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Transformers are pretty easy to substitute, in my opinion. All you need to know is the primary and secondary voltage, and the full-load current drawn by the load. If it's a center-tapped transformer, that's important to know, as well.
I'll assume the transformer in question is a "step-down" transformer - meaning it steps the high "primary" voltage down from something like 120V AC to a lower "secondary" voltage, like 12V AC. Then find your "turns ratio", R, by dividing the primary voltage by the secondary voltage.
A 120V to 12V transformer has a turns ratio of 10:1
Once you have identified the primary and secondary voltages, and turns ratio, you'll need to figure out how much current the load would ever possibly draw on the secondary side of the transformer. This is your "full-load curent" or FLC. If you re-upload the picture, I can try to help you figure that out. For a safety factor, I'd multiply the full load current by 125%. We'll call that A.
1.25 x FLC = A
Then multiply A by the secondary voltage. This will tell you what size transformer you need. Transformers are rated in volt-amperes, or VA.
Then all you need to do is search for a transformer that is rated for at least the calculated VA and at least the voltages on the primary and secondary coils.
Let's say you've done your calculations and found you need a 42VA transformer to step 120V down to 24V. Your turns ratio is 5:1. Say you find a 5:1 transformer rated at 60VA and rated for 240V on the primary. That would be more than adequate in most cases. Again, without knowing the exact application, I can't say for sure.
Lastly, use extreme caution if either primary and/or secondary voltage of your transformer is greater than 50 volts. Do not mess with mains voltage unless you have the skills and knowledge to do so safely. In a fight between you and the power grid, I'm betting on the grid every time.
Best of luck! br> br>