My 2400c had something similar with its backlight after I restored it. I assume that it had not run for several years before I did the surgery needed.  

I found that regular use seemed to cure it of whatever ailed it. Glad to hear that the 550c lives again.

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Hey beachycove, more good news and a little surprising too I guess, - at least I wouldn't have expected all these things in the course of a week. Your positing that regular use seemed to cure your 2400c ailments has applied to this 550c in several ways. After days and days of gentle daily use, indeed, the fading screen backlight issue seems to have now disappeared completely. In addition, the Control Strip's battery charge indicator bars have reappeared! Ever since doing a clean install of OS 7.5 early last week, they never appeared. Of course, the batteries are very old and even though the charge indicator bars quickly went to "full", they barely keep it running long enough after pulling the AC power cord to render the typical message about going to sleep in 10 seconds. Then yesterday, for the first time, the date & time and the brightness/contrast settings were retained even after being shut down with the AC power cord removed for a couple hours. We get severe thunderstorms at this time of year, so the ritual of shutting everything down and unplugging everything occurs sometimes several times daily, inadvertently creating another "test". It's hard to believe that the clock battery is playing ball at any level after all this time, we'll see. Anyway, I appreciated your encouragement very much.