H89 lives - but for how long?

Many thanks to Patrick Rigney for his help in getting my rickety H89 (it is literally held together in places with baling wire and wallboard screws) back on its feet - last nite it loaded and ran CP/M for the first time since ???. Here's my question - after the machine was running for a minute or so, the screen got very bright - the "black" background became green and the screen characters were so over-driven that they became "fuzzy". The brightness control worked (i.e. changed brightness levels) but the available range only went from "too bright" to "really too bright". After another minute or so, the brightness level gradually subsided back to normal and everything seemed ok for the next 20 minutes of operation. What caused the "glow" and what incipient failure does it suggest? [unpaid politically-correct endorsement]BTW - although Patrick is a regular participant in classiccmp, I "met" him through the Vintage Computer Market and his offer for NorthStar boot disks. Thanks Sellam and Patrick! Jack