Electronics > Projects, Designs, and Technical Stuff
Adding composite input to an old B&W TV
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richard.cs:

--- Quote from: schmitt trigger on April 22, 2020, 12:59:55 am ---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.


--- End quote ---

Well, 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.
flimshaw:
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.
vk6zgo:

--- Quote from: richard.cs on April 22, 2020, 10:32:11 am ---
--- Quote from: schmitt trigger on April 22, 2020, 12:59:55 am ---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.


--- End quote ---

Well, 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.

--- End quote ---

Nope! 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,
richard.cs:

--- Quote from: vk6zgo on April 23, 2020, 03:17:24 pm ---
Nope! 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.


--- End quote ---

The 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.
james_s:

--- Quote from: flimshaw on April 23, 2020, 02:52:57 pm ---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.

--- End quote ---

That'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.
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