I played around with this yesterday with my Yeasu ft-857d. I tapped the first I.F. at 68mhz, but of course it knocks down the receive on the rig big time when I split it with the RTL dongle Receive on the dongle side isn't that hot either.
you probably overloaded the point where you tap the signal out. It is the question of high or low impedance tap point.
What I read on the experiments from the Italian guys, the FT-817 is working without any issue and problems, without degradation of the signal but I am not sure that they manage to do the same with the FT-857 or 897.
Check the site, the photos are quite self explanatory, I have no problems with Italian language but they write (and talk) too much :-) so use the google translator for more info.http://hamradio.selfip.com/i6ibe/panadapter/panadapter.htm
I have two of FT-817d but still did not make the mod. I have to do it before the microwave RS season..