"Michael Black" [email protected]
wrote in message
GavinI (GavinI.2l7hk[email protected]
Yukio YANO Wrote:-
Check for a blown INTERNAL fuse in the Transformer housing !
Thanks for the suggestion Yukio.
If there is a fuse it's hidden well inside the transformer
I don't think you are looking for an actual fuse. It would be
of wire (or the whole primary winding) that has such high relative
that if too much current is drawn, the winding opens. This is th
principle as an actual standalone fuse, in that inside their casing
there is a small bit of wire that will open up if too much current is
put through it. But the transformers are designed so they'll open up
if too much current is drawn.
The result is not a "blown fuse" but an open winding, which is what
happened to you.
Of course, that leaves open the question of why the transforme
so before replacing it, you'd want to be making sure nothing on
the secondary has gone bad to draw too much current.
As for voltages, you can at least get an idea by looking at the
electrolytic capacitor(s) in the power supply. The secondary will
go through some rectifier, likely a bridge but maybe some other
configuration, and then to some filter capacitors. Those (and they
are relatively big) will have a voltage rating marked on them, and
that rating will be higher than the transformer output voltage. How
much higher is why it only provides an idea.
It usually is a *proper* fuse in as much as it is an encapsulate
with a temperature rating rather than a current rating. It's called a
"thermal fuse", and that is the reason that it is buried in th
close to the transformer core, where you can't get at it. Many such
transformers have the legend " Thermal fuse fitted " stamped on them
the fuse element is actually wired across two terminal pins on the
transformer. One of the power input leads goes to one of the terminals
whilst one end of the tranny primary winding goes to the other. A thir
has the remaining winding connection. In these cases, a safety resisto
convential wire ended fuse can be TEMPORARILY wired across the pins t
the thermal fuse is connected, to allow the rest of the unit to b
for problems. It is however, very unusual for there to be any othe
particularly on Denons. Their transformers just go open for the hell o
I replace probably 5 or more a year.
As far as calculating winding voltages from smoothing cap ratings, i
valid technique, but a couple of "rules of thumb" need to be applied
should first take a value of about 2/3 or 3/4 of the cap voltag
if it's a 63v cap, a figure of say 45v. This then needs to b
about 0.7 to get back to an RMS value for the winding voltage, becaus
rectifier will produce a peak voltage from the RMS voltage, and the
smoothers will be rated to withstand this. That gets you back to 31.5
Knock another couple off to allow for unloaded over-voltage, and yo
come up with a transformer winding of 30v RMS nominal, which woul
nominal unloaded DC rail of about 45v.
However, as I said before, you are unlikely to find a suitabl
tranny with the correct ratings, physical size and safety approvals
the shelf ', unless that shelf is in a Denon component supplier'