I recently acquired a 1997 vintage Radio Shack HTX-10 transceiver. It features all-mode operation on 10 meters and has received very positive reviews. However, there seems to be a lot of confusion over the PL/CTCSS tone generation function and adding to the mystery is that it's not even mentioned in the manual: http://www.repeater-builder.com/radio-shack/htx-10/htx-10-manual.pdf Despite this, it is possible to activate an 88.5Hz tone by pressing the Function button and LCR/4 (one of six multi-function buttons). Supporting this, the display shows "RPT" when this is done and I have measured an 88.5 Hz tone on the carrier. It can be turned on and off just as easily as if it were a normal function despite there being NO mention of it in the manual, although this IS mentioned on pages 8 and 9 in the Magnum 257 manual which is a nearly identical transceiver: http://www.cbtricks.com/radios/rflimited/magnum_257/graphics/magnum_257_om.pdf Unfortunately, every copy of the HTX-10 service manual has a very poor quality schematic, but a nearly identical rig is the AE485S. That schematic can be seen in higher resolution here: http://www.repeater-builder.com/radio-shack/htx-10/ae485s-25w_schematics_and_board_layouts.pdf As far as I can tell, the Sanyo LC72322 CPU either generates the tone or turns an oscillator circuit on and off. This is obviously a highly useful feature for 10M repeater use, and being able to change the PL/CTCSS tone frequency would be extremely beneficial to all owners of this rig. The answer, however, has evaded me. Anyone able to shed some light on this mystery?