I used to use a mixture of dishwashing liquid diluted in water and baking soda. Baking soda is mildly abrasive when not fully dissolved, so you have to be careful, but it was the chemical action I was after. Put it on, let it sit for a minute or two for the chemistry to work, then very GENTLY wipe, then rinse.
This worked beautifully for several Marantz 22XX recievers, a Sansui 9090, and several other items I tried it on. Then I tried it on the Marantz AT-5 Digital Timer. Before I knew it, some bits off some of the lettering were disappearing! :sigh: :tears: I've often wondered if it happened because by the time that unit came out, Marantz was starting to cut corners and used a cheaper protective coating, or because that unit had previously been exposed to something (maybe years of direct sunlight?) which weakened it , or...???
Anyway, after that experience I have been wary of returning to baking soda mixtures, despite the fact that its chemical action (alone, not to mention the mild abrasive action) melts away cigarette-smoke residue and gives a great shine to metal surfaces. Now I'm using a bluish (ammonia-free) spray-on cleaner called "Swipe", which also works well and so far hasn't damaged any lettering. They claim you just wipe it off, no need to rinse, but I make sure to rinse very well, just in case. After a thorough rinsing under running tap water, I then dip/rinse in filtered or distilled water, to largely eliminate mineral-salts residue that could be left from tap water, before waxing for protection.
I also use a toothbrush (and some other small brushes), and sometimes q-tips, as well as cloths and very occasionally toothpicks to get stubborn dirt out of corner spots, but I only use those under good magnification and lighting, because they could scratch off the protective layer if used too vigorously. Sometimes DeOxit helps to eliminate stubborn spots of oxidation.
On that topic, there is nothing like detail-cleaning a brightly-lit surface under magnification. Anything that looks really clean viewed that way, looks NEW to the normal, naked eye :yes: (except, of course, for the scratches, which are then at least clean scratches!).