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video tx problem

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bond000

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i have built a video tx. i can tune to its carrier in the uhf band using my analog mini tv. but,there's no video from the camera,except for the carrier.i am using a small 1 inch by 1 inch camera. i am enclosing the circuit. please see the video amp stage in the circuit. is there anything wrong with it? i mean at video signal input, ground connection is not used.
 

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You do need to connect the video input ground connection to your video source.

You might also want to adjust the value of 10k resistor on the base of the video modulator transistor to a larger value, or add a much smaller resistor between that transistor's base and emitter.

It would be a nice touch to add a 300 Ohm resistor across the 100 Ohm pot to bring the video impedance down to 75 Ohms. This is not what is stopping the circuit from working. Get it working first, then worry about image quality.
 
Well, from the schematic, the video polarity is correct for the U.S. (negative going sync tip makes the video amplitude greater), but your monitor is not in sync. Could it be that:

1. You are not in the U.S.? (as an example in the U.K., the sync tip corresponds to the lowest amplitude of the carrier).

2. The monitor and camera are not basically compatible (different line rates -do you know they are compatible?)?

3. You need to adjust the horizontal hold on the monitor? Instead of connecting the 10k from the base of the video modulator transistor to +9V, try connecting it to the base of the RF amplifier transistor. This is supposed to be a Gilbert Multiplier, in which a few hundred millivolts difference in base voltage should be all that's required to obtain 100% modulation.

4. That you need some bias on the video modulation transitor?

By the way, this schematic is drawn "upside down", making it really difficult to read. Maybe it was done this way on purpose.
 
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Roff

Well-Known Member
You do need to connect the video input ground connection to your video source.

You might also want to adjust the value of 10k resistor on the base of the video modulator transistor to a larger value, or add a much smaller resistor between that transistor's base and emitter.

It would be a nice touch to add a 300 Ohm resistor across the 100 Ohm pot to bring the video impedance down to 75 Ohms. This is not what is stopping the circuit from working. Get it working first, then worry about image quality.
I think video input GND is supposed to be connected to +9V. It would not make sense to do it the other way round.
 
I would say that + 9V is the ground, and yes connecting the video return anywhere else would suck (a lot of current).

As I said this schematic is terrible.

By the way, in the photographs of the tv screen, I think I can spot the vertical blanking,but no vertical sync. And it would appear that if there is horizontal blanking, that the horizonal is locked.

It could be that the vertical sync is not visible because it is clipped of or blanked inside the tv set, in which case, the sync pulse may be there, but not visible.

Quite puzzling.

Here are some ideas to try and find the problem:

It could be that the vertical sync is not possible because the bias on the video modulator is not right. Have you experimented with a resistor from the base of the video modulator transistor to -9V?

It could be that the that the video output of the camera is not not composite video (in which Horizonal and Vertical sync pulses are mixed with the video signal) -that the sync signal is supplied separately, either as separate H. and V. drive pulses or as mixed or composite sync pulse from the camera. If this were the case, the monitor could synchronize horizontally on horizontal blanking (which would be part of a separate sync type video signal). Do you know if the output of the camera is composite sync?

It could be that the video signal is inverted from what the monitor expects (though I can't see evidence of this in the pictures of the screen, but then I don't know what image to expect). Are both the camera and the monitor made for the old analog U.S. broadcast stadard (NTSC for the TV receiver, RS-330 or a variant for the camera)?

What kind of transistors are you using?

What make and model is the camera?

What make and model is the TV receiver?

Can you look at the circuti with an oscilloscope?
 

Hero999

Banned
PCB is critical with UHF design, you didn't build it on a breadboard did you?
It's not UHF, it's VHF, those transistors are no good at UHF.

How does this transmitter work anyway?

It looks like an FM bug style transmitter.

If I remember rightly you need FM for the sound and AM for the picture. I assume this is a video only transmitter so it's probably AM. Heck you probably get both AM and FM as the capacitances of the transistor junctions change.
 
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Looking at the circuit again, I would say the Gilbert Cell analogy is a bit off. Its more as if the video modulation transistor is modulating the emitter current in the RF amplifier, which in turn modulates the emitter resistance of the RF amp, and therefore the degeneration on the emitter. I know, its kind of a stretch, but the emitter of the RF amp is already bypassed, and that's the only thing I can think of.

Got any other ideas?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Yes, it's AM modulating the RF amplifier transistor, shouldn't be any FM as the oscillator isn't modulated.

Hero999 - the sound in the UK is 6MHz FM - but it's an FM modulated 6MHz carrier, which is then AM modulated on the RF output, along with the video.
 
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Hero999

Banned
Isn't vieo supposed to be Single Side Band (SSB)?

This transmitter will produce Double Side Band (DSB)?

If the TV is tuned to the wrong SB then won't the image appear scrambled as the spectrum will be in the wrong order?
 

bond000

New Member
thank you very much gentlemen for all your replies.

i am using a china make analog portable B&W TV
i am using a PAL camera,which works on 9v.

the transistors are : BC547 for video stage. and 2N918 for oscillator and rf modulator stages.

sorry,i don't have an oscilloscope.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Isn't vieo supposed to be Single Side Band (SSB)?
No, it's vestigal sideband, reduced carrier.

This transmitter will produce Double Side Band (DSB)?
Yes, as all domestic modulators do.

If the TV is tuned to the wrong SB then won't the image appear scrambled as the spectrum will be in the wrong order?
No, try it with your VCR (if you still have one?).
 
i am using a china make analog portable B&W TV
i am using a PAL camera,which works on 9v.
have to wonder whether the TV receiver is compatible with the camera. If it is, for example, a NTSC (59.994 Hz) television set it won't work with a PAL camera.

Is that a PAL television receiver?

In what country was this television sold?


the transistors are : BC547 for video stage. and 2N918 for oscillator and rf modulator stages.
Those look like a good choice for the RF transistor, and probably ok for the video transistor. The transistors are not likely to be the problem.


It could be that the designer means for the video transistor to diode clamp the video signal using the input capacitor with the forward conduction of the video amp's Vbe junction. In this case, changing the transistor type might require a change in the base bias transistor. And the BC547 can have an unusually high current gain.

Did you try changing the bias on the video modulator?

If my guess that the modulation takes place by the video transistor modifying the Gm of the RF amp is correct, then a negative going sync tip would decrease the RF amp's Gm, resulting in a reduced carrier modulation. This is consistent with the polarity of modulation in the U.K. Would somebody please check that analysis?
 

bond000

New Member
i connected the camera directly using a cord to the tv. it can receive PAL camera signal.
i changed the bias on the transistor base. but,no use.i am now getting a plain white screen,when tuned to the tx signal.anybody know what it means?

actually,i built a circuit. but it is a simple one transistor circuit.it worked for me.but the quality is poor. please take a look at it. any chance of modifying the present circuit to work like this?i chose the second circuit,because it has different stages and quality and range will be more.but,its not working.please help
 

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i connected the camera directly using a cord to the tv. it can receive PAL camera signal.
i changed the bias on the transistor base. but,no use.i am now getting a plain white screen,when tuned to the tx signal.anybody know what it means?

actually,i built a circuit. but it is a simple one transistor circuit.it worked for me.but the quality is poor. please take a look at it. any chance of modifying the present circuit to work like this?i chose the second circuit,because it has different stages and quality and range will be more.but,its not working.please help
In the U.K. the video is modulated such that white corresponds to maximum carrier amplitude (Positive modulation).

This takes



If the screen is all white when you tune to the transmitter, then the carrier is there, most likely (but not necessarily) the sync pulses are being transmitted, but there is no video modulation -that is to say that the darker parts of the video signal (if there are darker parts) is not affecting the carrier amplitude.

An alternative explanation, though less likely, is that the camera is starting at a white card or into a light.

This takes us back to adjusting the bias and the video level on the video modulation transistor.

The differences between the transmitter that works and the complicated circuit is that the complicated circuit has a buffered RF oscillator a buffer for the video signal.

I think the main advantage of the more complicated circuit (without deep analysis) is that the output power is likely to be a little higher and have a less frequency modulation in the signal. The video buffer transistor (which I referred to in previous posts at the video modulation transistor) adds a little temperature stability to the modulation, but perhaps more importantly, may (if my guess is right) diode clamp the blacks so that the black level doesn't sift around as scene content changes.

I am reluctant to try a design by "remote control" over the web and not having access to PAL equipment, I am not able to take on this as a design task.

Most likely, your best course is to assure that you have built the circuit according to the schematic and then to continue to experiment with the bias and video level adjustment. Alternatively, look for another design on the web.

If you can get your hands on an oscilloscope, it will probably help you a lot.

Whether you can get a scope or not, being familiar with the TV signal modulation waveforms will at least let you visualize how changes to the circuit can affect the picture.

Have a look at this site (from which I lifted the image above).
World Analogue Television Standards and Waveforms
 
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