Re: //e power supply voltage levels

[mjmahon@aol.com (Michael J. Mahon)]

Bill Garber wrote:

>I don't mean to sound insulting, but why isn't anyone
>suggesting checking of fuses. That is the first place I'd
>look. Also in most power supplies there is a rather large
>resistor in the line between the AC cord and the main
>transformer. Another fault could be the main capacitor
>just after the transformer. 

The Apple ][ supply is a switching power supply, so there is no
transformer on the line side of the supply.

There is an internal fuse, then a half-wave voltage doubler,
and then the switcher.

Usually when the fuse blows, it means a capacitor or a
switching transistor, or both, are shorted, so replacing
the fuse has a low probability of success.

The most common cause of old Apple supplies failing is
loss of capacitance or increase of equivalent series resistance
of one or both of the capacitors in the voltage doubler.  If
they go too far out of spec, the supply will not start under
load.

If you are justifiably confident of your ability to repair a
supply, try replacing the capacitors with ones scavenged
from old one-time-use flash cameras--they have 330+ volt
ratings and are about 100 microfarad or so, so should do
the job nicely.

-michael

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