src@cs.odu.edu (Scott R. Chilcote) (07/13/90)
I had a problem recently where my Mega 4 flat-out DIED while booting up
recently. It got about ten seconds into my hard disk boot sequence, and
went out like an expired coupon. No display, no LEDs lit, no fan running.
Let me explain: I know something of hardware. Before I panic, I look for
blown fuses. I've had my Mega open before (installing TOS 1.4) so that
helped. I unscrewed the cover, unplugged the battery, untwisted the metal
shield, and stared at the power supply. Blown fuse! I plucked out the
tiny thing and zoomed off to Radio Shack. It's a fast-acting 2 Amp fuse,
Cat. No. 270-1244.
I get home, plug in a new one, reconnect the Mega, and turn on the switch.
NOTHING. With the ``impending doom'' feeling I dig my way back inside the
unit, and as expected, the fuse is blown again.
It's time to get technical now. Fortunately, I've had some experience. To
rule out the computer, I unplug the power supply from the mega, and physi-
cally remove it from the system. I install another fuse, and plug it in
separately. By the way, the Mega's switching supply has the power switch
and the AC plug incorporated, so when it's removed from the box you still
have these. I plug it in, turn it on, and poof! The fuse goes again.
Once again, experience helped. Now I knew the problem was in the power supply
itself. In power supplies, one of the things that seems to go most often is
the rectifier diodes. The Mega's switching supply feeds the AC line to a diode
bridge directly, for full-wave rectification. With a meter, I started testing
the bridge (one unit with four leads) and sure enough, one of the AC lines is
shorted directly to the + output line!
Well, I'm using the Mega now. I made another trip to the Shack and put in
a nice big replacement bridge. This part is labeled DB1 on the power supply
PCB, and the part itself is a small black cylinder labeled `W10M' in tiny
white letters. The one I replaced it with is a much beefier model, which
handles 400 PIV at 4 Amps. When in doubt, overestimate.
I'm not saying that all suspicious deaths of STs with switching supplies will
be caused by this part, but it'd be a good place to start looking. With a
little bit of careful checking and soldering, you can save a lot of waiting
and $$$!
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Scott Chilcote
src@cs.odu.edu