As for the bigger picture, learning about what goes on inside the radio, I recommend two books written by Lou Franklin. The Screwdriver Expert's Guide is a good intro. His other title, "Understanding and Repairing CB Radios" goes into more detail.
His web site is at
http://www.cbcintl.com
The info for your Galaxy radio is posted at
http://www.cbtricks.com/radios/galaxy/dx959/index.htm
A link to the clarifier mod is on that page.
In general, if you plan to use the radio mostly on AM, this is not necessary. But if you will use it on SSB a lot, unlocking the clarifier makes it a lot easier to use on sideband. If you don't feel confident that your skills are up to this procedure, find someone who can help. That radio is probably not a good "first" project. The foil traces in that radio are spaced pretty close together, and it's easy to accidentally 'bridge' a glob of solder across a gap between them. Soldering skills are something you can acquire through practice, mostly.
As far as learning about radios, I would recommend collecting cheap yard-sale CB radios for ten bucks each or less. This is a cheaper way to learn the things NOT to do. If you blow up a ten-dollar yard-sale special, no big deal. If you blow up your new 959, that's different.
73
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