17 Jul
2000
17 Jul
'00
1:41 p.m.
----- Original Message -----
From: "Adrian Graham" <agraham(a)ccat.co.uk>
To: <classiccmp(a)classiccmp.org>
Sent: Monday, July 17, 2000 7:17 PM
Subject: Mac Classic analogue board
I mentioned this in another thread so I thought
I'd ask the question
properly; I've got a Classic that will fry the glass fuse (and do the
sound
and lights bit too) when one is inserted. Is it a
common fault? I
should've
guessed something was wrong when I bought it - there
wasn't a fuse in
the
plug and it had no case screws, but then I didn't
spot that till I got
it
home.
Haven't seen this in a Classic myself, however violent fuse blowing of
that nature generally points to a problem in the mains side of the
supply. Frequently the diode bridge, (or one the 4 diodes arranged in a
bridge) that rectify the mains voltage is/are shorted. This is
straightforward to replace and generally there are no other components
affected. It could also mean however that the switching
transistor/scr/whatever is crook, they are a little more expensive, but
provided you can procure the right one ok, no real fuss to change,
however you should bear in mind that when this happens, sometimes there
are other components that get fried. I have seen one switch mode supply
where fuse vaporising was caused by a very dead electrolytic (mains
rectifier filter cap) but that's a little unusual, usually they just get
a bit leaky if anything.
Cheers
Geoff Roberts
Computer Systems Manager
Saint Mark's College
Port Pirie,
South Australia
geoffrob(a)stmarks.pp.catholic.edu.au
netcafe(a)tell.net.au
ICQ: 1970476