Retrofitting AV input on an old B/W TV.

[vampi-the-frog]

Hey guys. I'm looking for some advice on how to fit a Video input on an old Sport 251 TV, produced in Romania. This is a small, portable TV set, made for camping, for people who want to escape the boring life of watching TV indoors, and watch TV in the forest! It even has 12V input.

It only takes RF input, VHF/UHF. I got a hold of the schematic, it's in polish: https://drive.google.com/file/d/0B0D5h65QGhZzZDJ2emtIOEpxLVE/view?usp=sharing

As far as the audio input goes, it's clear, I just put it on the point marked M203.

The demodulator stage is very similar to the first schematic on this page (can't hotlink): http://www.azp.ro/scheme-electronice/schemetvalbnegru/109-scheme-electrica-tv-alb-negru-pentru-televizoarele-an-romanesti.html - See "MODUL FI CALE COMUNA" (which means IF (intermediate freq) Common Path Module)

The demodulator circuit seems to be inside the TDA440 (datasheet here: http://www.classiccmp.org/rtellason/chipdata/tda440.pdf), and it seems the video signal comes out of pins 11 and 12. My problem is that I can't apply the composite video signal to just one of the pins, because they are inverted versions of each other. if you look at the second schematic, there are some oscilloscope drawings at the bottom. Drawing 1 and 2 show the outputs of pins 12 and 11. They just look like inverted from each other, and also like regular composite video (black and white only), because there's a sync pulse, a backporch and so on. I am confused, however, why pin 12 (oscilloscope diagram 1) has the sync pulse going up, it looks like it would be the inverted version of a normal video signal.

Either way, how could I make it so I can plug in a normal video signal. Would I need an inverter circuit, to get the inverted version of the signal as well? Would this do?

Thanks!

[SeanB]

How about getting an old video recorder or old game console and getting the RF modulator out of it, and feed the video into it, give it a 5V feed from the set using a 7805 fed from the 10.5V rail and put the RF into the antenna input?

[vampi-the-frog]

I could do that, but it would not be as fun, and as "correct".

[kolbep]

More Fun :
Pull out all the insides.
Put one of these LCD Monitors (that you get for a car DVD Player) into the box where the screen was.
Put some weights in it to compensate for the missing tube.
You will also need to put a 12v power brick to run it.
Then in the screen settings, turn the color down until you get a black and white picture.

For more realism, you will need to grind off (beware) the front bezel of the tube (or maybe it is loose), and have this in front of the LCD Monitor. So it will look like a black and white tube.

Or you could just pick up an RF Modulator at any Chinese shop, and use that....

[vampi-the-frog]

You guys are so full of ideas!

But I think my solution is the cheapest. Plus it's not even my TV, so I can't go that extreme.

So, any ideas how I can plug the composite video signal into it?

Cheers!

[deephaven]

Looks like the output of the chip which shows the inverted video waveform (pin 12, labelled video positive in the data sheet!) feeds the sync separator and the other output  (pin 11) feeds the video output stage which drives the CRT.  So you're right, it does look like you need both polarities of signal. The level shown on the waveforms is 2.5 Vp-p which means, if you want to feed it with standard 1V video, you will need a gain of 2.5 for each signal. Also, check on the DC levels of the signals you generate, you might not be able to get away with AC coupling like you have shown.

[vampi-the-frog]

Looks like the output of the chip which shows the inverted video waveform (pin 12, labelled video positive in the data sheet!) feeds the sync separator and the other output  (pin 11) feeds the video output stage which drives the CRT.  So you're right, it does look like you need both polarities of signal. The level shown on the waveforms is 2.5 Vp-p which means, if you want to feed it with standard 1V video, you will need a gain of 2.5 for each signal. Also, check on the DC levels of the signals you generate, you might not be able to get away with AC coupling like you have shown.

Yes, it looks like I'll need some amplification circuit. The oscilloscope traces don't show the DC offset, so I'd need to measure with a real scope, which I don't have, just to see the DC offsets.

Do you think I could get away with a simple transformer? One that multiplies the voltage by 2.5, and would isolate the input as well?

Also, looking at the diagram inside the chip itself (page 11-74 on this pdf http://www.classiccmp.org/rtellason/chipdata/tda440.pdf), it looks like the signal is simple, then, with some transistors, is transformed into normal and inverted. The wire coming into R54 is from pin 14, which is just V_S supply voltage.

Another idea is to find an existing TV set schematic with a TDA440 and AV input, and see how it handles that.






Thanks!

[SeanB]

You likely can AC couple the signal in provided you disconnect the chip outputs, you will need about 2V of video to operate the sync detector and about the same for the video amp. That circuit you showed with the phase splitter will be fine for that, though you might need to amplify the video with another transistor to get enough signal in to it.

[deephaven]

A transformer wouldn't be a good idea because you need a frequency response of below 50 Hz up to 5 MHz. The other thing is you really need a 'DC Restorer'. This means making sure the black level stays at a constant DC level - AC coupling will make the black level go up and down depending on the average image level down the frame. The fancy way to do this is to have a clamp which uses a transistor which turns on during the blanking (burst) period. A simpler method is to connect a diode between the AC coupled video and the desired DC level. This then puts the bottom of the sync pulse at a fixed level.

[vampi-the-frog]

So, I should be able to get the desired DC level with a resistor divider?

Thanks, I'll try breadboarding this on monday. What would be a good transistor for the phase splitter schematic? Perhaps BF199?

[deephaven]

This explains about DC restoration with some sample circuits: http://tinyurl.com/dc-restoration-circuits

You can probably get away with the transistor you mentioned if you have some already.

[vampi-the-frog]

Now there's a good book! It even has troubleshooting.  Thanks!