Thanks for the email and information. The picture of your setup shows that there may be a sync problem for using our RGB converter.
Our AV-1 model is designed for taking care of the standard CGA type RGB signals, which were the most popular spec among all RGBs. CGA has 15.76Khz for the horizontal frequency and 60Hz vertical frequency. If your RGB input signal does not follow this spec, our converter can not process the signal correctly.
Can you look into the spec for H freq and V freq of your old Spectrum +3 and see if it is compatible with standard RGB CGA ?
For troubleshooting purpose, a good source of providing such compatible 15Khz RGB signal is DVD player with 480i or 576i YCbCr component video output(not 480p or 576p which are YPbPr and progressive). If such a device available for you, you can use it to test out with our RGB converter to find out if the converter itself has a problem.
Sincerely,
Support