Video Transmitter Problem

Discussion in 'The Projects Forum' started by deepak007, Jul 27, 2008.

  1. deepak007

    Thread Starter Active Member

    Sep 30, 2007
    59
    0
    hello

    i have built a video transmitter circuit.its not working. the last transistor (Q4) is not turning on. the voltage on the base is zero. i don't see where the (Q4 TRANSISTOR) base gets its voltage from. is the circuit correct?is the biasing correct? please give me some suggestions.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    You're correct - there is no path for bias current for Q4. Someone forgot something.

    Q1 doesn't have a bias path either, nor a DC blocking cap.

    Ask the author to publish an updated schematic. Preferably, one that's viable.
     
    Last edited: Jul 27, 2008
  3. Gadget

    Distinguished Member

    Jan 10, 2006
    613
    0
    Q4 is a Class "C" amp, and requires proper operation in all previous stages and correct tuning before it will begin to function. An RF probe applied at various points to assist in the peaking of the RF stages may help.
     
  4. deepak007

    Thread Starter Active Member

    Sep 30, 2007
    59
    0
    could some one correct it?
     
  5. Wendy

    Moderator

    Mar 24, 2008
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    A class C amp is meant to turn on only during half the cycle from the oscillator, and usually uses a tank circuit on the collector to make the pulse "ring" and reshape it into a sine wave. This design doesn't quite jive, but it could work as is, though the output would be way too rich in harmonics.

    My first question is, is the amp oscillating? This has to happen for anything to work.
     
  6. deepak007

    Thread Starter Active Member

    Sep 30, 2007
    59
    0
    i have checked the voltages at the transistors. Q4 has no base bias. so it is not turned on. so, some one please correct the circuit for me. i want to receive the signal on my portable tv set.i am enclosing it again.
     
  7. Wendy

    Moderator

    Mar 24, 2008
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    Something in the neighborhood of 2.4KΩ will bias the transistor on, but I don't think it's what the designer intended, and you run the risk of smoking the transistor. If you must add a resistor bump it up to 24KΩ or more. You won't see anything but bias on a class C amp, it spends most of its time off, it's part of the design. Are you seeing any sign of a carrier wave? My first suspect is the oscillator, what is the value of your inductor?

    I also don't see what would set the frequency in this design, unless it the 5pF cap and the coil. Usually a tank circuit is used on the collector.

    I have to agree with Wookie, this is a seriously flawed schematic.
     
  8. deepak007

    Thread Starter Active Member

    Sep 30, 2007
    59
    0
    thanku for ur reply mr. bill marsden !

    this is my first video transmitter circuit. i have built many audio transmitters using FM. but i don't know much about video.( AM modulation.)

    please help me with these problems:
    1. as you said, i will include a 2.2kohm base bias resistor on Q4.
    2. will add a trimmer cap. parallel to L1, also with L2 to tune the circuit.please give me an inductor and trimmer cap. value for a channel with in 1 to 14 channels.

    3.as mr.wookie had said, there is no cap. at the Q1 signal input.i am gonna add a cap.please suggest a value.

    i have worked very hard on this circuit and even etched a pcb with out testing the circuit! its my mistake. i don't want to drop at this point. so, please help me.
     
  9. deepak007

    Thread Starter Active Member

    Sep 30, 2007
    59
    0
    this is what the circuit designer has put on his website....http://www.newcircuits.com/circuit.php?id=rfr007


    "This circuit is a powerful video sender in VHF band. The modulator section is designed to operate on either channels 7 to 13 or 14 to 29. For channels 7 to 13, L1 and L2 are 3 turns of #22 wire wound on a 3/16 inch form. For channels 14 to 29, L1 and L2 are 2 turns of #22 wire wound on a 3/16 inch form. Video signal is modulated as AM."

    what a description! its not working.
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    Basic video is a form of AM. What coil did you opt for? He didn't say this, but it should be solid wire, not stranded.

    I suspect the author is planning on parasitic capacitance to provide the C in the LC values, your PCB may be part of the problem, in that it has values the author didn't plan on. Personally I think this is his design flaw, not yours.

    High frequency circuits are extremely finicky, short lead lengths are usually required for proper operation. Can you post your PCB layout?

    I notice they have a forum for this website, have you done a seach or looksee on it?

    I've built video transmitters, but from kit form, Ramsey I believe. Been a long time, decades.
     
  11. deepak007

    Thread Starter Active Member

    Sep 30, 2007
    59
    0
    actually i am a beginner to EAGLE. this is my first pcb in eagle cad.i actually hand draw the layout. it's more easier to me.but, this i have drawn in cad.

    i made a mistake in the layout,because i am not familiar with eagle. i used a long trace to connect L2 to the base of the Q1.it might act as an antenna.will it have any effect on the circuit?

    i used a double sided board and i used some ground plane on component side also.i have used a some via's to connect both sides of the ground plane. it's not that professional,but that's all i can afford.
     
  12. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Most circuits from the www.newcircuits.com website seem old and use obsolete parts.
    The "1W Bass Amplifier" uses an LM390 IC that has not been made for 12 years.

    In this video transmitter, CV1 is in parallel with and tunes L1.
    C2 and the capacitance of Q3 are also in parallel with L1. Don't add another capacitor.

    Q4 is a small RF power transistor and operates in class-C for high output power.
    Maybe its interfering harmonics are not noticed in that country.

    C8, the capacitance of Q4 and the antenna's capacitance tunes L2.

    The video input is DC-coupled to ground for good black level control. 0V input creates max AM modulation for full power on sync pulses. The pot P1 adjusts the brightness of white.
     
  13. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    The Eagle Layout Editor is somewhat quirky, and takes time to get used to it.

    The "names", or reference designators of your components all start with "E" for some reason. I usually use the "rcl" library; when placing components, the name normally starts with R or C as appropriate.

    You can change the reference designators using the Name icon button. It's on the menu on the left side. It looks like a horizontal European resistor with R2 on the top and 10k on the bottom; the R2 is black and the 10k is greyed.
    If you click on that icon in the schematic and then click on a component, you can change the name. Each name must be unique. I suggest changing yours to match the original schematic.
    Similarily, you can use the "Value" icon button to add the values of your components. It's next to the Name button and looks just like it except the R2 is grey and 10k is black.

    It is a good idea to always have the grid on. I usually have mine set to 0.1" when placing parts. Many of the supplied library parts won't line up properly as they were drawn; you may have to temporarily decrease the grid size to 0.05" or even less.

    When you are routing traces, use the right angle or 45° drawing tools, not the freehand drawing tools.
     
  14. Mu86neer

    Member

    May 1, 2009
    23
    0
    i'm building the same Audio Video transmitter and i think if i used MC44BC373 chip modulator then followed with RF amplifier (using opamps) then radiated by antenna..!!!
    Thank you :)
     
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