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The problem is multifaceted, look at the block diagram page 5, AGC uses pulses from horizontal output to control tuner gain, if horizontal oscillation is not running, or not correct, or AGC itself is not working, then tuner gain will be wrong and you won't be able to tune anything.
Sometimes, when AGC is not working for whatever reason, an external bias can be used ( battery or the like ) to force the tuner into the right range of gain for test purposes.
You don't know at this point if the horizontal oscillator is stable at the right frequency, or the right amplitude, or the right waveshape, all of this matters.
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