arjoll - you might have seen this ,but just in case
https://www.allaboutcircuits.com/new...n-zx-spectrum/
The modulator shown is an UM 1233E36. I take it that the E36 refers to the TV channel ,as the blog states UHF ch36.
Interesting is the composite video mad, and the ULA mod. I've got both a 48 & a 128a, which possibly use the same (UK) modulator, but as is usual ,Swimbo has put them somewhere safe and finding where usually ends up with a tiff, so I'd rather not.