Blown power transformer in Denon DRA-345R Tuner Amplifier

Check for a blown INTERNAL fuse in the Transformer housing !

Yukio YANO

On Mon, 29 Jan 2007 02:06:30 +0000, GavinI Has Frothed:

The rail on a 350 is 35 volts IIRC. Cant help otherwise

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Pierre Salinger Memorial Hook, Line & Sinker, June 2004

COOSN-266-06-25794

The internal thermal fuse is not a replaceable item. Also, the power transformer is a designated safety item, so should not be substituted, if the original approvals of the equipment are to be retained.

Arfa

Thanks for the suggestion Yukio.

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

Gavin

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GavinI

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

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GavinI

The VFD requires two supplies. The main one is in the region of negative 29 ( -29v ) and is usually derived either via a seperate regulator from the main negative rail or, more commonly, from a voltage multiplier fed from one of the transformer general windings. The second supply is approx 3v AC for the VFD filament. This is almost exclusively fed from a separate winding on the transformer, the reason being that it is 'floated' on the -29v DC, to reduce internal inter-electrode stress. I suspect that this is your third winding, and the reason that it has such a 'high' resistance.

Arfa