Questions and minor issues

[fbachofner]

Thanks again, all! If I decide to do the resistor replacement I'll post my experiences with it.

Please post photos of the process and a hardware list (and sources) if you can. One of my two MVPs displays consistently dark video and I would be willing to try to fix this with a guide!

Thanks in advance.

[zaphod7501]

This mismatched resistor is really starting to intrigue me. I work for an industrial control company and we do a lot of in-house hardware work. I wouldn't be hard for me to have the 220ohm resistor "professionally" replaced with a 75ohm... Would this really fix it? I'm debating whether it's worth it or whether I should just stop being such a perfectionist since the video as it stands now is more than watchable.

Thanks again, all! If I decide to do the resistor replacement I'll post my experiences with it.

Oh, if it were only that simple. I am an electronic tech. I've been servicing camcorders since '85, VCRs since '76, and TVs since '72; and I did quite a bit of experimenting with impedence matching on my MVPs. I believe that it's a basic design problem. Hopefully sub won't mind the use of bandwidth this could take to explain.

Normally a video output circuit connection will measure 75 ohms to ground, both on the input and output connector. If you were to scope the output without anything connected, it will be well over 1V P-P, but after the cable is connected to a 75 ohm input connection, the level will drop to 1V P-P (standard video level). With an MVP, you would measure 220 ohms and about 1V P-P on the output and after the cable is connected to a standard 75 ohm input connection, the level will drop to about .6V P-P (too low!). The chroma does not seem to be affected as much for various reasons (frequency, DC level, no sync, AGC, etc) leaving a symptom often described as oversaturated color. It's actually low luminance levels.

The simplest solution would be to change the resistor on the TV's (or a powered A/V switch's) input connector. This would allow the video level to be closer to the 1V P-P standard.....but it still won't work correctly because the output of the MVP is not stable if it does not have a sufficient load on it. The video will fluxuate from OK to washed out, depending on the video content. I found that a 100 ohm load works pretty well. I took an four way A/V switch apart and put a variable resistor in place of the 75 ohm input resistor and adjusted it for best operation (ended up about 100 ohms). This way the MVP is on V1; a DVD on V2, etc and the TV has about the same brightness on all inputs as a result. No modifications were needed on the MVP or the TV, only on an inexpensive A/V switch: but it has to be an electronic switch, a mechanical switch won't work. A distribution amplifier would work also. A distribution amp with input level adjustment would be perfect (I haven't found one with this feature).

The reason is because the output of the MVP is connected directly to a video processor IC and not a video buffer circuit. If you looked at a schematic of video output circuits, you would generally see the output of a chip going to a transistor amplifier (buffer) before connection to the output jacks. This is for "impedence matching" so the output has the same electrical characteristics of the input on the TV (or VCR, or switch, etc) and to protect the chip from static discharge when plugging the cables into it.

The best solution would be to build a video buffer circuit, connect it to the S-Video output, and produce a video output that would connect correctly to standard devices. You could model the circuit after designs found in VCRs and DVD player schematics. I had time to modify A/V switches and the inputs of a Receiver/Amp but not enough to design a full correction circuit. You need to be able to adjust the gain of the amplifier not just the input level like I did with the A/V switches.