NotTaR of Television Sets : The wandering black blob on old Sony    
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The wandering black blob on old Sony

"I had a Sony KV-1920 TV (very old) that suddenly started to displayed a black blob on the screen. The blob was anywhere from 1" around to almost the size of the entire screen. It had a sharp, not fuzzy, outline, and it would shrink in size as the TV warmed up, usually disappearing completely in 30 minutes. It shrank in sudden jumps, not gradually. Sometimes the blob would be stationary, other times it would tumble around rapidly all over the screen."

(From: Raymond Carlsen (rrcc@u.washington.edu).)

Measure the regulated +130 volt line... I think you'll find it has drifted upwards just enough to trigger the "protective" blanking circuit. In those sets, if the B+ (and consequently the HV) went up, the screen was automatically blanked so you couldn't use the set. It was before HV shutdown. Older RCA TV sets used to throw the horizontal out of sync. The low voltage regulator is an analog type with a pass transistor that is probably leaky, causing the high B+. Changes in line voltage and loading (with brightness changes) cause the partially blanked picture to change (the black blob moves around, sometimes blanking the entire screen). When you replace the defective component, reset the 130 volts with the pot, and you're back in business. The pot itself may have a bad spot... just move it one way or the other to get off that spot.