No signal from LFTX when data sent from gr.file_source

Hi all,

I have been testing USRP and Gnu Radio for a while and it’s been a very
interesting, educational experience.
One of the important things for me is to get capture to / playback from
file capability to work.
I simplified a bit and essentially what my script does

usrp.source_c --> gr.head --> gr.file_sink

It produces huge files (with complex samples, I understand) which I
intend to “play” back through LFTX
daughterboard. So, another script does something like:

gr.file_source (gr.sizeof_complex, fname,true)
-----> usrp.sink_c (0, interp)
-----> fftsink2.fft_sink_c

Gain on the LFTX is set to -1.02 dB - as read by self.u.pga(0). The
problem is that I don’t get any signal
from LFTX although I can see the expected spectrum in the FFT window
(which is connected in parallel
to the same stream). The signal is digital radio transmission with
estimated S/N around 40 dB.
Since the spectrum matches what I had seen when tuning the radio with
some receiver script which also
provided FFT window, I assume data stream is OK. Just to check my
hardware I tried
and checked the outputs of LFTX with an oscilloscope. I could see the
expected sine wave nicely
phase shifted by pi/2. Hardware is OK, but somehow my “playback” script
doesn’t want to push data out.
Do you have any suggestions?