Xerox D-series workstations

From: Don Maslin <donm_at_cts.com>
Date: Sat Nov 6 17:19:46 1999

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]
>
> > >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
>
> 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...
>
> > 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.

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
 
> 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
>
>
Received on Sat Nov 06 1999 - 17:19:46 GMT