I attempted a mod within the ZX Spectrum to generate a blanking signal so that the display would only show the RGB output.
Whilst it did not seem to be causing an issue with the RGB display TR5 and TR7 were fitted the wrong way around at the time of manufacture. So I swapped the TR5 and TR7 transistors with new ones and soldered them back the right way around. TR5 generates the RGB vertical sync and TR7 is responsible for generating the ULA IOREQ signal.
I then attempted the blanking signal mod which involves
- Moving the link from LNK2 to LNK4 position
- Adding a 470Ohm resistor in position R9
This mod disconnects the Composite video signal on pin 1 and replaces it with +12V, which through the voltage divider provided by R9 provides 1.5V on pin 16 of the Scart plug which will switch the TV into RGB mode.
Unfortunately, after the mod the display did not work and was blank both on my TV and on the monitor using a SCART to HDMI adapter.
After rechecking everything the issue could not be fixed so I moved the link back to position LINK2 which restored the picture but the ghost image was still there.
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