other than 50 Hz, what are the characteristics of the source?
what is the turns ration of the transformer?
what voltage do you expect on the output side?
how much current does the 555 circuit draw?
on the surface, it looks like the capacitor should hold up the
rectified AC, but the scope trace looks like it isn't holding up the DC
as Easy peasy says, look at pin 8 of the 555 carefully
please show the time scale of the 'scope
what is the reference for the 'scope traces?