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Re: opening the 2c brick psu

Roger Johnstone wrote:

In <1176613311.401890.12900@o5g2000hsb.googlegroups.com> phil wrote:


You don't mention whether the 12v power supply you used was regulated,
or what the actual voltage was when the //c was switched on, but if it can supply 12v under load, it should work. 

OK, I found the problem..
When I crimped the strain relief in the back of the power in DIN plug,
It shorted the plus with the housing ground, so plugging in the lead
in shorted my 12V psu. Oops. The fuse f1 (yellow, behind power switch
inside 2c unit) did pop also, so if anyone knows what to replace it
with, that would be good. I'm using a 2A one for now...


Glad you found the problem.  That shouldn't have blown the //c fuse,
though.  Have you checked the original brick after replacing the fuse?
(Or, *maybe*, some fault in the brick is what blew the fuse in the
first place...)
According to the Apple IIc Technical Reference Manual it's a 10 amp fuse. That seems ridiculously high to me, but it doesn't look like a misprinting and 1.0A would be too low. The manual says the internal switching power supply can run from 9V to 20V and consumes a maximum of 25W, so at 12V it could draw peak currents of around 2A. 

I haven't checked the schematic, but I would expect there to be a
large-value electrolytic capacitor just past the power switch and
fuse (probably inside the power supply).  The high value for the fuse
may be used to prevent early fuse fatigue due to repeated inrush
current surges.

In steady state, 3-4 amps should be fine.

My recommendation would be a 4A Slo-Blo fuse.
Check that the diode CR1 isn't shorted. It's a reverse polarity protection diode connected from the positive to negative supply input lines immediately after the fuse. 

Assuming thats a power diode (1A or more forward current), then it
should quickly blow the fuse if a reversed supply is attached,
protecting itself from overtemp damage.

If I wanted to blow that diode, I'd apply a high DC (or AC!) voltage
of the *expected* polarity, to cause it to break down in zener mode.
Then in less time than the fuse could blow, the diode would be toast.
Of course, the internal power supply would also be toast.  ;-)


My 15V brick outputs no power at all, so if anyone can please give me
a hint on where the plastic casing is welded together... Thanks.
I had to open a IIc power brick up a few years ago. From memory the two plastic halves are glued together around the entire seam. I used a vice to repeatedly squeeze the brick around the seam until it broke and I could get a screwdriver in to lever the two halves apart. The only other option is to use a fine-bladed saw to cut it open. Designers (or accountants) who save a few cents on screws by glueing cases together: first against the wall when the revolution comes! 

I hear you!  But, as you can imagine, sealing line voltage circuits
is regarded as a virtue in the consumer electronics world.

-michael

NadaNet networking for Apple II computers!
Home page:  http://members.aol.com/MJMahon/

"The wastebasket is our most important design
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