Why would an isolation transformer add noise to the waveform?

[sprok]

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[ataradov]

It is possible that you are reaching core saturation. Saturation usually does not look like this, it typically affects shape of the slopes, not peaks.

[T3sl4co1l]

Hmm, the combination of leakage inductance and filter capacitance may be making a resonant circuit, that's being excited by your mains' flattened tops.  Or put another way: your mains voltage contains a small amount of higher harmonics, which are being magnified by a resonant filter.

Try a 1uF film cap in series with a resistor of 10-100 ohms, placed across the terminals.  Does that dampen it?

Tim

[System Error Message]

The circuit you measure is the secondary of the transformer. Since you connected it to ground and the oscilloscope is usually grounded as well, a loop can be formed or signals can also travel to ground. Some circuits filter noise by grounding so your oscilloscope picks out the noise from ground.

[Delta]

Are you sure that capacitor is directly across the secondary?

7uF across 125VAC will allow 0.33A to flow! 

[Arjan Emm]

Maybe the isolation transformer was intended for a specific inductive load. In that case they may have added the capacitor to compensate for that and improve cosine phi with it.

[BravoV]

Abit off topic, but I guess its still related.

I can see you used old CRO to measure mains voltage, interested on how or what probe did you use to get those results ?

[Mechatrommer]

Just a cheap Chinese 100mHz probe P6100 using 10X mode... possibly unsafe but I'm no pro.

we can understand case insensitive typing out of laziness but since H is capital....
mHz = milliHertz
MHz = MegaHertz