Hi,
I’m working on re-documenting the MX-12 Luma Mod. (Which I will upload when complete)
It was done in 2006 by Brian Murray however he was killed in a bike crash before he could verify his documentation. MGCJerry did add some notes to the documentation, correcting a few small errors. I managed to scrape it off the waybackmachine recently.
I’ve re drawn the schematic for the extra multiplexer chip which should be installed.
Everything makes sense except one part I am confused about, this is unclear in the documentation also.
I am concerned that if directly connected to, the existing multiplexer chip IC4 (PIN 12) (cmos4053) could be overloaded. I am not sure if the pin would need isolation from the existing circuit. It is fed by a transistor with a very low AC voltage. It is not a question of it receiving 2 x +5v or shorting it to ground.
I would think that if IC 4 (PIN 12) did need isolation, this would be mentioned in the original documentation, (as the chrominance section is isolated and documented) however I would enjoy confirmation that it’s fine to direct connect, because this step is super vague.
Part of this step uses a ceramic cap which bridges two pins on the new multiplexer. I was planning to use something in the pf range for this, maybe 470 / 220, though I’m not certain. I don’t think it’s a critical value or it would (probably!) be mentioned.
This concerns the luma, Y part of the mod, the pins connecting to (IC4 PIN 12) are luma signals, either from source1 or source2, or the original. The truth table for the 4052 output selection is shown in the schematic.
If anyone has any advice on these two points, I would be happy to hear your insight!
Here are the pics of the sections in question - I can send on pdfs if a deeper insight is needed.
Ta friends,
(black text is B.Murray, red text is MGC.Jerry)
