Quote:
Originally Posted by Semir_DE
- Sync tip clamp for DC restoration.
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Well done. This is very important and sometimes neglected. The DC axis of the video must be stabilized prior to the modulator, with clamping at the sync tip or the black level. If not there are serious implications for sets which DC couple the signal from their detector to the CRT. (In some designs of sets that re-insert the DC later at the CRT grid or cathode, often popular in American designs, it is not a problem).
For my modulators they are modified Aztec units for Xtal control, which is also essential I think and I organized the appropriate drive circuits with the video sync tip clamping.
But, the problem is not over there. Building proper modulators is only part of the problem. The RF outputs of the video and audio modulators need to be combined and then level controlled and metered (in my opinion to have a proper generator). This is also very handy as a tool to help service and test & verify the performance of the TV set. It is often neglected though.
I have attached an image of my mixer/combiner and level meter system and the schematic. It was built into a shielded enclosure with smc connectors and uses a hand made double sided pcb with a ground plane built into an old tuner casing. The level control device is a professional mini circuits RF attenuator.
The unit should ideally also have video and audio level controls & metering prior to the modulators too.
The article on the converter unit I assembled with my modulators in it is here:
http://worldphaco.com/uploads/RF_MOD...TELEVISION.pdf