Why does the screen get too dark on my Bang & Olufson Beovision MX 5000?

I was just given a used Bang & Olufson Beovision MX 5000, but no manual or remote. The picture seems to be okay when first turned on, but gets progressively darker the longer that it is on. Eventually (after about half an hour) it's too dark to comfortably watch. How can I fix this problem?

(The tv is generally finicky, for example the sound does not turn up very high, but I've solved that problem by running it through my stereo. I only mention it in case it's related.)

Thank you all for your help!