[old_tv_nut]

I looked at the control panel of the lcg 400 and apparently there is no way to turn off the B-Y or R-Y phases independently. With a full screen color bar pattern, you should get patterns like the attached on the CRT
(left to right):
cathode will be inverted luma (left waveform, but inverted))
Blue grid will be the middle waveform when B-Y phase is correct;
Red grid will be the right waveform when R-Y phase is correct

Relative amplitudes of the three waveforms may be different due to need to drive red gun harder.

It's much harder to get this exactly right this way than using the gated rainbow and adjusting for the correct nulls, which are very sensitive to correct phase; but it can be done close enough.