Jump to content


New Members
  • Content Count

  • Joined

  • Last visited

Community Reputation

16 Good

About simon.plata

  • Rank
    Space Invader

Recent Profile Visitors

2,402 profile views
  1. It should have enough "power" given that the output impedance is matched to 75 Ohm which is the standard for video. So any splitter which works with standard output video, should work with this mod.
  2. @the_crayon_king made a very good point, you could still have audio signal interference. Did you removed ALL the components indicated in the instructions? Otherwise audio signal could still have a path to mix with video. Also, the indicated components needs to be removed in order to leave a clean chroma and luma signal. The RF oscillator is still in place? Or was removed? I believe all the above makes sense because both the mods you referenced as working properly, are taking signals directly from TIA pins. Bests,
  3. Ok, Colors and saturation looks good, then I can say the mod is working properly. The issues with the image are related to gosthing and image post processing by the TV DSP. BC548B/C and BC558B/C are good replacements. Be sure there are BC5x8B or C, the last letter defines de hfe of the transistor. Letter A or no letter are low gain devices. I suggest the following: The 75 Ohm resistor is very important. Be sure it's 75 Ohm +/- 5% The video cable is the responsible of most of the problems. Please ensure you're using a good video cable and connectors. Check cable resistance, should be less than 1.5 Ohms. Check ground. Try to find a CRT TV set (old TV) and check the video image on it. Several new LCD/LED TV sets have several image processing algorithms that can distort the video. If the gain change didn't affect the brightness, it's an indication that the TV DSP is normalizing the video and further post processing is being done. After checking the image in a CRT and tuning the gain, go to your LCD/LED TV set and disable ALL the image processing or enhancing features. This should improve the image. Let me know how it works for you. Bests,
  4. Typical audio line input impedance is 10K and line output impedance is 100 or 600 ohms. Due to short distance of the cable in comparison to the wavelength, impedance matching is not required. For maximum audio signal level you may choose 100 to 600 ohms for R9. However I prefer to use 10KOhms (same as line input) for short circuit protection and to avoid input saturation. Please let us know how it works for you and the video results. bests!
  5. Hi, I’m pleased you find it useful. First of all the trimmer only adjust the gain of the amplifier. That means it varies the brightness of the image only. Ghosting and other distortions are not related to the trimmer adjust, but to impedance matching. The best trimmer adjust could be seen from different standpoints. One of them is obviously according to the NTSC or PAL standard. Which is 714mV Or 700mV between the black and white level correspondingly. You will need an oscilloscope and a test cart to generate a color bar test signal. My favorite adjust point is which “Looks” better in your TV set. I use to set the TV at factory settings for video. Then I adjust the trimmer until I get the more pleasant image. Lower than optimal gain will give a dark image but well defined colors. Higher than optimal gain will give an overdrive signal. The brightness isn’t increased but the yellow will change to white. Optimum gain should be near maximum brightness but having well defined yellow. Hope it helps!
  6. Ghosting could be produced by overdrive of the video signal, try to reduce the gain an see if the ghosting persist. If it persist then could be a mismatch of coupling impedance. Try a different video cable and different lengths the shortest the better.
  7. It sounds like a good idea. My concern is about the bandwidth and roll-off frequency. Darlingtons tends to have multiply the base-emitter capacitance of the hidden base, but should give a try.
  8. Hi ChildOfCv, In respect your first comment, you´re absolutely right . It's really easy (nothing to do really) to design the mod using an integrated video buffer in place of a discrete one. Indeed I ordered some video buffers to do another AV-mod for my VCS consoles. In this case I selected the TSH122. https://www.st.com/en/amplifiers-and-comparators/tsh122.html However the objective of this post, was to give an easy to built AV-mod which really works and was affordable for almost everybody in the world. In some places it's not easy or affordable to order uncommon electronic parts. Also I wanted it to be possible to assembly in an universal PCB or even a bread-board. Regarding your second comment, I might don't really understand your question. However I will try to respond. In the first post of the tread, I exposed two options: The first one is a one stage amplifier in common-collector configuration, which is the best approach as you said. However, given the high output impedance of the resistor network DAC (about 10.9KOhm), the current gain needs to be into the hundreds, which is in the edge of the capabilities of a general purpose transistor at 4.2MHz. The second one is a modified common-collector configuration like the first one, plus a preamplifier stage in common-emitter. I choose the common-emitter configuration for the preamplifier to take advantage of the high input impedance and the convenience of embedded biasing of the common-collector second stage. I hope I have responded to your question appropiately, otherwise please let me know your comments and I will try to do my best.
  9. Hi @slaanesh, PAL requires a higher video bandwidth (5.0MHz) than NTSC (4.2MHz), and this is the reason why you get the better results with your trim-pot all way down, which is the point of the highest gain for the circuit. To optimize the circuit for PAL's signal, it’s needed to move the knee point or the roll-off frequency point to the right (Higher frequency), which requires a complete redesign of the circuit, because it its DC coupled. The roll-off frequency is the result of the superposition of the RC low pass filters formed by the transistor's parasitic capacitances and the input/output equivalent resistances. You may try to change the C1 capacitor from 47pF to a higher value like 68pF or 100pF to increase the gain in the high frequency end. That trick could work but it’s not warranted. Unfortunately I’m being too busy at work, otherwise I would try to redesign the circuit with PAL bandwidth in mind. I will try when I have a truce from work.
  10. Hi @slaanesh, I'm pleased it worked for you. It was designed for NTSC so the optimum gain could be in the edge for PAL. Also I want to thanks @Djoulz for making it easy and accessible for more people.
  11. Hi and thanks to everybody who tryed this mod. May be I forgot to suggest to adjust the trimmer until you obtain the better picture in your set-up. The calculated value is for 0dB gain and 1Vpp output which is the standar composite video signal span. However due to the tolerance of the components in the VCS board and the in the mod, it could be lower or higher than that. The trimmer was included to correct that deviation. Best regards and enjoy!
  12. Hi stlouisrod, have you tried to reduce the gain of the amplifier? Gosthing could be produced by several factors, including unmatched impedance and overdriven signal. The RCA cable could increase the impedance and generate gosthing. I tested it on different LED TV and I have got nice video image. Best regards,
  13. By the way, the link to the mod is lacking tha "a" at the begining so is not working. Here is the correct link: http://atariage.com/forums/topic/271678-simple-diy-composite-video-mod Enjoy
  14. Hi mogul345, thank you for posting my mod in your list!!! Just one coment: my mod comes in one and two transistors versions. However two transistors version is better. Also I posted the PCBs if someone want to fabricate it. I wanted to fabricate a lot and have them available but due to my work I couln't ship them quickly. I have the Eagle files and would be fantastic if someone could fabricate a lot and sell them online. Best regards,
  15. Despite the mod is just a simple video amplifier, it exists several reasons to not work properly: One of them could be amplifier saturation to any or both rails. The real problem can't be found easily without an schematic or a signal analysis with an oscilloscope. The alternative is to revert the mod you did and try with the mod proposed in this tread which is proven and has worked for several people including myself. Best regards,
  • Create New...