Hi!
Forgive me for stating the obvious - but have you checked the preset adjustments for Intensity/readout intensity (refer to the full service-manual) - it looks to me like it's smearing due to the intensity being far too high - most scopes of this type I've seen, Hamegs in particular, aren't designed to operate with such a high degree of readout intensity, and there may be a fault in the CRT grid bias circuits!
Turn the intensity right off on both the trace and readout, then gradually increase the trace intensity until it just becomes visible - if it is still badly smeared, try disconnecting the readout board again, if you get a clear trace, the fault may be in the interface between the digital parts and the intensity-mod circuitry!
Can anyone upload the complete HM1507 Service Manual or PM it to me please? I can't find it anywhere (to buy even!) and R & S only send me user manuals every time I've asked them for one!
Chris Williams