Electronic – How to use the Z-input on the oscilloscope (full range of contrast / brightness)

I guess all scopes are a little different and don't provide any adjustment knobs for the Z-axis input. Mine is a Protek 6502A and it's Z-axis spec says

Z Axis Modulation:

• Sensitivity: 5V P-P (Intensity increases with a negative going input)
• Bandwidth (-3dB): DC to 2MHz
• Input Impedance: Approx. 33 KΩ
• Maximum Input Voltage: 30V (DC or AC peak)

The input is the normal BNC probe plug and no separate GND. So I am already apprehensive that I will need a second voltage supply. My setup is TTL digital and I don't have a bench PSU, just drive everything off 5V DC cell phone chargers.

So, I just went ahead and added a second charger and connected GND of first to +5V of the second, and now I have a +5V – GND – -5V setup. Then I plugged a 1 MΩ trimpot between the two extremes and lead the middle to the Z-input.

I have hooked this up to my brand new 256 x 256 raster scan generator that I built with counters, buffers and R-2R resistor network (and it comes out pretty flawless, I'm happy to say.)

Now if I have the intensity knob just past the middle, say at 1 o'clock I can get the screen to go blank if I turn the pot over to the +5 V rail. When I turn the pot over to the -5 V, I get the brightness maxes out before the full brightness I can get by turning the intensity know further up.

I'd like to have the full contrast range on the Z-axis, but the specs seem to tell me that 5 V peak-to-peak is its range, but I doubt that. It also says 30 V max, so I guess that means beyond 30 V I would destroy it. But will I benefit from +/- 9V? Or +/- 12V?

And what if I were to build myself an auxiliary power supply with +/-15V, would that mean that I am then pushing the 30 V absolute maximum specified?

Here is a photo of what I'm getting if I hook the Z-axis into my X-axis ramp output (which goes from 0 to +5 V). It's not bad, but I like more brightness on the bright and more contrast range, with the black still black.

UPDATE: I bought a pair of 9 V batteries and with the pot between -9 V and +9 V I get about the full range. I suppose +/- 12 V would reach the full range. So, now I need to figure out how to convert the TTL > +2 V to -9 V and < 0.5 V to -9 V. A comparator / op-amp comes to mind. But it wouldn't work, not yet.