18 May
2009
18 May
'09
5:59 p.m.
Be careful with the high voltage ones in the input
filter before the
diode rectifier. (A couple of uF, 400 or so volts AC rated - encased in
a clear plastic case) I had an apple 2 recently that had one of these
Mains filter capacitors (these are the non-polarised ones, AC rated,
connected across the power lien, or from one side of the power line to
earth) often fail spectacularly, but do no further damage. They are
certainly not the cause of any output-side problems.
fail spectacularly - tons of brown goo that smelt of
cigarette smoke
(tar?) spewed forth. It was hard to remove. But It is happier now.
Also - regarding the drop the 12V rail - isn't that the one that is
monitored to regulate the switcher?
The 12V line powers part of the regualtor circuit (on the schematic I
have, it's based round Q4 and AR1 -- AR1 is an optoisolator, and the LED
secion appears to be drawn backwards!). But the actual regulation
compares the votlage on a zener doide/normal diode circuit (applied to
the base of Q4) and the 5V line (applied to the emitter). So in fact the
5V line is the one that's actually monitored.
Does anybody have the schematic for these PSUs?
There is a schematic on page 93 of the 'Apple ][ Reference Manual' that I
have in front of me. I can't find any edition number in said manual. This
is a schematic for a 110V-input only version, the 240V version is
presumably different, but may not be very different (different
transformer turns ration, but it might be the same circuitry).
I have never had to repair an Apple ][ PSU, so I have no idea if this
schematic is even close the real thing. I have found some PSU schematics
is technical manuals to be total works of fiction, though (the one in the
TRS-80 Model 3 techncial manual is hilarious!). So you might want to
check it carefully agianst the actual PCB.
-tony