1. Removing the top cover is simple - first take off the shoulder strap brackets (4 screws) and then remove the 5 screws in the top cover and it pops right off - don't undo the two screws nearest the speaker grille, those hold the speaker in place on the underside of the top cover. Be sure not to damage the speaker wires or connector. The wire from the speaker is pretty short but there's just enough room to get to the connector to undo it if you tilt the top cover up at the rear.
   
   
   
   
2. The filter goes in the space at the front of the board.
   
   
   
   
3. Here's the filter correctly oriented ready to be put in. Note that the 3 pin connector is on the LHS and the 4 pin connector on the RHS. The filter is installed writing side up as shown.
   
   
   
   
4. Here's the filter after installation. It should look like this.
   
   
   
   
5. You can go ahead and put the top panel and shoulder strap brackets back on and turn the rig back on. Press the "F" key and hold it for half a second to take you into menu mode. Turn the SEL dial until you get to menu #38. It should say "OP FILTER" and "OFF" just above that. Rotate the main tuning dial until you see "CW" (not "OFF" or "SSB"). Important note - as you go through the "SSB" choice, you will notice that the rig switches to the newly installed CW filter - DON'T PANIC, all is okay. This threw me for a while until I understood what was going on. Make sure that menu #38 is set to "CW" (ignore what the receiver actually does!) and then press the "F" key for half a second again to exit menu mode.
   
   
6. You're almost done. You will probably panic at this time (as I did!) because as you switch through the modes from USB/LSB/CW/CWR etc., the rig seems to be stuck in SSB filter mode. There's one more thing you have to do to get the CW filter working. Check out page 14 of your FT-817 Operating Manual - number 7 at the bottom right of the page you will see the "C-Key" setting for "NAR". You have to set this in order for the receiver to switch to the narrow CW filter when you switch the rig to CW or CWR modes. To do this, first put the rig into CW or CWR mode then tap the "F" key once quickly. The FUNC Keys menu items for the A/B/C pushbuttons should appear. Rotate the SEL control until you see "IPO ATT NAR" over the A/B/C buttons. Then press the "C' pushbutton once to set the filter to Narrow. A small right-pointing arrow should appear and the rig should switch to the CW filter - you should hear a marked difference in the receiver noise "tone".
   
   
7. That's it. You can swap between the SSB and CW filters anytime by revisiting the FUNC Key menu and toggling the NAR function.

How does the filter sound?

I did some preliminary tests with my HP8642B signal generator - the receiver is very sensitive indeed. The CW filter is an ABSOLUTE JOY to behold....it has a superb frequency response - very very sharp skirts - great stopband. WOW!!!! I'm blown away by it....I'll have some real measurements to back up my first impressions for y'all sometime soon. It's late, so I tried it on the bottom end of 40m - sounds REAL NICE!!

Folks, if you are going to do CW seriously with the FT-817, you will simply HAVE to get the YF-122C filter option. It's a must have IMO.

This modification can also be found on http://www.ad6a.com/FT817_CWfilterinstall.html