Transformer Voltage Measurement?

It depends upon the rated capacity of the <...5 volts ac...power supply...>, which has been withheld from the forum. If the load has swamped it, then that might be an explanation; it seems at first glance that it is not capable of suppling the magnetising current of the <...transformer...> to which it is connected.

Stay calm everything's gonna be alright. ofcourse a transformer does a thing, it withholds some like the IRS. Easy peasy

It sounds like your 5v power supply has insufficient amperage available... If you want your final supply to be something like 40v at 1 amp, you would need a 40 watt + loss from conversion as a minimum...that means you would need at least 8 amps@ 5 v....and your transformer would have to be rated above that min...

Thanks for the reply. I think that you have answered my question.

If your voltage, V, steps up 1:8, the current, I, steps down 8:1. Any load impedance on the secondary of the transformer, Z=I/V, appears as Z/64 to the power supply. For example, a resistance of 64 ohms on the secondary of the transformer would appear as 1 ohm to the power supply.

You have measured 5 vac, but failed to consider its power capacity, how much current it can supply?

Your step up transformer will also require certain a minimum amount of current to energize, excite and be able to produce the corresponding magnetic lines of force that in turn correspond to the step up voltage..

That is your basic reason why your expectation did not materialized...