Oddly enough I just had the same thing. I use the RF output to transmit to the kitchen, and it died - while the RF decoding was still working fine. I bought myself a new tuner (and a spare for the future) from tivoland (other tivo parts sites are available), and replaced it myself.
It's not for the very faint-hearted - but isn't that difficult if you're a bit used to the inside of electronics.
In any case, it all worked very well, and all is now well. If uncertain about the process then someone might be able to let you know if pacelink can do it.
Well the Tivo Tuner is a ZIF tuner but (as all manufacturers do) , they solder it into the main board to save fractions of a penny on the ZIF sockets!
If you've ever repaired a Digibox tuner before its the same process. It involves desoldering around 20 pins (can't be bothered to count them) which is easy, the tricky part is desoldering the shielding mounts at either end of the tuner. A lot of heat is required to get the holes clean enough of solder as the tuner tin can absorbs a lot of the heat.
Once you've removed the old tuner, I would recommend not re-soldering in the mounting points, the tuner has lips on the mounting points to hold it in place and make contact with the earth pads. Once the new tuner is soldered in, is is well secure, especially as it is screwed to the tivo case, so there is no need to solder in the mount points at all!
More generally, before you get to the soldering bit (I don't know how much detail you wanted) :
Allow a while between unplugging the Tivo and moving it to avoid head crashes, and give it a while for the PSU to discharge. You shouldn't be touching any part of it anyway but accidents do happen.
Try to earth yourself occasionally during the process to avoid static doing more serious damage to the boards.
Take the lid off the Tivo, remembering from when you upgraded it that it's stiff.
Unplug the various leads from the motherboard remembering where they came from and which way round they are.
Take out all the screws which go into the board through the back around the scarts and RF plugs. There are a few which hide under plugs and things.
Take out the screw or screws (can't remember) which hold the motherboard down.
Carefully straighten the twisted tabs which also hold down the board. I'm guessing these can be a bit temperamental so be gentle.
Board will carefully lift out at which point it's time to warm up the soldering iron, solder and solder sucker, and replace the relevant bit.
Once done put it all back together, turn on, and bask in the impressed adoration of lovely wife who has been contemplating life without Teletubbies in the kitchen at breakfast time (for our son you understand - not her - honest)