What's this transformer secondary?

[iXod]

I want to save an old small Japanese-made single-board LED alarm clock.
The transformer primary has opened and I’m searching for a
replacement.

The small off-board transformer is marked EI-28 and E313214. I found a
page that describes a series of “EI-28” transformers as 12v:

https://www.alibaba.com/product-detail/ei-28-power-ac-adaptor-class_60676334117.html?spm=a2700.7724857.main07.83.7fec116aAXDo9f

(See table “Transformer Series”.)

Farther down this page the EI-28 is apparently listed as having a 36vac
secondary (if we ignore the column header “vdc”). Is that +/-18? The
page doesn’t say. The large LED display is attached right on top of
the 1-sided PCB which makes tracing the circuit difficult.

How can I know the v of this c-tapped secondary?

Thanks.

[Niklas]

I am afraid that those numbers you listed are not enough to find out anything about the voltage.
EI28 is the physical size and configuration of the transformer core. E and I shaped parts are joined into an 8-shape after the copper windings are added. 28 is the physical size which also relates to the power rating.
E313214 is the UL file number for the transformer manufacturer.

[iXod]

Thanks for the news. 

Anybody have suggestion how to possibly power the circuit with AC or DC and determine when the proper voltage is reached? Or other method?

Thanks.

[Whales]

You should be able to work out an acceptable secondary voltage through a bit of analysis of what you have.

(1) What's connected directly to the secondary?  A bridge rectifier?  Then a voltage regulator, or some discrete parts?

See if you can reverse engineer this part of the board.  Draw a little bit of a schematic out.

(2) Do you have a variable DC supply handy?  Once you find where the DC gets made, you can try attaching your supply and slowly winding it up until the device comes to life.

(3) Other hints may be in the battery voltage (if the device can also be battery operated) and the max voltage ratings of the chips used on the board.

How can I know the + and - of the primary?

It's AC, it does not matter.  Assuming they haven't done something silly like attach the core itself to one of those terminals.

[albert22]

With enough patience you can disassemble the transformer and count the number of turns for the primary and the secondary. Knowing the primary voltage and the turns ratio you get the secondary voltage.
Those clocks used the AC frequency as the timebase, the clock may not work if you feed it with DC. Although it may turn on.

[iXod]

How can I know the + and - of the primary?

It's AC, it does not matter.  Assuming they haven't done something silly like attach the core itself to one of those terminals.

Meant to say + and - v of *secondary* (it's c-tapped). ie, +/- 12v.

Corrected original post...

Thanks.