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Monitor conversion of B&W TVs

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Posts: 289

I'm new here and I'm from Oz so apologies if I'm covering old ground.

I saw a post about adding AV inputs to vintage TVs. I know a bit about this subject having built a substantial business in the 1970s around doing just this to colour TVs.

Here are some salient points:

1. Firstly and most important, never attempt this on a live chassis TV. It can be done but it requires either a mains isolation transformer or a well-designed high speed optical isolation circuit with critical safety measures. The life you save may be your own.

2. It's easy to make it work, it's difficult to make it work properly.

3. Many older B&W TVs use the IF AGC for contrast control. Bypass the IF, you bypass the contrast control, you will need to add a variable gain amp and interface it to the original control. TVs with high level passive contrast controls are better candidates.

4. In most cases, if you wish to retain black level (trust me, you do), you will need to include a DC restoration clamp in your design. To be authentic it needs to be a driven clamp and operate on the back porch. Unless carefully designed which may mean a split into two channels, sync and vision, this clamp will interfere with sync.

5. Your video buffer will need to provide the gain to match the original 0 - pk level and offset at the vision detector. Skimp on this step and the result can be bad sync and/or white crushing.

6. Rapid APL changes from what is an AC coupled source may result in dropped frame sync for brief periods if the sync time constants are not adjusted accordingly.

You get the idea. It's not as simple as it looks.

A well-designed modulator is a better choice unless you have the time and the knowledge to engineer a proper solution.



Topic starter Posted : 30/06/2020 6:49 am
crustytv and ntscuser liked