads keep
all of it from getting to the motherboard. Somewhere beyond 2 Amps, the resistance
of these leads + connectors, etc. is enough to produce a significant Voltage drop
and the Voltage at the motherboard will be less than 5V.

     Meanwhile, you run into another problem: the Voltage across the PS leads is
changing to match the switching of logic circuits. So, now, you have Voltage
spikes propagating throughout the system. The higher the current pull, the greater
the drop across the leads, the worse the spikes.

     Upon noticing that the voltage at the motherboard is too low (say, after
plugging in a new accelerator board) some users will try tweaking an adjustment on
the PS to get a voltage increase. (You can do this easily on II+ suppies-- on
others you may need to add a mini pot.) The user tells the PS to regulate at 5.3V
or whatever in order to allow for the drop across the leads. 

     There is no particular reason for the PS to mind being told to regulate at a
slightly different Voltage. It should maintain the new output and take care of
things like over-current protection with no problem. And, so long as the user does
not pull out the accelerator board or some other heavy power user, the Voltage at
the motherboard as read by a meter should be 5V.

     The catch is the Voltage spikes. They are still there only, now, they're
bigger.

     Basically, whether or not you try adjusting PS output, the more current you
push through the resistance bottleneck, the bigger the spikes. These can cause all
sorts of problems in the logic circuits-- for example, a Soft Switch may be
flipped ON when it is supposed to be OFF. Naturally, programs do not work
correctly and the system may crash.

     Evidently, the power supply has problems with the spikes, too. It monitors
the output Voltage and adjusts current flow through components to maintain the set
Voltage value. The spikes could confuse the control circuitry resulting in
repeated brief over-current and over-Voltage situations which might damage some
components. Or, possibly, the Voltage sensing circuit is damaged by the spikes.

     In any case, the evidence is clear. Many, many IIgs power supplies with
under-size leads (including some expensive "Heavy Duty" models) are in the junk
heap. Our original (1986) power supply isn't. We installed fat leads-- 10-12 gauge
+5V and Ground leads plus a heavier +12V lead-- years ago when an accelerator card
was plugged in. Since then, we've added a Focus drive and RamFAST.

     So, if you wanna avoid glitching and dead power supplies, you gotta get Fat
leads!



Rubywand