Seems like there should be an actual horizontal input, not a trigger.
I do have a circuit from the Handbook to put a capacitor and 5 megs of resistance between the modulated HV and the scope horizontal.
Do I need a different scope to get the trapezoidal display?
The frequency response will not matter much because that is a gain vs. frequency problem. It is not a linearity problem.
As long as you get adequate vertical deflection, and as long as earlier scope stages do not overload from being overdriven, all will be well.
The problem is you can **NOT** use internal sweep for a trapezoid. The sweep has to come from the modulation source at a point where the modulation is very clean and not distorted. This would generally be in an early audio amplifier stage.
If you think of how the display works, you will understand why. If you just swept with a triggered sweep on the X axis, every RF cycle would show as a deflection related to RF amplitude. You would see a standard waveform.
To see the trapezoid, the horizontal sweep has to vary with audio level and frequency. You would have a little dot at rest that would move at a slower rate left and right with audio positive and negative, and up and down with RF envelope at a very fast rate. If transmitter modulator and RF phase was in phase with the audio and you had negative left and positive right, the RF envelope would expand on right sweeps and contract on left sweeps. When you hit 100% zero on negative peaks, you would get a point. If the audio input continued negative but RF was already zero (100% negative peaks) it would trace a thin line left out of the point. The positive audio voltage peaks would drive the X axis sweep right, and the modulated envelope would increase. This would draw a wide vertical base to the right.
So you see, while you can and should trigger off the audio (to prevent burning the screen), you have to sample the undistorted audio for actual sweep. Then the RF becomes the other axis.
Jimmy Hendrix even sang about this axis, as he distorted his Marshall amplifiers. :-)