Re: Upgrading ... a IIGS-- power mods

[Rubywand <rubywand@swbell.net>]

Obsbedia2 writes ...
>
<Rubywand wrote>
> > Is the ROM 3 a better machine? Yes, it is. On the other hand, once you add
> > the motherboard Slot jumpers to improve +5V and +12V power distribution,
> > the ROM 3's quieter bus advantage disappears. (In fact, ROM 3 users who
> > pile in Slot cards should probably add the jumpers, too.)
> 
> News to me.  Please, tell me more about these Slot Jumpers.
 ....

     The jumper mod consists, mainly, of #18 gauge wires run from the power
connector to Slot 3 and from Slot 3 to Slot 7 for +5V and +12V. The idea is to
reduce the resistance between the voltage sources and Slot cards in order to
reduce voltage drops and noise. Noise spikes can produce errors and crash the
system.

     On a ROM-01, the jumpers are a nearly certain requirement if you use an
accelerator and/or boards like a PC Transporter or Second Sight (not sure about
boards like the RamFAST and Apple HSS SCSI controllers).

     The ROM 3 is supposed to have heavier power distribution traces (as
evidenced by audibly less noise through a Stereo Card).  Even so, it is difficult
to believe that the same designers who continued to install wimpy lead power
supplies got it right for the Slot bus. (About the only way to be sure is to load
the Slots with a few power-sucking boards and measure voltages at the boards plus
look at noise levels with a scope.)

     Adding the jumpers usually goes along with removing the bottom metal shield
(for more space below the motherboard and to eliminate a shorting hazard).  The
mod is discussed at http://home.swbell.net/rubywand/Csa2POWER.html .


     In any case, the jumpers are not the most important power-related
modification. The +5V and GND leads from the power supply to the motherboard are
too skimpy for nearly any IIgs setup beyond a 1MB system with a Stereo Card. The
+12V lead is less critical but could prove too skimpy when using some boards
which add a 12V load.

     When a power supply lead (mainly +5V and GND) is too skimpy it acts like a
resistor across which significant noise spikes develop. DC supply voltage is also
reduced, which lowers noise margins at IC inputs. The result is likely to be
random system crashing.

     Meanwhile, your power supply is getting mixed signals about loading, which
could lead to excessive currents inside the power supply which could, eventually,
ruin it.

     The fat leads power supply mod is described in the same FAQs as the jumpers
mod.

     What's annoying about the power supply lead problem is that it's such a dumb
design flaw. The IIgs power supply has plenty of capacity for just about any
setup; but, the undersized leads cut the useable capacity nearly in half.

     If a IIgs seems to work okay, no power supply or added jumper modifications
may be needed. If there are 'inexplicable' glitches and crashes, especially after
adding some new Slot card, the mods are a pretty likely cure.



Rubywand