Adding Composite Input (Capacitor Question)

[Electronic M]

Originally Posted by ARC Tech-109

That's what I had in mind, DPDT relay to feed the 1st video amp while cutting the DC to the IF amp, composite goes in after the 4.5 sound trap so I still maintain the DC integrity of the grid, did some tests just poking a 1V Hi-Z video signal in and got excellent results.

I've found it unnecessary to cut power to the IF, it doesn't really change the opperation of the Deknis circuit on the CT-100 or 21CT55... If you build it with the socket adapter and use a clip lead to harvest the B+ that goes to one side of the rear most contrast pot next to the HV cage you can make the video mod a completely bolt on/plug in mod...
...With audio that becomes more involved. On the customer set I added a stereo headphone jack to a bracket that screws to the tuner... the jack is configured to short the detector output to the audio input (return the circuit to stock) if the plug from my preamp module is pulled out, but pass both lines to that module in isolation if the plug is in.

If life wasn't so busy I'd build a few more and sell them. I think I did a really good job with it.

[ARC Tech-109]

The reason for doing it this way is so I can mechanically isolate the video input line from the grid of the 1st video amp when using the tuner and then cut the IF amp off when using the video input so I don't get swamped with mixer noise. Not cutting the grid out of the existing circuit as I see no need to and this way the modifications are kept to a minimum.

[Electronic M]

Originally Posted by ARC Tech-109

The reason for doing it this way is so I can mechanically isolate the video input line from the grid of the 1st video amp when using the tuner and then cut the IF amp off when using the video input so I don't get swamped with mixer noise. Not cutting the grid out of the existing circuit as I see no need to and this way the modifications are kept to a minimum.

I haven't seen mixer noise doing it the way I have. Neither circuit noticably interferes with the other when the video amp grid is switched.

[ARC Tech-109]

Thread Revival Time... long post

Been running my Magnavox T940 on a steady diet of both composite video and real RF for the better part of the summer and aside from a few unrelated issues with the focus circuit I'd say it's a win.

Okay so how this all works is on the rear of the set is a 3-position 4-pole rotary selector switch that I installed the summer of 1985 on the existing tape I/O strip and the function of this was to select between internal detector audio that was actually provided by an internally mounted Recoton "FRED" V622 MTS decoder that picked the 4.5 SIF using a coil of wire wrapped around the detector tube, not exactly elegant but it worked for over 20 years. On the internal stereo receiver it was a matter of cutting the two resistors feeding the original detector audio to the main selector switch to get a left & right which was brought to the wipers of the new rear selector switch and the second set of selected contacts were wired to a new set of RCA jacks on the I/O strip to provide a simulcast function, RF video and external stereo audio from a laserdisc or stereo capable VCR which at the time was a Sony VO-2610 U-Matic. This was during my high school years and the set has been following me through life since then.

Now over 40 years later with the everything back in working order a composite input has successfully been introduced. basically its a SPST relay that's been wired to the video test point TP-201 on the IF board to switch the input without loading the 1st video amplifier from the detector with shielded coax. I've found that even a 3 inch piece of wire or coax is more than enough to smear the details and chroma.

So what's happening here now is the 3rd position of the rotary selector switch is not only handling the input audio but also controlling the +200V source to the 3rd video IF plate and closing the relay to feed composite from a rear mounted BNC connector, the video amp works best with about 2.6 volts P-P so I'm using an output from a Kramer VM-1045 distribution amp that's been turned up to provide this and that gets fed from a Leitch A/V switcher that I funnel my sources into.

I constructed the RCA CT-100 video adapter circuit following everything to the letter but for some reason it would not play nice with the Mag no matter what I did and found it worked well with the video provided by the Kramer so this is what I'll use in the house.

The reason for the switched +200V is the IF would leak into the video detector even if the diode was shorted or lifted so shutting down the stage was the best solution.

So viewing results.
Both the RF and IF alignments are very good after spending some 8 hours going thru the procedures and making little compromise adjustments along the way and honestly it looks nearly as good as a composite signal with only a slight bit of color beat on the fine details while composite seems a little better despite a full 5 MHz of bandwidth from the digital betacam, type-c is just as good coming from a NOVA 800 digital full-frame TBC. Biggest limitation seems to be the coarse RGB trio count of the delta gun CRT and it only shows just under 300 lines properly focused.

Now my biggest complaint is videokarma is not allowing me to upload any images no matter how small.