"EADGBE" wrote in message
ups.com...
I have discovered a fault on the power supply board - in fact, it's
the only possible fault there could be, other than the transformer.

And it looks like my initial panic about a defective transformer MIGHT
have been an overreaction.

First of all, I must apologize for my hasty (and incorrect) assumption
that there were TWO 3.5v taps, connected to common ground.

This was NOT true. The two blue wires coming from the transformer are
ONE complete AC supply.

The GOOD news: This 3.5v tap is NOT dead. It DOES produce voltage.

The PUZZLING news: Instead of the 3.5v AC specified in the service
manual, I am measuring a steady 7.97v AC.

HERE IS THE FAULT I DISCOVERED: As I mentioned before, the two wires
of the 3.5v supply (which is actually measuring at 7.97v) are each
connected to their own 1-ohm 1/4-watt "R-FUSE" before proceeding to
the jumper wire that goes to the flourescent display.

I don't really know what an "R-FUSE" is, actually. They just seem to
be regular dull blue 1-ohm metal film resistors. But the schematic
symbol shows a small rectangle with symbols for a fuse and a resistor
inside the rectangle, connected in series.

Is it possible that these are indeed metal film resistors BEING USED
AS FUSES as well as being a voltage drop for the "3.5v" supply which
is actually 7.97v?




Fuse resistors are pretty common. They are not meant to be replaced in the
same way as a normal fuse as they do not normally burn unless there has been
a serious problem. (Fuses often just get tired and blow when they haven't
been told to). You have to correct the problem, then replace the fuse
resistor. If yours are in series with the filament supply and one has
blown, it is because FAR TOO MUCH CURRENT HAS GONE THROUGH THE FILAMENTS AT
SOME POINT.

AND EXCUSE ME FOR SHOUTING AS LOUD AS I CAN.

So it does look as though the unit has suffered some kind of over voltage
thingy, and the filamant may now be a piece of curly toast. Unless of
course the problem is just that the cabling and circuit board from
transformer to display has a fault instead. Go and check the resistance of
the filament right now, as I suggested earlier on. This may all be academic
if you need a new display.

Incidentally, fuse resistors are not just resistors. They are flame proof
so they wont make a lot of alarming smoke and a cringeworthy smell, and have
been designed as a fuse, not a resistor.


Gareth.