UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

Argus25 · 2nd Mar 2018, 1:47 am

Keith,
I have built many video modulators over the years. For your application I would recommend:

Use crystal control 41.5MHz and 45 MHz with two separate amplitude modulators and mix them in the correct ratio with the sound one 6dB down. Then a buffer amplifier and level control is helpful.

Don't forget, in the case of the video one, to sync tip or black level clamp the video at your modulator's IC's input (if you are using an IC) or otherwise on many sets (eg Bush TV22 and others) you will have retrace lines appearing at times.

Also, make a board with a ground plane, or you will find any measurements you make with the scope will be confounded by where you connect the scope earth along with other issues of not using a ground plane at these frequencies.

Hugo.

2nd Mar 2018, 1:47 am	#7
Argus25 No Longer a Member Join Date: Oct 2016 Location: Maroochydore, Queensland, Australia. Posts: 2,679	Re: Modeline problem Keith, I have built many video modulators over the years. For your application I would recommend: Use crystal control 41.5MHz and 45 MHz with two separate amplitude modulators and mix them in the correct ratio with the sound one 6dB down. Then a buffer amplifier and level control is helpful. Don't forget, in the case of the video one, to sync tip or black level clamp the video at your modulator's IC's input (if you are using an IC) or otherwise on many sets (eg Bush TV22 and others) you will have retrace lines appearing at times. Also, make a board with a ground plane, or you will find any measurements you make with the scope will be confounded by where you connect the scope earth along with other issues of not using a ground plane at these frequencies. Hugo.