Adding composite input to an old B&W TV - Page 1
Forum section: electronics / projects

author: flimshaw
author_rank: Contributor
timestamp: April 20, 2020, 01:48:29 pm
content: Hello hello,I saw a video on 8-bit-guy about adding RGB inputs to a color CRT, and was wondering how difficult it would be to convert an old black and white RCA tv I have (AXR 120Y -https://www.radiomuseum.org/r/rcataiwan_solid_state_axr_120y.html) into a composite monochrome monitor for vintage computers. I've done a fair bit of tinkering with CRTs and know the safety precautions and general principles, but I can't find a schematic for this one and I'm not sure if the signal I want is there, and if so how to find it. Here's the block diagram I'm working from:My assumption is that somewhere on this board antenna signals are decoded to a normal composite signal ("video detector") and then sent off to get amplified and sprayed on the CRT. and at that point between "video detector" and "amplifier", I could cut the trace and inject an external composite signal. Is that a safe assumption, or would the signal on the board not necessarily be a standard-level NTSC composite signal? I don't want to bother going too far if I'm vastly simplifying the problem.Thanks in advance!

author: Muttley Snickers
author_rank: Unknown
timestamp: April 20, 2020, 02:16:23 pm
content: Couldn't you just use a modulator to convert the composite video signal from the computer to an RF channel in the same manner a VCR would ?.

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 20, 2020, 02:35:49 pm
content: You'd probably have to work at the "video detector" level. I'm not completely sure its input (which would be the output of the IF amplifier) is a proper composite signal. If it is, you know where to insert your composite input. You will likely need some signal conditioning.To figure this out, first thing IMO would be to locate the "video detector" block input on the board, and take a look at the signal on a scope.

author: flimshaw
author_rank: Contributor
timestamp: April 20, 2020, 05:52:13 pm
content: Thanks, I poked around with my scope and found a video signal eventually. So it seems like the detector has nothing to do with the IC, and is instead implemented with some discrete components inside that little can. I snipped what appeared to be the location where the signal left the can and went into a transistor amplifier circuit and then on to the TV, and I was able to then inject a signal into the TV and it seems to mostly work.The image is not *too bad*, way sharper than the RF adapter, but there is still a fair amount of snow on it. I'd expected to get a pretty snow-free image once I bypassed all the tuner circuitry. Does this mean there's some additional source of noise coming in that I would have to track down and cut? Or perhaps my expectations were high. Could it still be getting a noisy sync or something from the RF bits?

author: bob91343
author_rank: Super Contributor
timestamp: April 20, 2020, 06:23:29 pm
content: Perhaps the level you are injecting is too low. And you need to make sure the detector output isn't added in with the signal.

author: schmitt trigger
author_rank: Super Contributor
timestamp: April 20, 2020, 06:33:16 pm
content: Are you absolutely sure that the baseband signal that you are injecting into the TV is completely stripped from its chroma component?From what I can see on your photo, the snow appears to be precisely that, chroma.

author: flimshaw
author_rank: Contributor
timestamp: April 21, 2020, 03:35:50 pm
content: Thanks, that signal *should* have been pure luma from a 5-pin c64 output, but it's a noisy early one, and it already was flakey and stopped working yesterday so, I probably shouldn't have been testing with it.So at this point, I have tapped into what I am pretty certain is the right point to inject, and I built a small arduino tvout rig as a test, as that will be putting out pure luma with no colorburst or chroma, and it looks fantastic. I've mounted an RCA jack where the antenna used to go in the back, and I'm grounding it to the same chassis post the previous antena module plugged into. The problem is that if I connect an RCA plug to any other device, the GFCI in my outlet trips immediately. The signal part can connect fine, but as soon as the ground is connected, it's over.On this board, there is no large isolation transformer I can see, the mains cord just solders directly into the board. I have a feeling that the "ground" i'm connecting to is not actually the ground, but is rather a hot/neutral ground. Is that possible, and if so, where do I find / how do I create a safe signal ground to attach to this plug? In a design like this, would the signal ground just be the neutral leg?? Feels not great.

author: Renate
author_rank: Super Contributor
timestamp: April 21, 2020, 08:33:56 pm
content: Yeah, there were/are a lot of TVs with "hot" chassises.It's easy enough to check, use an ohmmeter between the plug and the chassis.If this is the case, you should stop right there.Screwing around with this might kill you.Plug it into an outlet with no GFCI and the phase reversed will put full line voltage on the "ground".(I originally wanted to just say that that noise is not chroma, it's somewhere around 1 MHz, maybe AM radio?)In my day we had Conrac SNA broadcast monochrome monitors, 10 MHz video channel.That's what you would want if it wasn't a collector's item now.

author: schmitt trigger
author_rank: Super Contributor
timestamp: April 22, 2020, 12:59:55 am
content: Indeed, as a cost saving measure, many TV chassis were “hot”.If you really want to use this TV, you will have to employ an isolation transformer.I remember the Conrac monitors. Fed from a pristine video signal, the image was simply amazing.

author: james_s
author_rank: Super Contributor
timestamp: April 22, 2020, 05:28:35 am
content: That's the first thing that came to mind. Most TV sets use a "hot chassis" design with no isolation and these are very dangerous to modify without knowing exactly what you're doing. People have been killed by adding headphone jacks and many oscilloscopes have been blown up by poking around in non-isolated equipment.That's kind of a nice little vintage TV, if it were me I'd keep it intact and use a modulator to give it a signal.

author: richard.cs
author_rank: Super Contributor
timestamp: April 22, 2020, 10:32:11 am
content: Quote from: schmitt trigger on April 22, 2020, 12:59:55 amWell, maybe. Isolation transformer on the TV power input would be utter overkill. RF isolation transformer on the new input would be reasonable, as would just coupling both sides with low-value Y rated capacitors. Both were a common approach on live chassis equipment.

author: flimshaw
author_rank: Contributor
timestamp: April 23, 2020, 02:52:57 pm
content: Thanks again, I have learned a lot! At this point, I've got my RCA jack mounted on the back and a proper polarized plug on the cable 😅. The image is pretty good with my arduino test rig, but as someone mentioned most normal devices seem pretty dim, so I guess it's expecting something a bit hotter than your average composite signal.So currently, the gnd of the RCA jack is just the neutral line, and the signal taps into the base of a transistor in the video amplifier. I'd love to learn more about the y-rated capacitor coupling. Despite my mods this thing still trips my GFCI outlet whenever I plug in the commodore. I can measure basically no voltage difference between the ground on the commodore and the TV, so I'm not sure where the current imbalance is coming from.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 23, 2020, 03:17:24 pm
content: Quote from: richard.cs on April 22, 2020, 10:32:11 amNope! RF isolation transformers will distort the composite video signal.The coupling capacitor route would be better, but it does cause a loss of the video DC component.This may not matter if the TV uses DC restoration in the video amplifier.It's a quite small TV, so the isolation transformer won't be very large.Some TVs which were fairly obviously designed to be transformerless, had an "Australian" version where a small 240v:120v transformer was "shoehorned" into the cabinet.Later small TVs use SMPS, so the Mains connections are isolated from the device "chassis" by the high frequency transformer incorporated,

author: richard.cs
author_rank: Super Contributor
timestamp: April 23, 2020, 03:39:58 pm
content: Quote from: vk6zgo on April 23, 2020, 03:17:24 pmThe TV must have dc restoration in some form because the DC information is not transmitted, the only question is whether it's before or after where he's tapped in.Regarding transformers the problem is the low frequency cutoff, and I'll admit to not having calculated it before posting...The lower end of the frequency range required depends on what content he's displaying on the screen, as a minimum you need to pass the horizontal sync pulses through so you need to pass the line rate (17 kHz or so), but unless the application has a huge brightness differential from top to bottom then you can probably get away without going down to the 50/60 Hz required to pass full (half)frame information without distortion and a minimum pass frequency maybe 500-1000 Hz might give an acceptable amount of distortion (granted so it's not RF). Looking at what he's displaying he's got maybe 30 characters a line which need perhaps 6 pixels horizontally, so the required upper frequency is around 4 MHz. I guess 4 decades is a lot to ask from a transformer and it's probably not that practical.Coupling capacitor value would depend on the impedance of the circuit node, and yes a new DC restoration circuit may need adding if the existing one is further back down the chain.

author: james_s
author_rank: Super Contributor
timestamp: April 23, 2020, 08:47:11 pm
content: Quote from: flimshaw on April 23, 2020, 02:52:57 pmThat's still very dangerous, you should *never* rely on the polarized plug to provide safety, it's not uncommon to find a receptacle that is wired backwards and you never know who might eventually get ahold of that TV and assume the baseband input is original and completely safe.You need isolation, and it sounds like you need a circuit to amplify the video signal to whatever level the TV is expecting. If you power it via an isolation transformer you can scope the video signal out of the tuner and see what you have to work with.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 24, 2020, 01:46:57 am
content: Quote from: richard.cs on April 23, 2020, 03:39:58 pmActually it is!Analog Broadcast Television systems go to great lengths to maintain the black/blanking level (In PAL, blanking & black level are the same, so I will use that system)Sometimes DC coupling can be used, but it is usually impractical, so ac coupling must be used.The time constant of the RC coupling used is long enough that there is no discernible "tilt" in the waveform over the duration of a line.This is not the case, over a field, however.To restore the blanking level, a device called a "keyed clamper" is used, which works at line rate.Keyed clampers commonly generate a narrower pulse from the line sync pulses, which is then delayed till the "back porch", operating some form of switch which connects that point to zero volts for the duration of the pulse.As the colour burst is also on the back porch , the device requires colour subcarrier "traps" so as to not mutilate the burst.This regime is maintained up to, & including, the transmitter modulator, although the reference clamped to may be other than zero volts.Thus, given levels of Modulation are representative of the original video DC levels.The result of this is that a simple diode detector at the receiver will produce a video signal with extremely low "tilt" over a field period.Old style "high level modulated." transmitters went a step further, with "Blanking Level Feedback", where video from diode detectors on the antenna feeders was used to adjust the Tx clamp level, so as to maintain "Blanking level stability".QuoteWhich is why they are not used in practice!QuoteThis dependence upon impedance was used to good effect in the early BW TV days, where vacuum tube circuits were mostly high impedance.Some very cheap TVs relied upon large coupling capacitors to minimise tilt, & didn't bother with DC restoration at all!More "upmarket" sets used simple,"diode clamp" DC restorers, which only clamped at field rate, but still did quite a good job.The set under consideration was made around 1984, so it probably does have DC restoration built into a large IC which also does the job of sync separation.The date of manufacture of the TV set also casts doubt upon the idea of it being a simple "transformerless" unit, with the "chassis" connected to Mains Neutral.The dubious advantages of such architecture to tube TV receivers are pretty much non-existent for solid state designs, so it is more likely to be a SMPS design.If it has a "figure 8" power cord, & a Symmetrical Mains filter with caps connected from both sides to chassis, there is every chance that the chassis will be elevated to 60v w.r. t. Neutral.Grounding that in some manner will cause a pulse of current in the ground,conductor, perhaps sufficient totrip the RCD/GFCI.

author: james_s
author_rank: Super Contributor
timestamp: April 24, 2020, 05:30:42 am
content: Quote from: vk6zgo on April 24, 2020, 01:46:57 amSorry but this is just wrong, and dangerous advice. I worked on a LOT of TVs back in the day and I can say with certainty that a great many solid state TV sets up into at least the late 90s used a hot chassis design. The Flyback/LOPT doubled as the transformer in the SMPS, with the whole rest of the circuit being completely non-isolated. Arcade monitors were similarly designed and virtually all required the use of an external isolation transformer in the cabinet. Only small portable sets with a DC power option and large fancier TVs had isolated power supplies. I don't think I've ever seen a CRT TV other than a small portable AC/DC set that had a removable power cord.

author: flimshaw
author_rank: Contributor
timestamp: April 24, 2020, 02:54:04 pm
content: OK I think I am sold on the isolation transformer route. I would rather go overkill than become killed, and I have a healthy fear of mains voltage and TV flyback nonsense in particular so I don't want to mess around.I watched a few vids on the subject and had a few questions.1. would this transformer be a fair match:https://bit.ly/2yK3jGs?the wattage ratings match the tv's specifications, and it's pretty small and I bet I could fit it into the body of the TV.2. when the AC cord comes into this device, it solders directly onto the PCB. So I assume that is the point where I insert the transformer. then I would also tie the ground from the RCA input to the tv side of that transformer, correct?3. once an isolation transformer is in place, would this also protect against the plug polarity issues? ie if it goes into a something with neutral/ground switched, what would happen?

author: schmitt trigger
author_rank: Super Contributor
timestamp: April 24, 2020, 04:01:14 pm
content: The transformer shows a white lead (shield) which for optimal isolation should be grounded.Since you are going to perform a "surgery" on the equipment anyways, why not go all the way with a replace the power cord with a modern three-prong connector and cable?You could salvage one from an old computer.

author: flimshaw
author_rank: Contributor
timestamp: April 24, 2020, 09:24:47 pm
content: Oh I definitely will. I was more asking based on the advice above to assume that the outlet itself might be wired backwards.

author: SiliconWizard
author_rank: Super Contributor
timestamp: April 24, 2020, 09:28:42 pm
content: Definitely be careful whatever you do.There are some dangerously high voltages in those CRT TV sets.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 25, 2020, 02:31:55 am
content: Quote from: james_s on April 24, 2020, 05:30:42 amMaybe this was the case with US made TVs,It is strange, indeed, if TVs made in Asian countries would follow that design.I also have worked on a lot of TVs from that era, & I have not seen one which used the type of SMPS you describe, especially, small BW sets.I believe such designs were used in Europe & the UK for larger Colour TVs.The only reason, apart from showing how "clever" they can be, as far as I can see, is to make any interfering signals from the SMPS in sync with the horizontal frequency.In the same time period, Sony, Sanyo & others used free running SMPS in the models sold in this country, with no interference problems.In Australia, "hot chassis" design was never popular, as in earlier times, power transformers were neither expensive, nor prohibitively heavy, & later, normal SMPS were exclusively used, even in imported TVs.Considering the TV under discussion.Its origin in Taiwan makes it more likely to follow standard SMPS design, with an isolated chassis.Of course, it may be that RCA, for some arcane reason, insisted upon a "hot chassis" design."Figure 8" in Australian vernacular does not imply a removable power connector, but refers to the appearance of common twin flex, and has done, back at least into the late 1950s.In any case, I wasn't offering "generic" advice, just suggesting a bit more research by the OP.If they have a DMM, they can, using the ohms range measure between each pin in turn of the plug on the power cord to see if the chassis is indeed, connected to one side of the Mains.To the OP, & any "noobs" who are "playing along at home":-"The plug on the power cord" implies that the TV is unplugged from the Mains, & the measurement is made between each, in turn, of the pins on the end of the power cord which normally plug into the power point & the chassis.