Injecting video in DuMont RA-102?

[Phil Nelson]

After much travail, my DuMont RA-102 works reasonably well, but I'm curious to see how it performs if I inject video & audio signals directly. The audio injection is simple and it sounds peachy. The video is another matter.

My first instinct was to lift the 1N34 video detector diode from pin 4 (1st grid) of the video amp tube V4 and inject video there, but the results were poor: wobbly sync and strange contrast, sometimes with almost-negative images. To check whether it's a polarity issue, I unhooked my DVD player source and substituted my BK 1077B Analyst. The results seemed equally rotten whether I switched the video polarity negative or positive.

This set has only one stage of video amplification. The pertinent schematic snippet is here:

http://antiqueradio.org/art/DuMontRA...oSchematic.jpg

The RA-102 Riders manual calls for a nominal -1.5 volts at the video amp grid (pin 4).

Will video injection require adding a little video preamp? I built one such years ago for my RCA CT-100 (http://antiqueradio.org/VideoAdapter...Television.htm) with good results. I'm not hell-bent on doing this now, but it would be nice to have a clearer idea of what the RA-102 would require.

You can find the full schematic at these links:

http://antiqueradio.org/art/DuMontRA...chematic01.jpg
http://antiqueradio.org/art/DuMontRA...chematic02.jpg

Regards,

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html