PSU keeps popping fuses

[Bo.b]

Hi all,

Noob here with a number of successful "paint by numbers" builds under my belt. Still don't really know anything about anything and would be most appreciative of all guidance.

I've got a external PSU for a channel strip that keeps popping fuses. These are both off-the-shelf units that have never been opened/tampered with.
The PSU worked well for years and just went down mid-session(obviously). Fuses are popping with the psu both connected and disconnected from the preamp.
The Fuse holder is in the IEC socket, one side goes to a spst power switch, and feeds the primaries of the transformer. This all checks out fine.

Now, the transformer is popping fuses both with the secondaries connected to the psu board or with the secondaries disconnected from everything(and each other of course!!). The voltage(before the fuse pops) across the secondaries is pretty to spec (24.5v on a 22v secondary) both connected and disconnected to the psu board.

The fuse normally pops after about 3-4 seconds. After this, the transfomer is warm to touch.

I'm getting continuity both across the primaries and the secondaries of the transformer, no continuity between primaries and secondaries though


Am I correct in surmising that my transformer has gone bad? What should my next step be? Should I swap out the transformer? Is it likely that something else caused this or does it sound like the transformer just went(maybe a power surge from the mains or something?)  Am I in over my head? Should I call a tech? Shipping it to my gear pimp would likely end up costing lots and taking ages.

Thanks for reading this. Any suggestions or guidance massively appreciated.

Thanks,

Rob

 

[david-p]

Sounds like you may have a shorted turn in your transformer -- though I have no idea how this would happen if it has been working for a while.

You could connect a mechanical (moving coil) ammeter in series with the primary and watch the current when you turn on (secondary unloaded).  Obviously it should be low, but if you have a toroidal transformer there may be a surge.

David

 

[college101]

your probably replacing the fuse with a lower rated fuse- and the current is too much, which causes the fuse to blow...check the specs on what type of fuse you should have in the unit, and then replace with the proper fuse...

 

[Bo.b]

Thanks for the replies guys.

I'll see if I can get my hands on a moving coil ammeter and have a look for any surge.

With regards to the replacement fuse, I couldn't read the rating on the fuse in the unit, so I mailed the company and they gave me the rating. It's a 250v 1A slo-blo

 

[david-p]

Slo-blow is to allow for any initial surges.

David

 

[abechap024]

If the transformer is blowing fuses not connected to anything, then the transformer is probably bad. And you say it gets warm after just a few seconds? Yea thats not good.

Are you sure the primaries are hooked up correctly? I only ask cause that seems to be the only other thing that could possibly be causing the TX to malfunction with nothing hooked up to it. But I imagine it would have a hard time re-wiring itself mid-session.

Easiest probably just to order a new transformer.

 

[MagnetoSound]

... replace with the proper fuse ...

A quick blow fuse, rated too conservatively, may blow at switch on if the load is connected.

With the secondaries disconnected this should not be the case. This, coupled with the transformer getting hot, suggests a shorted turn as David says.

Why did this happen? Possibly a short downstream, such as the rectifier. These are often spec'd on the cheap and eventually short due to running hot over a period of time.

Why did the primary fuse not protect the transformer? Possibly the rating is too high. 1A is pretty hefty for a single channel strip.

A quick test for shorts at the rectifier and regulator stages should confirm whether the transformer has failed due to overload.

 

[JohnRoberts]

Sounds like a possible shorted turn that may not measure as a fault with an ohm meter but will cause high current draw in use (a shorted turn is like shorting a secondary).  These are generally a failure in wire insulation caused by over temperature.

JR

 

[Bo.b]

Thanks for your replies all. I took a while to deal with this, but yesterday a new transformer arrived. Popped it in and everything's working perfectly. If I've got this right, there was a short in the primaries. It's a dual primary and I'm getting continuity between my primaries.

Don't know why it happened, and I'll keep an eye on the psu/transformer heating up.

As for it being caused by a mains spike, the unit went down mid session while a whole bunch of things were running, and nothing else popped, so I don't know.

So anyway, problem seems to be solved. I kind of wish I'd managed to figure out exactly why it happened to learn more about this stuff - I'm very much a self taught noob. I might in fact unravel the transformer to see if anything looks bad - might as well since it's already dead.

Nevertheless, thanks all for your help.

 

[audiomixer]

that's a good idea, taking stuff apart is key to learning something....

cheers,

- Michael