Preamp - Overheating Xformer (Without Load) but Not Blowing Fuse

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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,


  • b&ktrans1.jpg
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Look for shorted diodes, bad capacitors and such, if they're part of the circuit. Most likely it has a shorted turn somewhere. Also, if that bolt through the center touches the disk, and the disk somehow touches the chassis, that would create its own shorted turn. If you put it on a bridge, you could see the shorted turn by the high dissipation factor. Anybody got any other simple methods?
Joined 2003
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I'd go with th e shorted turn theory - but strange that th eprimary fuse is not blowing.

Another suggestion: disconnect the transformer secondaries from the rectifiers (i.e., secondaries open circuit). If it still gets hot, its th e transformer. If not, its in the rectifiers or caps.

good luck.
Joined 2003
Paid Member
I just took anothe r look at th e phto - looks like there is no rectifier or caps in the PSU box. If this is indeed true and its getting hot with no output connection, the problem mus t lie around the transformer.

If you run the supply as shown in the photo with the transformer out of th e box and it still gets hot, you probably have an internal transformer problem - shorted winding etc. If it run s cool - your transformer mounting is a problem as described earlier in this thread.
So you think the insulation has failed and it is shorting out on the core or adjacent/overlapping conductors?

The voltages are still 25V and 10V as specified.

Does this shorting affect the output voltage????

I still can't understand why the fuse does not blow when connected OR NOT connected to load. It heats up enough to melt plastic!

Thanks in advance,
Yes you are correct there is just the noise filter and fuse on the primaries and nothing other than the 5 pin connector (to mate to preamp on the secondaries. The bridge & the PS Caps are in the preamp and will not affect the load if the cable is not connected from this outboard PS Box.

Did you try disconnecting all the secondaries? There could be a short in the cable or connector. If a secondary with relatively low power is shorted that winding can overheat without blowing the primary fuse.

A shorted turn won't change the output voltage much. It is possible for the fuse not to blow, the fuse will allow 50W/100W (depends on line voltage) to be dissipated somewhere. If dissipated in a localized spot even less than a tenth of this will be more than enough to melt the insulation. If it has a shorted turn the heating would tend to be localized. Is the whole transformer getting hot after a little while or is it a localized area that is getting hot? A shorted turn will get hot pretty fast compared to overloading a transformer.
WTB: Sanken SAP15N & SAP15P (or STD03N & STD03P if compatible)

I have the Cambridge Audio A500 integrated amp with the blown output devices.

I cannot find a source to purchase unless I buy 1000 min. quantity.

The distributor will NOT supply samples to me/my company of either set. (assuming the SAP versions is obsolete)

Does anyone have 2 pairs of either devices to SELL in order for me to use in my amp.

Can I use the STD03 series without too many mods ? (Add a resistor on the emitter etc.)

Any help would be appreciated.

Ignore last post as I entered in this thread versus an new one in the Market Place.


Anyways I will be spending some time looking at my problem with the Power Supply.

I will take measurements to determine if it is the transformer shorting out or the cable has a problem.

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,
Other possible causes

For anyone reading this with a similar problem (I understand Dan´s problem is solved) an overheating transformer could also come from:

- DC component on the mains (especially bad for toroids). Possible cure (not tested myself): antiparallel diodes with big caps in parallel
- if there are two primary windings, check for their respective orientation, could be the other way than you think (from experience ;-) )

Maybe others can add...

Hope this helps someone
If you're getting a reading between the primary and secondary windings then the transformer is toast, the insulation between the windings has failed somewhere. This creates a hazardous situation other than the overheating because depending on where along the winding the short is, you may have a path to the mains 120VAC on you output, electric shock hazard by touching the amp and ground!

Normally these transformers should be isolated (ie, no conductance between the primary and secondaries.
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