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I think video amplifier IC's are available that have a 75 ohm input impedance. You would need a gain of at least 2 to offset the termination voltage decrease. You could use diode clamps on the output to help protect from a tube short, one to ground (or negative supply) and one to the positive supply for the amp. Those diode would have to have a low capacitance so that they didn't effect the video signal. Any DC restoration would have to be applied to the video output tube circuit, once you pass the video through a capacitor you nullify the effect.
Nice set! I can imagine you would love to restore it, but as we know, you have so many projects going on! Will look forward to seeing and hearing the slow power-up when you get to it, just to see where it stands and what it might do, if anything.
Sllers think you are going to fix it up and then turn around and sell it on ebay for a large profit. You can't get more money for one of those televisions then it was brand new Any more then that then the person who is selling it is out of their minds.
Really enjoyed this video!...great idea to show the picture with a direct injection signal!...always wanted to see how the color information would have displayed on an early B&W set capable of 4.5 MHz bandwidth!!! Were many early sets capable of full bandwidth? Have you seen the color dot pattern interference in any of your other set restorations?
Yes and yes. Going way back over 10 years ago I showed it in several similar Admiral sets. Early RCA, Dumont, Zenith, Stromberg, etc had excellent bandwidth. All the high end brands in other words.
composite outputs expect a 75Ω load. Without it (and assuming the grid/bias circuit of the video amp is a much higher impedance), you’re essentially piping in double the 1.14Vpp amplitude. This isn’t necessarily going to damage anything, but it would explain the choppiness of the window-circle pattern and excessive contrast.
Cool looking set Bob! When I first saw it, I assumed someone bolted a speaker box to it to improve the sound quality. I can't get over the size of that power transformer, looks like it is good for 1,000 watts and probably accounts for half the weight of the set. From the overall condition of the set, this should be a more pleasurable restoration.
Great video Bob! I would terminate the composite video input with a suitable non inductive resistor to ensure voltage levels are correct; also, using this resistor may help dampen any flashover voltage transients within the video output valve that may damage the external video source. As a simple form of DC restoration, maybe think about using a diode to clamp the video input during a negative synch pulse?
Great job getting this done in one night! We always run into stuff that ends up taking longer or not quite coming out as perfect as we hope. Really looks excellent though, and I'm surprised the direct injection worked that well with just one video amp stage. I have done it once on a 19" Zenith porthole a few years ago, it worked well but I guess the color information herringbone effect explains why those patterns were appearing in the picture, despite having great bandwidth and definition. The Zenith did have 2 stages of video amps but perhaps less gain per stage, can't remember the exact tube types. The audio was a bit trickier because the Zenith used only an output tube to drive the speaker, and the previous stage was a gated beam detector (with buzz control). I was able to make that tube a regular audio gain stage by replacing the quad coil with a resistor and feed the combined mono audio to its grid, and it worked well.
To elaborate on why I had to modify the gated beam audio tube to make it an audio only gain stage was because injecting the audio to the volume control yielded insufficient volume, because the volume control feeds the output tube, and that tube by itself did not have enough gain.
Direct video injection is a more complicated question than it seems. I can understand the purist viewpoint for sure and the direct injection would make more sense if the set was going to be used a lot but I suspect that most of these are not. Having the option for both sounds like the sensible solution. It's a beautiful set too 😊👍 PS: I am about to take the back off a 14" Panasonic (1999) to check the caps. Wish me luck! 😄
