Does anyone know for certain the specications for the power plug which goes into the side of the radio? It appears that it's a 4mm barrel, but I'm unsure of the center pin. Manual doesn't say. I'm trying to make a custom power cable for my radio.
You're looking for an EIAJ Type 3 plug. Take a look at the Radio Shack Adaptaplug Chart. Google is your friend on this plug search.
Shouldn't be necessary. My home power supply for my 651 (same as your 106) actually came from Menard's, it was a variable/selectable voltage wall wart with interchangeable tips a la the RS Adaptaplug method. The tips are only marked for polarity, I just matched up the one that was identical to my RS lighter plug supply. Cost about $10 IIRC, and works well, no switching-supply RFI that I can detect. I'd think that any big box store would have something similar if you don't want to do the online thing.
That just made it A LOT better.
The Whistler price of $9.95 is decent also. When Radio Shack used to sell all of their various adapters and plugs before bankruptcy, the price used to be $15 for the 9V 800mA adapter (#2730355) with the Adaptaplug C (#2730336) used for the PRO-106....The Whistler PS made this a whole lot simpler.
The plug is a EIAJ standard it IS NOT RARE. If you find a 9v SONY power supply it will have this plug on it.