Adding a composite video input to this 1977 portable TV

hello everyone and welcome back to adrian's digital basement on today's video we're going to take a look at this little panasonic tv that i showed on a super mini mail call on the second channel i'll put a link in the description to it if you haven't seen the video about this tv one of the problems with this tv is while it works fine is it only has rf input hence the two tuner knobs here i think this thing would be a lot more useful if i had a composite video input on it so on today's video i'm going to add a composite input to this tv so without further ado let's get right to it [Music] this little panasonic tv is just so chunky and awesome looking i want to be able to use this a little bit more especially because it's just so easy to move around because it's portable and of course it runs on dc 12 volts means it's very easy to power with any old 12 volt power supply that's lying around so this rf only input was a product of its time and certainly was normal for back then but it just means that it's really hard to use this tv these days especially with the use case of what i want to use it for which is retro computers so i think it really is logical to add the composite input to this thing to make it more usable and for me the more likely to actually use this thing and not just have it sit on the shelf and collect dust i'm just going to start removing the screws on this set now i will talk about something lots of people say when i'm working on these old tvs or monitors that i should rgb modded or composite mod the tv and i often write this in the comments but a lot of times that is just not possible to do cheaply or safely and really the reason for that is a lot of these sets well the larger versions at least run on mains power and don't have an isolated power supply design that means on those sets if you take the plug it's a two prong plug a two pin plug and you stick it in the wall you are relying on the fact that you are using a polarized plug to make sure that metal things on the outside of the case now not the antenna jacks because those have a transformer but say i added a composite jack the ground which would be connected to the chassis ground of the set could potentially be live at mains voltage if you plug the plug in the wrong way i don't have to say of course that that would be extremely dangerous and even if i go and add a three-prong plug to the set so you can't reverse it you're still relying on the wall outlet to be wired up correctly and that's just not 100 certain so when you look at computer monitors for instance ones that have composite input and ones that are from this time period those computer monitors at least the ones i've always seen always use a transformer inside to take the mains voltage bring it down to some other voltage and then there's a little power supply that generates the b plus when you have a transformer everything inside the set is now completely isolated from mains voltage and no matter how you turn the plug around there's just no danger of you getting a shock now set like this it already runs on i'm sorry i'm just looking for all the screws here are there any on the back no it doesn't appear there are a set like this runs on dc there's no ac at all involved with this set so that means that you're pretty much safe to go ahead and add a composite mod okay so i bet you these knobs here have to come off to get the back cover off there we go you just pull them out i'm going to try using a little spudger here to help and how about these down here oh nice okay you have to have strong fingers to get these knobs off so sometimes a little tool goes a long way to help there we go this bottom one had a little plastic spacer of sorts and interesting is the top part of these knobs is actually rubber see it's kind of squishy in fact no the whole thing is kind of rubbery kind of neat and it's nice that that rubber is not solidified okay so let's see does this come apart oh yeah look at that that was the that was the key right there all right so there's a couple wires here because this is the speaker and there's a headphone jack there so i'm assuming these connectors come right off so there's one all right and the other one there's a connection inside the case all right so here is the design inside this thing so a little crt neck board it's just so super cute look at this little five inch black and white crt i love it one thing that is totally coming across by inside this set is it's really really clean inside now it's it's very likely that a set like this never got used very much because of course who's watching a ton of tv on a five inch tv not a lot of people but the flyback transformer here is well there's a little bit of stud on it but it's really clean compared to what you might expect continuing to look inside you see this metal can here and there's another metal can right there those are of course the tuners for this set which translate to the knobs on the front there i'm not going to be touching these or taking them out but the tuners feed down to this board down here at the bottom which will be taking the rf signal that's coming through the tuners and then decoding that into a video signal and then this board right here which is loose because all the screws are out that board will be taking the video signal from the tuner and then turning that into something that can be displayed on the monitor so it'll handle decoding to sync signals from the composite signal and then doing the deflection and then also doing the cathode drive to create an actual image almost certainly panasonic did something like this because the tuners and this uh if board probably are going to be slightly different from one region to another and that way that you could just still use the same video board here at least for ntsc regions to display a picture and just swap out the tuner section now as far as audio goes i haven't really talked about that so the speakers here in the headphone jack notice here there's sort of yellow and black wires here that go to this connector that connector went to this connector here down on this if board on the tuner board so i'm pretty certain it's this tuner board down here that is generating the audio and has the amplifier on it and i don't want to modify this monitor too much in case i ever want to restore it back to the way it was so i'm probably not going to be doing audio for input on this thing just video i gotta give props to panasonic for a pretty nice layout here the inputs to this board are up at the top here and we have the various sections that are labeled so i went and bought the sam's photofax schematics for this set and i'll show that in a second but i want to see it by just looking around here if i could figure out potentially a way to do this mod without even having the schematics like say i couldn't find the schematics so down here on the tuner board there's this connector right here now this is just going to the tuners here so we know that that is just an rf connection for for these things but i need to look at what connects the tuner board up to this board and i see some connections right here and it looks like those run down here now when i look up here at the top at these wires coming from the tuner there's something here that gives away what might be the video signal that i want to tap into let me just snip off the zip tie here so we can get better access to the wires so ignore this red wire which says b1 that might be like b plus like voltage or something take a look at this wire here sort of a brown purple wire it's shielded so e1 is a shield and then there's a white center that comes out here c9 almost certainly this is going to be the video signal that comes from the tuner board up to this board outside of rf signals inside tvs there's very little reason to have a shielded wire for any other purpose other than video back here where these wires are connecting this is the volume control and also the power switch for the tv and that red wire is coming off of this going up that top board so almost certainly that's going to be the main power like the dc input that is going up to this board anyhow i think i might be in luck that these video wires are so easily accessible right here because this mod might be as simple as hooking this up to an rca jack that i screw into the back of the case potentially with a capacitor to block some dc voltage stuff like that we'll we'll have to measure and take a look at what what we see on this wire here is the sam's photo fact for this model tr555 from panasonic okay scrolling down originally this foot effect if you had the actual paper would have been one giant sheet so really easy to look at and unfortunately uh they split it when they scan it would be nice if they didn't do that these are the tuners here and then it outputs here and then comes in right here i think if from tuner so there's all the tuning and the decoding and whatnot that happens anyhow onto the second page of the schematics here so this section right here is tuner related stuff and what's going on right here is what we're looking for test point 13 on the tuner board notice here c6 there's a white wire and see this little loop to ground that is that browned cable up here that i was talking about with the shielding that is what's going on right there and that is almost certainly the video signal and it comes into that analog board into this analog board here it goes through a resistor and it splits off and this wire that's going off down here was going to that sync section we were looking at before the sync separator but this with the video signal continuing on there's a 4.5 megahertz filter and it says right here that the amplitude is about one volt peak to peak which is correct for a video signal that's typically what ntsc is one volt peak to beak so i have to say there's a good chance that from a level perspective the input on this board is already matched and expecting a regular video signal now this 4.5 megahertz filter is going to be here to try to remove like chroma information from the video signal because of course this was out when color tvs existed so to prevent that dot crawl you get on ntsc they installed this filter and looking at the board here this green thing is almost certainly that filter i'll have to just check the part number there it says x141 and on the back yes it's the green thing that says x141 so i'm going to have to remove that part to bring back the sharpness of this set because with that in place it's going to be pretty soft when trying to display any graphics from a retro computer all right so first thing i'm going to do is i'm going to remove the white wire and this grounded cable so this should come out there we go so that video signal that comes from the tuner should be disconnected now and while i'm here i'm going to remove this filter as well and i think what that does is anytime there's part of the signal that's over four and a half megahertz it just sends it to ground now as far as a cable to bring that video signal from the back of the case which i'm going to have to install jack to this point i'm going to use this which i saved out of an old crt or an old monitor and these are shielded coax type cables so very similar to the one that was on here so i just stripped some of the wire off you notice there there's the shielding and the center conductor will be carrying the video signal and this just helps eliminate interference on that cable cool just like that it's connected up i'm just gonna apply some heat shrink onto this existing cable from the tuner so it doesn't short out to anything so let me strip off the end of this and we need to test this i mean maybe i'm on the wrong track here and this isn't even going to work at all anyways so i went and grabbed a 75 ohm resistor i'm just going to connect it between the input and ground this resistor in the monitor along with the one that's in series in the device you're connecting create a voltage divider which is also part of the termination of the video signal this rca cable here comes from the test pattern generator my tektronix okay we're all clipped in and we're ready for testing moment of truth time are we gonna get a nice clear video signal or are we gonna get something that looks like garbage oh okay i'm getting absolutely nothing here nothing at all interesting sometimes it's a good idea just to rule out that your connections aren't a problem with the cable or whatever so why don't we uh plug this into this apple monitor here okay and turn out the cable i was using was bad here we are connected to the monitor now we have a nice sharp and clear picture there there it is so let me clip on again and try this one more time hopefully i haven't gone down the wrong path with this with this monitor okay we do not have a good picture okay interesting oh wait when i unplug the resistor we do get an image although i gotta say it looks horrible there are retrace lines completely visible in the entire image for some reason and if i hook up the 75 ohms it does just completely blank it out okay as you can see i have an image here on the set again but this is actually hooked up again through rf i removed that heat shrink on the wire and i clipped it all back together because i'm doing now what i should have done from the very beginning i should have looked at the signal that comes out of the rf stage before it goes to the analog board because even though we have that picture it was really too dark now here is the video signal coming through that coax wire up to the analog board from the tuner and you'll notice that this red mark here is ground and that the video signal is far above ground now basically it looks like what was happening is that the analog board i'm calling the analog board the sport on the side expects to see a video signal that's about two volts above ground so if we look back at the schematics here there's where i cut this stage right here seems to be where that dc offset is coming from so one possibility is i could attach the video input signal over here on this side of this transistor the problem with doing that though is it would invert the signal you can kind of tell that's happening look here at this uh a little oscilloscope graph here you see the lines are up near the top and if we scroll over here here on this other stage here they're down near the bottom incidentally on the emitter right here of that transistor you can see here 2.38 volts which pretty much matches what we're seeing here on the output of it so what i need to do is figure out how i can apply a dc offset to that input video signals of around 2 volts and then we should have a good picture all right take a look at that we have a really really nice image and this is a video input directly from the pattern generator and it looks perfect now now let's take a look at the oscilloscope here notice we now have the entire video signal a little bit above ground well it's about a volt above ground and that makes the monitor very happy now i am supplying the extra voltage which is four volts right now from my bench supply and i'll show you the circuit in a second but i'm using a couple resistors and a capacitor here to do this dc offset now if i turn off the power supply look at this the image fades away in fact let's switch to something that's a little bit more obvious there it is now if i turn this back on look at that it gets bright those scan lines go away it now looks good and when we turn off the power supply it will fade away i have to say as well the sharpness is amazing what we're looking at here is a resolution sweep so these lines get closer and closer together and there is still definition in the lines all the way over on this side and i can tell you if that little filter were in place this would just be a solid gray mass we would not see those lines and here is my rough sketch so video comes in there's a 47 microfarad cap that i stuck there we have ground down on this side with a 47k resistor and we have 100k resistor up to b plus now well i have my bench power supply right now but i will use the b plus in the monitor which is about 10.9 volts so i have the bench supply set to 11 volts right now just for testing and then this wire goes off into that board now for my understanding this arrangement with the cap and this resistor creates a bit of a low pass filter so that could cause some high frequency roll off which could potentially make this a little softer but i tested removing that resistor to ground and turning the power supply down otherwise uh kind of runs away the voltage will slowly drift up and while it's slightly brighter in this section it's very minor there's almost no noticeable difference alrighty first things first i'm going to disconnect the positive from the video signal and i'm going to install this cap next up the 47k resistor that goes to ground i'm just going to install that right on the back here i have the cap right there on this side of the board and that's the white wire that will be the video input i need to connect that to this lead of the cap and i'm going to put some heat shrink on it and then we have a zip tie just to hold everything together but we're not quite done in this section i need to find uh the 10.9 volt b plus which literally might be that pin right there b and then i can attach this 100k resistor to it okay i just put the power in the tv let's power this up and see if that is indeed so this is ground over here oh that's 11.5 so that's actually i've been coming for the power supply i'm just going to use this 11.5 volt rail since it's right here it's the most convenient so the oscilloscope is connected i don't have anything hooked up to the video input let's see what voltage we see when i turn on the set all right so we're getting about 2.7 volts or so i think that should be fine i don't know why this is jumping around a bit though maybe i need to hook a video source up to it and when we look at the oscilloscope there is the signal coming in from the signal generator so that looks normal it's all around ground and when i turn on the set no change so the tv is on right now and yet it is still at the correct level and when we turn around the set there it is it's looking good perfect absolutely perfect i'm ecstatic about this and there's the convergence grid looking fantastic turn the brightness down so switching through all the various test patterns i'd say the only problem i'm seeing right now is maybe the video signal is a little too hot and i'm wondering if that's because it doesn't have a 75 ohm resistor to ground that i need to add let me experiment around see if i can get the picture a little less hot a little less overdriven and yes the video signal needs a resistor and i'm holding one on place right now 75 ohms between the center pin or the actual video signal and ground and this needs to be on the other side of the capacitor not right here on the board but actually on the back where the connector is where the video cable will plug in and that makes everything look a lot better on the set here like the brightness and contrast work properly without this i had to have the contrast knob turned all the way down otherwise it was just overblown and this is the resistor in place so i put vid there this is actually the ground on the video signal coming in and it also connects to ground on the set and a 75 ohm between the video there and this side here and that's basically part of the voltage divider in the video signal that ensures termination and whatever correct levels but it has to be connected on the left side of this 47 microfarad cap right here alrighty so the next step is just to implement this on the back of the case so i can actually put this thing back together and plug in a video input into it and there it is there's the video input now if i could do this again i would do a better job and center that i do have a punch i can't find it i don't remember where i put it and i could have put a center punch that way the drill bit wouldn't have wandered over to one side but being off center won't cause any kind of issue so uh why don't we give this a test okay let's hit the power test pattern generator is connected to the back we'll try some other sources in a second this set does take a little bit of time to warm up it's a bit interesting but there it is nice all right so let me put this tv set back together okay we have the apple 2c out and it's connected to the monitor which is on right now the focus set manually let's turn it on all right hopefully i have the camera in focus i gotta say it looks pretty amazing it is so sharp now there it is with attack of the petsky robots and it's pretty amazing how absolutely sharp this monitor is it's fantastic for a five inch screen i can't really show what it looks like with the rf but i can tell you it's going to be very very soft and i think a lot of it has to do with this filter that i took out so even if i had left it rf just removing this probably would have sharpened it up dramatically now there is one issue with this set and you can kind of see it going on right here the cathode drive on here unfortunately is the type where the contents of what's on screen affects the overall brightness of the set very common for black and white tvs to be like this i'm assuming it had to do with maybe making it easier to see what was on television but it's not ideal for a computer monitor and here's what i mean when i reset the computer this text is quite bright but if we fill up the screen with something white so if i go to text you'll notice it'll be bright and it will get dim there it is it's a lot dimmer now now the camera might be adjusting but overall it's a lot dimmer than it was now this has nothing to do with the capability of this crt to produce a good image i could turn up the knobs on the back of the tv and then it would look better but then the problem is if i make this screen look bright by let's see turning up the contrast okay so there we go that looks nice and bright still very sharp if we clear the screen now what happened is the background looks kind of gray and i sorry i turned up the uh the brightness control to get that so if we turn this down so it's not like that then if we fill up the screen again it's still kind of dim now this is completely to do with the design of the cathode drive in this set and it's not a fault it's just the way it's designed it's just not a very fancy design now if we go pr number three here now if i adjust the brightness so that it is at the correct level it's not overdriving that is amazing it is 100 percent clear and readable for a 5-inch tv set not designed to display high-resolution text this is coming out absolutely fantastically so am i pleased how this thing turned out absolutely let's switch to some regular video from this vcr and see how that looks so there's the vcr menu that we did see before hooked up through the rf input and it looks super amazing you do see the dot crawl happening that is normal because that's the color signal showing through in the luma that's just the way a composite ntsc works they pop in a tape into the vcr and there it is a certain movie with airplanes in it from the 80s looking pretty good and i have rewound the tape and we're just starting at the beginning here so there you can kind of see that thing i was talking about where the match was bright and the background get darker and there just did it there so it's it's not just the apple ii it happens on everything but again that is exactly how these sets work and with that i'm going to end this video here this little tv has this little video input on the back now and it is so much more useful to me now because i can just carry this around so easily and use it as a little set for computers even though it's got that inaccurate brightness on the screen or whatever that's just part of the design and it's still useful for just showing text and stuff like that anyhow i'm really happy how this turned out i think it makes this thing so much more usable so you'll probably see it on the bench in the future i want to thank the original viewer who sent this in i'm sorry i don't remember your name i'd have to go check my spreadsheet for that and i want to thank my patrons their names are scrolling up the screen the support to my channel means so much to me if you want to become a patron yourself you can do so at the link in description below hit subscribe if you haven't already all that other youtubey stuff and don't forget to check out my second channel there is all sorts of interesting stuff there so i guess that's it so stay healthy stay safe and i'll see you next time bye [Music]

Channel: Adrian's Digital Basement

Views: 72,288

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Id: -R7165iOMw0

Length: 27min 13sec (1633 seconds)

Published: Sat Jan 22 2022