Trying to match small AC transformer - 240V primary 12.8V secondary

Matthew Kirkcaldie wrote in news: snipped-for-privacy@news.sydney.pipenetworks.com:

That's strange. Transformer primaries don't open very often. I would be inclined to dig deeper to find out why there's an open circuit (internal fuse open? Bad connection? Melted wire?) and whether there is any fault downstream of the transformer (shorted component?).

Don't know the reason, but I have removed it from the board and there is infinite resistance across the primary terminals and no visible damage to the transformer or the board it's on. Fuse is intact. The unit would be about 5-6 years old I reckon.

Any small transformer with an approximately 12V output should be fine.

Small transformers are impedance protected, the windings can be damaged by power surges or an overloaded secondary. If the unit works after replacing the transformer, it was probably just a surge that popped it.

Mebbe it has a faulty or tripped thermal protector ?

Thermal protectors are fitted to prevent overheating etc. Usaully you can by pass same, and "fix" them, but as theyre fitted for a reason( not used to our extreme heat?) they often fail. The tranny suggested is probly close enough.

The markings will be house numbering, either from Yamaha or their chinese part supplier. The 0118 will be a date code - week (probably) 18 of 2001, and googling that will not uncover anything helpful.

As others have said, any comparable unit with a 12v6 secondary will be fine. Any downstream elctronics which are voltage sensitive will have some supply regulation, to guard against mains input variation.

Apart from the Jaycar unit mentioned, Altronics have one which has dimensional info at

Even Trickie Dickie has one

Which one is for you depends on how critical size is, and how near you are to the supplier.

In my professional repair life, I replace trannies in hifi units all the time, that have primaries that have gone open for no apparent reason. Yes, sometimes there is a thermal fuse in the primary, and yes, sometimes it's connected across a couple of external pins so can be temporarily bridged for test purposes, but I find it rare to then come across a fault downstream, that has caused a genuine overheat that has led to the failure.

Arfa

I've not had cause to use them for audio parts, but their computer side is one of the worst I've ever come across and put me off them as a company.

very small 50/60Hz transformers can be a right pain, as the wire gauge is so damn fine. Its extremely hard NOT to slightly damage, say, 44AWG magnet wire when winding such a transformer.

Low power transformers must have extremely high magnetising inductance AND low volume, so require silly numbers of turns of astonishingly (vanishingly more like) fine wire.

One I used in an OEM UPS' had, IIRC, 11,000 primary turns, of (again, IIRC) 44AWG. Now 41AWG is about 0.09mm outer diameter; 11k turns is thus around 70mm^2, ie 8.4 x 8.4mm, which is about the cross-sectional area ISTR for the primary winding.

And we had quite a bit of trouble with those - although they were made in Mexico, for a very low price - all of which was related to the termination.

Cheers Terry

Is there just the one for all Yamaha products? In the UK I found the one which dealt with their pro audio fine but not the one for computer stuff. It was some time ago when CDW was newish and expensive. But I've got a long memory. ;-)