Strange: Preamp Xformer Overheating but Not Blowing Fuse and Supplying Right Voltages

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dtm1962

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Joined 2003
#1
I have a B & K preamp with an outboard Power Supply ( Basically a transformer with 25V + 25V & 10V + 10V secondaries) that is overheating (extremely hot melting adjacent plastic fuseholder and its own clear plastic insulation BUT..... still supplying the correct voltages and not blowing the small 0.5A primaries side fuse.

It gets hot even with the preamp load disconnected!!!!!

Can someone tell me what is happening???

Has anyone experience this situation?????

Is the insulation on conductors shorting out or what???

I included a photo of the damage as well.

Any help would be appreciated.

Thanks in advance,
 
#2
First, that fuse is too big for your app- obviously it isn't doing its job if things are melting and the fuse isn't blowing. Let's see, 1/2 A at 120V is 60 Watts. 20 watts is plenty enough to heat things up, so you'd better use a smaller fuse. The whole point of a fuse is to prevent things from getting hot enough to melt or start a fire.

It seems like you have a short circuit, probably on the secondary side- look for a shorted rectifier or filter cap- you may get nearly the right output voltage, but it will have a lot of ripple under load.

I_F
 
#3
Hi,

If the secondary components and voltages are OK, it is probably due to a shorted turn, probably on the primary side.
There is little you can do, except replace it (unless you're ready to dismantle and rewind the whole transformer).
LV
 
#5
I finally got around to measuring the primaries and secondaries.

And guess what I am measuring resistance of about 20 ohms between the primary windings and the secondary windings which I believe is not right.

So I assume this is the problem of why the transformer gets extremely hot and eventually blows the fuse.

Any comments on my findings.

Thanks in advance,
 
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