6 Nov
1999
6 Nov
'99
9:19 a.m.
On Sat, 6 Nov 1999, Tony Duell wrote:
________O/_______
O\
I am sending this to the list, because I want to make
a small point.
Obviously I like old computers. And I don't mind receiving e-mail
(questions or help) relating to such machines -- that's why I am here ;-).
________O/_______
O\
[Transformer that blows fuses]
Don't worry about it.... We all make mistakes, me more than most.
The good thing is that now we know what's going on, and it makes sense.
So we can go forwards...
Would it not, perhaps, be a good check to disconnect the paralleled
windings and check them out individually to determine whether the two
input or output windings might be bucking instead of properly
paralleled. Resistance checks might also indicate if there were shorted
windings in one.
- don
Are you
_absolutely_ sure about those connections? I assume you've
checked them visually, and not relied on continuity tests (remember the
transformer might be shorted).
I was wrong, of course. I had done it visually, but missed a trace under the body of the transformer, *and* mis-oriented
the switch leads when I
checked them. The wiring is actually absolutely straightforward:
N------------+--1(| t |)5--+----fuse2--A front
L--fuse1--+--|--2(| r |)6--|--+--------B panel
| +--3(| n |)7--+ |
+-----4(| s |)8-----+
OK, this makes a lot of sense. I think it's safe to assume that there are
2 primary windings, which are connected in parallel for 120V mains (and I
would guess in series for 240V mains -- this is a very common setup).
And 2 identical secondaries, also in parallel. And the transformer is
always energised, which explains why the fuse blows when you plug the
machine in (without having to turn the machine on)
(Sigh. I'm still at the point where I
can't tell that obviously wrong
things are wrong.) This seems to rule out everything but the transformer
:-(
If fuse 1 blows even with A and B disconnected, then it's very likely
that the transformer has shorted turns. Still, all is not lost. There are
ways to make some likely guesses as to what the transformer should be. Firstly look at the physical size of the transformer
and compare it with
ones in catalogues. From the fact that it's a PCB mounting unit, I am
going to guess at 3VA or 6VA. Something around that, anyway.
Then remember that fuse2 is a 3/4A fuse, so the total secondary current
is <3/4A. If it's a 6VA transformer, that's beginning to sound like a 12V
0.5A unit or something close to that.
Look at what it's connected to. If it goes to a rectifier/capacitor
arrangement, what's the working voltage of the capacitor. That puts an
upper limit on the secondary voltage. Remember, of course, that the peak
voltage on the output of a rectifier = the _peak_ AC input voltage, which
is sqrt(2) times the RMS voltage.
If all else fails, dismantle the old transformer, unwind the wire, and
count the turns. Then work out the turns ratio between primary and
secondary. Although there will be more turns on the secondary than you
might expect (to compensate for voltage drop when you start drawing
current), this will give a good idea of the expected voltage. Given that,
it's easy to get a replacement transformer.
-tony