2
\$\begingroup\$

I'd need some advice on a circuit, which generates analog video (s-video) from composite and Chroma (a mod inside an old Sinclair Spectrum computer). I haven't designed the circuit myself, but I managed to build one :) The circuit is working, but for some reason the chroma level is apparently too low. I'm a real newbie regarding electronics, so I hope I'm asking something that is actually viable regarding this circuit design.

enter image description here

Would it be possible to raise the chroma output level (easily?) by changing some of the components of the circuit above? The transistors are all BC547B:s. The input level for the chroma part of the circuit measured with a cheap (rms) multimeter without load is ~4v measured directly from the ic (LM1889) that generates the chroma subcarrier (sorry, but that's all managed to measure..). The monitor has 75Ohm termination for the video signal.

All help highly appreciated :)

\$\endgroup\$
12
  • \$\begingroup\$ Always inline the image, if possible, so that the question makes sense if the link dies and to have all the information in one place. I've done it for you. \$\endgroup\$
    – Transistor
    Mar 15 '19 at 8:09
  • \$\begingroup\$ Thank you! Sorry, my bad :/ \$\endgroup\$
    – user161284
    Mar 15 '19 at 8:41
  • \$\begingroup\$ What is the level on the positive side of C165? \$\endgroup\$ Mar 15 '19 at 9:07
  • \$\begingroup\$ I guess you meant C65 instead of C165? Sorry, I forgot one important thing: The pad of C65 was completely destroyed by the previous owner, so as the idea of the mod is to remove the C65, I took chroma directly from the pin 13 of the LM1889 chip. The voltage measured there is ~4V. \$\endgroup\$
    – user161284
    Mar 15 '19 at 9:11
  • 1
    \$\begingroup\$ Did you actually use an electrolytic (polarized) capacitor for your C165? The chroma signal is DSB modulated and centered at 3.579545 MHz -- in other words, rather high frequency. I'm concerned that the ESR/ESL of an electrolytic capacitor might actually be blocking most of this. I would substitute a much smaller (100 nF or less) ceramic capacitor in this location to see whether it helps. \$\endgroup\$
    – Dave Tweed
    Mar 15 '19 at 11:26

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.