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Injecting video in DuMont RA-102?
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 Last edited by Phil Nelson; 01-30-2017 at 06:07 PM. |
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Phil-
Check out this thread about direct injection and my Crosley "Swing-A-View" http://videokarma.org/showthread.php?t=268178. My set has only one stage of amplification also.
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John |
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Thanks, that's interesting. I guess the executive summary is that you're close to making it work, but still have an interference issue. Is that a fair statement?
Regards, Phil Nelson |
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Phil,
The 1.5v you mention is likely a DC bias voltage, not a P-P video signal. Experiment with it, try negative and positive going video to see which it likes more. You may need a higher amplitude signal at that point for it to properly pick up, I've had varying degrees of success with different sets in the past when doing this. For example, the 52T-1634 Philco I had as my first set made a dazzling monochrome picture with nothing more than normal composite output from just about any source. Color sets less so, they tend to need a stronger signal hence the amplifier to boost it up a bit. You say it works 'reasonably well' right now, what exactly is keeping you from giving it the ole siskel and ebert? I ask because there's a good chance that what's keeping it from being outstanding now will likely still keep it from performing it's best later, so why not get it where it should be first then try injecting video? My experience has always been that small problems become bigger ones post modification, and become harder to trace down. Only exception of course being tuner/IF troubles, I eliminate a lot of problems by bypassing that early in a resto.
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Evolution... |
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Quote:
In light of that, here are the remaining issues: 1. Weak audio. Sounds OK if you crank it, but you shouldn't need to turn volume halfway up before hearing anything. When I injected audio on the RA-102, it was strong like my RA-103. (Yes, I did tweak the audio adjusters to get best (under the circs) volume & fidelity at the "best video" tuning point.) 2. The tuning is twitchier (narrower?) than my RA-103, and the tuning meter is pretty unresponsive. The needle does move a bit while you tune, but way less than I'd expect. Again, if bad alignment equals weak signal, I guess these symptoms aren't surprising. (I have cleaned the devil out of the tuner, its tube sockets, etc., and both tested and tried subbing all of those tubes.) 3. Vertical jitter. There is still a slight jitter, even after I replaced a vertical xfmr and most other vertical components. Not noticeable when watching ordinary program content, but visible if you display something like a cable TV program guide with lots of horizontal lines. Dumont published a service note with a modification to reduce jitter; I have read that, but not tried the mod. 4. The picture blooms and loses focus if you have set the contrast/brightness to correctly display ordinary content, and then the station displays a very bright scene, like a white ad screen with small dark lettering. Poor HV regulation? In the course of getting the HV to work at all, I replaced nearly every related component on the p-s chassis. Quote:
Regards, Phil Nelson |
Audiokarma |
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No that interference was not in the video amplifier circuit. I moved my discussion on that video amplifier to a thread about the restoration of the whole set. I should update the video amplifier thread to point to the new one. The circuit works really well, although I am using a 12v battery for the power source. I may circle back to the power supply after fixing other issues with the set. The trick is finding a DC power source "clean" enough to not inject problems of its own.
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John |
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Quote:
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Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4 |
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Try hooking your scope to your 1077's video output and verify that it's capable of about a 5 volt video signal. You can also place a jumper between the 1077's positive bias supply over to the video jack which will allow you to vary the positive bias that the video signal rides on. You'll want your scope set to DC coupling to see the bias level change. That bias supply is floating from ground, so you'll need to ground it's negative side for positive bias. You can also reverse the plus and minus and end up with an adjustable negative bias.
[Edit] I just got to thinking - The 1077's 25 volt bias supply is going to be pretty sensitive to adjust down around 1.5v, so you might put something like a 100k resistor in series between the bias supplies positive output and the video out jack. That should knock the DC bias down to a more manageable level. Last edited by Kevin Kuehn; 01-31-2017 at 12:43 PM. |
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Don't you have a B&K 1077 analyst? Those are great for test video injection...Give it a try and see what it does. It will be a lot quicker and easier than building a video amp.
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Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4 |
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Thanks, I tried using my 1077B analyst (see first post) without noticeably better results.
Regards, Phil Nelson |
Audiokarma |
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It's really odd that the 1077 isn't giving you results when you switch the polarity of the video output to positive. Did you try going "up the river" with the 1077 and injecting video on the output of the video amplifier with the gain cranked and the video negative on the 1077?
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John |
#12
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The thing expects positive going sync pulses at that grid, and an average
video signal that has a positive DC average value. And there is no gain control. You probably need a rather large video signal with an adjustable DC level to get the signal in the center of the range at the amplifier plate. Use your scope at the plate. (Yes, I'm a scope fetishist.) |
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I wonder if the transistor is too "slow". You might need a true video amp transistor.
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Quote:
Any suggestions? I know diddly about solid state. Thanks, Phil Nelson |
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I would modify it slightly so you're gain-controlling it from the input side.
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Audiokarma |
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