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HELP! fuse in one of my speakers keeps blowing...

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HELP! fuse in one of my speakers keeps blowing...

Postby Giscol » Fri Nov 29, 2013 9:15 pm

Hi All

I have a pair of active monitors, one of which has recently started to pop fuses. Having cottoned onto the fact that this isn't normal, I thought to check the other speaker (the that isn't blowing). I got them second hand so it's quite possible that they aren't the first set of fuses.

Anyhow, they're 5x20mm glass type, both marked T 1.25A L 250V, but the wire inside is visibly different. The one that doesn't blow is straight and thicker. The one that blows (I bought a pack of 10 thinking all fuses are created equally) has thinner wire with a blob in the middle. I believe the T stands for slow blow, which is what the manufacturer specifies. Can I have the wrong or inferior fuses in my speakers? Any thoughts before I go back to the manufacturer?

Thanks very much for any help.

Rod
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Re: HELP! fuse in one of my speakers keeps blowing...

Postby damoore » Fri Nov 29, 2013 10:59 pm

Have you tried one of the new fuses in the "good" speaker? That will tell you if it's the fuse.

When does it blow - at turn on, at random, or when being pushed?

It might be worth swapping the channels and seeing if the same speaker still blows, in case it's an issue with the channel. It shouldn't be, but it is worth eliminating the possibility.
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Re: HELP! fuse in one of my speakers keeps blowing...

Postby Giscol » Fri Nov 29, 2013 11:15 pm

Hi Damoore - thanks for your reply

The fuses always seem to blow at power on. They've never cut out if they survive that. Sometimes the bad one will be fine for a while, surviving a few power ups.

Right now, for instance, they're both working. I've put the 'good' fuse into the other speaker and it's survived power on fine. However, I've put one of the thin wire/blob fuses in and it too has survived power on. But I'm not confident (after going through 5 in a week) that it will continue to do this.

Is it possible that I should get extra slow blow fuses (TT rated, I think?)?

Thanks again.
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Re: HELP! fuse in one of my speakers keeps blowing...

Postby turbodave » Sat Nov 30, 2013 8:09 am

Hi, Stop looking to change the fuse and get the problem sorted! There is a reason......find it! Dave
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Re: HELP! fuse in one of my speakers keeps blowing...

Postby Giscol » Sat Nov 30, 2013 9:04 am

Hi Dave, I know what you mean but the problem isn't restricted to one of the speakers, I have tried swapping power cables, mains sockets etc but the only consistent thing is which type of fuse blows. It's just whichever speaker has this alternative type of fuse in it.

Last night I swapped the fuses and as expected, the good fuse is fine. This other type of fuse has now blown but in the other speaker!

I will email the company and see what they suggest. Thanks.
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Re: HELP! fuse in one of my speakers keeps blowing...

Postby dmills » Sat Nov 30, 2013 9:36 am

Are you sure your new fuses are T1.25 and not say T125mA?

Where did you source your fuses from?

Personally I would suspect a dud batch of fuses for the symptom of the fault following the fuse, but that does not explain the first failure.

Regards, Dan.
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Re: HELP! fuse in one of my speakers keeps blowing...

Postby James Perrett » Sat Nov 30, 2013 7:52 pm

While the T indicates that it has a slow blow characteristic, it doesn't necessarily tell you how slowly it will blow. I wonder if you need to check the fuse specifications a little more thoroughly to find out what the differences are? I suspect that in this application the original fuses may have been chosen to be slightly slower to blow than normal in order to withstand any inrush currents. Any decent supplier should have the full data sheets available.
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Re: HELP! fuse in one of my speakers keeps blowing...

Postby ef37a » Sat Nov 30, 2013 11:56 pm

Hi Rod,
I have just checked the fuse in one of my Tannoy 5a's and it too is T 1.25A.* The monitor is only rated at 100W (rating plate max consumption) which would imply a mains current pull of only around 440mA max. The reason for the massive fuse over rating is to cope with "inrush" current when the large internal capacitors charge up. You have not given the make and model of your monitors (please do!) so I can't look up their rating but my Tannoys are about as wee as decent ones get and so anything beefier I would expect to be fused to the next value of T1.6A.

Fusing electronic equipment is not that exact a science! The inrush current makes it hard to get a low enough rating to be protective but not cause nuisance blowing. Do get in touch with the manufacturers and ask if there has been any such problem. If they are 1/2 decent bods they should send you a couple of fuses free gratis, unless of course they have a good idea of a stock fault.

*The Tannoy fuse is of the coiled kind and they supply a spare in the back of the IEC mains carrier!

Dan! Know what you mean. About 3 month ago I wasted an hour looking for a short in a 200watter when I had in fact fitted a 315mA mains fuse!

Dave.
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Re: HELP! fuse in one of my speakers keeps blowing...

Postby Giscol » Sun Dec 01, 2013 2:48 pm

They are unity rocks. I emailed them yesterday so may get a response on Monday. The fuses I bought were cheap ones off eBay , can anyone recommend a reliable fuse supplier? Thanks again, all.
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Re: HELP! fuse in one of my speakers keeps blowing...

Postby dmills » Sun Dec 01, 2013 3:51 pm

CPC, Farnell, Rapid, RS, Maplin (in a pinch, more money then god) whoever your favorite components place is.

Regards, Dan.
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Re: HELP! fuse in one of my speakers keeps blowing...

Postby ef37a » Sun Dec 01, 2013 4:41 pm

Giscol wrote:They are unity rocks. I emailed them yesterday so may get a response on Monday. The fuses I bought were cheap ones off eBay , can anyone recommend a reliable fuse supplier? Thanks again, all.

Ok, found the manual.
I read that each speaker contains two 100W AB amps which is considerably more powerful than my Tannoys (30+20W IIRC) so it could well be that 1.25A IS marginal especially if your mains voltage is near top tolerance.

N very B! I am NOT suggesting that you fit an uprated fuse! You must never do that unless given the nod by the manufacturer.

If you get the fuses from Maplin take a glass with you to inspect them for value. I have had a lot of wrong resistors from mssrs Maps!

Dave.
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Re: HELP! fuse in one of my speakers keeps blowing...

Postby Hugh Robjohns » Sun Dec 01, 2013 4:48 pm

It sounds like the rating on your eBay fuses are either marginal or wrongly marked. Any of the mainstream component suppliers should provide more reliable fuses, bu I'd call Unity and ask nicely for a couple of factory fit spares!

H
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Re: HELP! fuse in one of my speakers keeps blowing...

Postby ef37a » Sun Dec 01, 2013 5:08 pm

Hugh Robjohns wrote:It sounds like the rating on your eBay fuses are either marginal or wrongly marked. Any of the mainstream component suppliers should provide more reliable fuses, bu I'd call Unity and ask nicely for a couple of factory fit spares!

H

I thought they might not include a spare because they would not want "user replacement" but page 5 of the manual gives you chapter and verse on this!

Dave
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Re: HELP! fuse in one of my speakers keeps blowing...

Postby Hugh Robjohns » Sun Dec 01, 2013 6:26 pm

I've just been re-reading your descriptions of the fuses. There are several different kinds of antisurge/slo-blo/time delay fuses, all marked with a T on the case, but with different ratings for the time they will support an over-current.

Some antisurge fuses are dual-element types, which look like they have a spring inside for half the length (MDQ). Others have a thin wire wrapped tightly around a thicker wire (MDL) -- and I suspect this is the type you're describing as the original fuse. There there is the type with a blob (or two or three blobs) on the wire (ATE).

The first two are usually rated to sustain twice the rated current for at least 3 seconds (and often 5) while the third is rated to carry twice the rated current a minimum of only 2 seconds... So if the in-rush current is over 1.25 amps for several seconds, it's possible that the ATE fuses can't quite cope and so tend to blow more often.

I'm sure Unity Audio will be happy to supply some fuses of the right type, but if not look for a supplier of the appropriate rated fuses with the dual-element spring inside, or the wrapped wire, rather than the blob!

H
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Re: HELP! fuse in one of my speakers keeps blowing...

Postby Trevor Johnson » Sun Dec 01, 2013 7:37 pm

Just to add to the expert comments.

The transformer / power supply topology determines the fuse type:
Toroidal - slow acting fuses, because of the inrush current at power on.
Clamp type - more commonly nowadays in valve equipment when you have secondaries for valve heaters, etc. - fast acting fuse.
Switched mode power supplies - fast acting fuse: there is a standard, IEC-60127.

Look at Rapid, Farnell or CPC for the appropriate fuse. Using the correct fuse is critical for yours, and others, safety,

Hope this is of help.
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Re: HELP! fuse in one of my speakers keeps blowing...

Postby ef37a » Mon Dec 02, 2013 9:23 am

Trevor Johnson wrote:Just to add to the expert comments.

The transformer / power supply topology determines the fuse type:
Toroidal - slow acting fuses, because of the inrush current at power on.
Clamp type - more commonly nowadays in valve equipment when you have secondaries for valve heaters, etc. - fast acting fuse.
Switched mode power supplies - fast acting fuse: there is a standard, IEC-60127.

Look at Rapid, Farnell or CPC for the appropriate fuse. Using the correct fuse is critical for yours, and others, safety,

Hope this is of help.

In general you will only find one T fuse type in "domestic" equipment, the "wire with spring" type (used to be called Anti-surge)

The bigger valve amplifiers will use them regardless of transformer type because the inrush heater current is large, the cold resistance of an EL34 e.g. being only 0.5 Ohms.

In practice getting "fancy" with fuses is a bit pointless. The only product I saw where I recently worked that had an "F" fuse in it was a 200watt amp in the HT supply just feeding the OP anodes. If a KT88 popped and blew the fuse they would invariably come back with a T fitted!

We now have reliable surge limiting thermistors and so fusing can be that bit tighter.

Dave.
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Re: HELP! fuse in one of my speakers keeps blowing...

Postby Giscol » Fri Dec 06, 2013 8:32 am

Thank you all so much for your informative responses I now feel overwhelmed at how much I didn't know...

Kevin at Unity suggested the speakers may need a service, but as they do work if the fuse survives power on I have sourced some different fuses (same rating) from another supplier. These appear to be of the coil (MDL - thanks Hugh!)) type - hopefully they will do the trick. Otherwise I think it may be service time!

Thanks again,

Rod
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