Re: Blown power transformer in Denon DRA-345R Tuner Amplifier


Arfa Daily Wrote:
"Michael Black" et472@xxxxxxxxxxxxxxxxxxx wrote in message
news:epmklr$omj$1@xxxxxxxxxxxxxxxxxx
GavinI (GavinI.2l7hkt@xxxxxxxxxxxxx) writes:-
Yukio YANO Wrote:-
GavinI wrote:-

-
Check for a blown INTERNAL fuse in the Transformer housing !

Yukio YANO-


Thanks for the suggestion Yukio.

If there is a fuse it's hidden well inside the transformer
unfortunately.

-
I don't think you are looking for an actual fuse. It would be a
piece
of wire (or the whole primary winding) that has such high relative
resistance
that if too much current is drawn, the winding opens. This is the
same
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 transformer
opened,
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.

Michael
-
It usually is a *proper* fuse in as much as it is an encapsulated
component
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 the
windings,
close to the transformer core, where you can't get at it. Many such
transformers have the legend " Thermal fuse fitted " stamped on them.
Often,
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 third
pin
has the remaining winding connection. In these cases, a safety resistor
or
convential wire ended fuse can be TEMPORARILY wired across the pins to
which
the thermal fuse is connected, to allow the rest of the unit to be
checked
for problems. It is however, very unusual for there to be any other
fault,
particularly on Denons. Their transformers just go open for the hell of
it.
I replace probably 5 or more a year.

As far as calculating winding voltages from smoothing cap ratings, it
is a
valid technique, but a couple of "rules of thumb" need to be applied.
You
should first take a value of about 2/3 or 3/4 of the cap voltage
rating, so
if it's a 63v cap, a figure of say 45v. This then needs to be
multiplied by
about 0.7 to get back to an RMS value for the winding voltage, because
the
rectifier will produce a peak voltage from the RMS voltage, and the
smoothers will be rated to withstand this. That gets you back to 31.5v
RMS.
Knock another couple off to allow for unloaded over-voltage, and you
would
come up with a transformer winding of 30v RMS nominal, which would
produce a
nominal unloaded DC rail of about 45v.

However, as I said before, you are unlikely to find a suitable
multi-winding
tranny with the correct ratings, physical size and safety approvals '
off
the shelf ', unless that shelf is in a Denon component supplier's
premises
....

Arfa



Thanks for your comments & v.good advice Michael & Arfa

I have worked out the supplies for the control electronics (around 15 -
20V) & the PA section (80 - 100V centre tapped). The 3rd o/p is proving
to be a little more problematic. It doesn't appear to be rectified &
only measures 15ohms accross the cct side ie - with the transformer
disconnected. It appears to supply stuff around the vacuum fluoro
display. I'm guessing the display needs a fairly high voltage, or it's
stepped up somewhere (perhaps this is the reason I'm measuring the
15ohms; ie accross a coil/transformer winding?).

I appreciate now that it will be hard to find an equiv. as you've said
Arfa.

Gavin




--
GavinI
.

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