Here's where to download the manual for your box: http://tv.manualsonline.com/manuals/mfg/apex_digital/dt250.html
It seems you're not alone with this failing product. Here's a forum on it: http://www.digitaltvconverterbox.com/APEX-DT250?page=1
I cannot find anything on repairing it. But I've found a lot of reviews.
http://reviews.bestbuy.com/3545/8888527/apex-digital-to-analog-converter-for-analog-tvs-reviews/reviews.htm
I don't think I've seen worse reviews since the Edsel came out