Yesterday, I measured the transfer function of the YARM PDH box.
I tested the electronic board and couldn't find a frequency dependent behaviour. So I measured the TF again and it looked nice.
Today's nice measurement could is/was reproducible. I suppose yesterday's measurement is just an artefact.
The electronic board is modified according to Kiwamu's wiki entry http://blue.ligo-wa.caltech.edu:8000/40m/Electronics/PDH_Universal_Box
Btw. The light could be locked to the cavity for ~3min.