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Hi,
On the Spectrum +3, disconnect R41 (150 Ohm resistor). The 75 Ohm resistor inside the LCD panel, will be in parallel with R41 inside the Spectrum +3, giving a combined terminating resistor of 50 ohms. The sync output voltage will drop from 1.2V to 0.48V. This may be too small a signal for the LCD panel to lock onto and sync strip.
It looks like the Spectrum 48K has a 75 ohm output impedance with a standard 1.2V signal, which could explain why the 48K works better.
Good luck.
Ian
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