Toroid trouble...

[mcfrede]

I was hoping to get a little help troubleshooting an amp I have been asked to look at.
Problem being it blows fuses faster than the owner can replace them.
I have opened the transformer and desoldered all the toroids secondary wires. Then I turned it on for about one second and true enough, the fuse blew.

This is a picture of the test setup.

I only know what is printed on the toroid, it is for a Little Dot amp and my chinese is not so good...


The fuses provided with the unit (the ones I received anyway) are "F1AL250V" which I gues means 1A, 250V. They look very thin on the wire making me believe they are the "fast-blow" ones (maybe that is what the "F" states? Could the blown fuses be caused by the inrush current when turning on the amp? It doesn't look all that big to me, but maybe the fast-blow fuses just blows?

I just tested for shorts between the primary side wires and they are shorted.
On the secondary side several wires also seems to be shorted:
All green wires as well as the black one test positive for continuity (any of the wires will test positive for continuity to any other of the mentioned wires).
All white wires test positive for continuity.
The yellow wires test positive for continuity.

None of the secondary wires test positive for continuity to the primary ones and no wires test positive for continuity to the casing.

Should I consider this toroid "dead"?

[vaskebjørnen]

The primary is 120 + 120 vac coupled in series and it is usable in 240v country's there are 3 secondary coil.
One 3.3 one 6.8 one 175v
How many ohms does the primary measure? That's what it's about. It should have an low ohm value but not shorted.

If it's shorted new toroid should be the answer

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[Solderdude]

A cause for concern is the 'F' for Fast acting or quick.
'F' types can not be used before transformers because of the inrush currents.
Certainly not when these transformers also have to charge electrolytic reservoir caps.

It must be a 'T' version (Time Lag) or shows 'Slow Blow' as you already suspected

Pop in a 2A/T fuse and test again if the fuse holds keep it connected and see if it warms up.
It shouldn't be without any load but might get a bit warmer.
When there is a short the 2A will blow or the transformer will get hot quite quickly (or become smelly )

Normally all individual windings should not be shorting amongst the others but the primary with a '+' could be connected (but are probably not) internally with a CT tap.
the 3.3*2 could very well be a CT winding (Center Tapped) and thus connect internally.

[AffeX]

Depending on the meter it could appear shorted. Think of each of the wires to e.g. primary side as two ends of a long cupper wire. But with an accurate meter it should register low ohm as there is a little resistance depending on the length (number of windings) as others have already stated above.

[mcfrede]

After quite some time of inactiveness I have looked upon this again.
This time I put in a 2A/T fuse, it lasted about 2-3 seconds before it blew. It just lit up and POOF it was gone.
I tell myself there is a small odour to the toroid afterwards.
Will it be safe to conclude that it is in fact the toroid that has malfunctioned or can i continue to troubleshoot?

Resistance on the primary windings are ~16 Ohm by the way, so no short there. The seems to be a few on the secondary side though.

[Solderdude]

Ditch the toroid in this case, It most likely has 1 or more shorted windings !
a short in a winding cannot be measured with a multimeter but can only be detected with a resonance meter (a specialistic device)
Say it has 200 windings in the primary and a few of them are shorted (layer upon layer or 2 windings next to each other) than you won't see this in Ohmic measurements.

[mcfrede]

Thanks for the help, I'll hear what the owner wants to do now.

What will cause a toroid to short, bad wire-insulation, or heat buildup due to something malfunctioning on the amp itself?
I am asking since I would not want to recommend buying a new toroid only to have that also blown by something on the PCB malfunctioning.

[Solderdude]

It could be a damaged enamel insulation when winding the trafo or when the trafo was bumped on installing e.t.c.
It could be heat build-up messing with the insulation as you say.
Can't really tell.
I have had 2 toroids having the same problem.
1 bigger one many years back and a small one about 2 years ago.

Both were not encapsulated.

I don't think it was under-dimensioned but rather would suspect 'bad luck'.
Replacing the trafo will most likely not have the same thing happening again.

[el_serpiente]

Being the owner of the amp I have contacted David from Little dot and asked if it is possible to acquire a spare.

[Solderdude]

That is probably the only route to get a similar one.
This one seems to have specific voltages for that kind of amplifier and is not an 'off the shelf' transformer.
I think it was custom made to specs for little Dot.

[el_serpiente]

David want 20$ for a spare and 19$ for mailing it to me. Then I just need to figure out how to get past custom...

[Solderdude]

That's actually quite a good price !
Both for the trafo (which probably costs more) as well as the shipping (considering the weight)

Customs could add around $10 if they intercept I reckon.

[mcfrede]

That's actually quite a good price !
Both for the trafo (which probably costs more) as well as the shipping (considering the weight)

Customs could add around $10 if they intercept I reckon.

Oh, I reckon you do not know the danish customs...

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[el_serpiente]

That's actually quite a good price !
Both for the trafo (which probably costs more) as well as the shipping (considering the weight)

Customs could add around $10 if they intercept I reckon.

yes, plus approx 30$ for "handeling fee". I don't mind paying the tax but I hate paying the 30$ to the post office for collecting the 10$.

This ends up in around 80$. I guess that still is ok and I will try and see if I'm lucky and the post office does not notice the package.

[Cowfoot]

You can ask the person that sends it, to declare it as an items worth 5$ or less. He can also declare it as a gift woth less then 15$. If he does on of those two, there shouldn't be added any customs, if i remember correct. Fred_fred2004 did this when i ordered cmoy parts from him in australia, and serveral others have done the same, and there haven't been added customs in any of those instances.

[Solderdude]

yes, plus approx 30$ for "handeling fee". I don't mind paying the tax but I hate paying the 30$ to the post office for collecting the 10$.

This ends up in around 80$. I guess that still is ok and I will try and see if I'm lucky and the post office does not notice the package.

In the Netherlands the handling fee is about E 15.- max and is lower for lower value shipments.
Hate paying that too !

Not all sellers want to declare false values as when a shipment is lost they can only claim that low value.
In the case of the trafo this difference is not that big so he might be willing to 'help'.

Hope it all works out.