X10 horizontal magnification can be found with many classical analog oscilloscopes. It's switching x10 gain to the X amplifier. We can expect a defective part somewhere in the X amplifier circuit.
HM304 is an upgraded HM303 with additional digital control, providing autoset and other features, if I understand right. The basic XY amplifier and CRT circuit should be almost identical between both models. At least the HM303 service manual is freely available from internet.