NotTaR of Television Sets : Loss of Horizontal Sync (also applies to..  
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Loss of Horizontal Sync (also applies to vertical) after Warmup

The problem lies either in the horizontal oscillator or in the sync system. If it really is a problem with sync pulses not reaching the oscillator, the picture will move around horizontally and can be brought to hold momentarily with the hold control. If the picture breaks up into strips, there is a problem in the horizontal oscillator. Rotate the hold control: if the frequency is too far off, the picture will not settle into place at any adjustment of the hold control. Look around the horizontal oscillator circuit: all of the oscillator parts will be right there, or check on the horizontal oscillator module. Another horizontal problem can occur if the set is an RCA made from around 1972-1980: these sets are designed to slip very far off sync if the high voltage is too high, to protect against radiation. Turning up the brightness will decrease the number of bars if this system is in question, as the high voltage is decreasing. In this case, check around the high-voltage regulation system on the deflection systems board. I've had 2 1970's RCA's with this problem.