Part Identification Help - EF25 Transformer

ef25 transformer

My Sony set top box recently spat the dummy,
DST-HD100A

I managed to track down the offending component
but having trouble searching for an equivalent part to
replace it with.

A transformer with markings EF25 11
and a bunch of numbers I can't make sense of.
packaging: 4 pins primary side, 3 pins used on the secondary

**broken link removed**
Looks like the transformer on this page,
(the one on the bottom left)

https://www.altronics.com.au/index.asp?area=prod&grp=361&list=0&pg=2&sort=code
This place sells transformers but have no idea which one if any is suitable.

The cost to replace the whole power board is ~50 according to the service center, maybe cheaper to buy a new box...
Any help would be appreciated

dst-hd100a

It's likely an OEM part that isn't available from any manufacturer for retail. You either can analyze the circuit or unwind the transformer to get a specification. EF25 is the core size, it probably has an air gap, that must be known. If you don't want to repair the transformer yourself, then the windings ration may help to get a replacement type. Some standard off-line switcher transformers are available from manufacturers.

BTW.: How do you know, that no other parts are defective with the power supply?

sony dst hd100a

I checked the supply voltage,
Fuse is still intact (continuity check)

Opto-isolater on primary side is getting 120V, secondary side 0 Volts
Well i can't tell if anything else is wrong on the secondary side without power.
4 pins on the primary side of transformer same voltage reference from Ground

No voltage anywhere on secondary side of the power board.
No physical signs of damage on other components although brown smoke marks can be seen on top of the casing...
(I did replace a leaky capacitor on the primary side)

Service Center can't supply replacement part for just the transformer, which is frustrating but understandable i guess.

A SD box costs around AUS$50 and a replacement board for this unit costs the same.
go figure...

Added after 13 minutes:

Was reading here about someone elses problem
https://groups.google.com.au/group/sci.electronics.repair/msg/58392eab215a70dc

"It is likely that an internal thermal fuse buried in the transformer's
primary winding has failed. This being the case, it should be an easy job to
measure the secondary of the transformer with an ohm-meter to determine its
configuration. I would expect the most likely to be a single centre tapped
winding, in which case, your meter will read pretty low ohms between all
three secondary terminals.

Have a look to see if there is a bridge rectifier, and two smoothing caps.
If there is, then the centre tap will be connected to system ground, and the
bridge will be producing a negative and a positive rail, with respect to
ground. You can get a pretty fair idea of the secondary voltage by looking
at the voltage rating of the smoothing caps following the bridge. Take about
20% of that voltage, and then multiply the result by 0.7. So, if caps rated
at say 25v, take off 5v = 20v and then multiply that by 0.7 = 14v, so
transformer would likely be rated either 12v - 0 - 12v or 15v - 0 -15v.
There may well be 12v regulators like 7812 and 7912 following the bridge,
and 20v input would be a good figure for these. "


Is this approach worth considering?

ef 25 transformer

The quoted discussion doesn't apply to your case, I think. It's dealing with mains frequency transformer supply, not a SMPS as in your case. A switcher supply is very unlikely to utilize a thermal fuse in the transformer, cause other parts have higher thermal load, particularily the switching transistor or HV switcher IC.

I still don't see why you expect the transformer to be defective. The basic check would be for DC voltage across primary capacitor, should be e. g. 160 V with no load for 115 V AC.

thermal fuse transformer

I'm not very familiar with repair work, especially power supplies and this is more or less the first time for me...

Primary Capacitor? would that be something that would be rated around 450V, 68uF?
Voltage was around ~120 AC last time i checked.
(supply voltage 240V)


But shouldn't I be seeing a voltage at least somewhere on the secondary side of the circuit board at the very least?
Pins on the primary side of the transformer shows ~120V