The resistors in the deflection amplifier output stages (in this case the horizontal deflection amplifier) have an annoying tendency to drift much higher in value and eventually fail open. This is also an issue with the HP853A, I have repaired three or four of these with essentially the same failure. You can check the resistors easily with an ohmmeter - if you measure higher resistance than the value shown in the schematic, then the resistor has failed. Suitable replacements are available from DigiKey or Mouser at low cost. If you find a failed resistor of a specific type, then change all of the resistors of that type in the deflection amplifier output stage (even the "good" ones). I have repaired at least one unit where all of the deflection amplifier output stage resistors were open-circuit on both the horizontal and vertical amplifiers.
