New here, if this is the wrong place, please do move it to a better candiate location, Sorry.
The intro (and my situation) can be found in the 'I need an AV switch; Any recommendations?' thread near the end of it.Symptom:
Joytech 240c a/v switcher introduces problems on the 'Y' signal of component video. (Some problems are horizontal lines, other include flicker and whatnot)Problem:
The IC used to switch the signals was not made for this purpose. They are using a HEF4051 8x1 mux. This 30 cent IC is not capable of properly handling the sync signal in the 'Y' of component video. Theres 6 of them in the 240c (one for each component signal, and one for the composite video, and 2 for the analog audio).Solution:
Replace the IC used for switching 'Y' with a better, proper, ic.
The candidate IC is MAX4315 8x1 video mux/amp.The fix
...(until i have time to create a proper one with full pictures and whatnot)
Replace the HEF4051 with the MAX4315. But the 2 chips are not pin compatible, so this means you will need to solder several wires to very small pins on the MAX chip (not for the faint of heart).
Grab the datasheet for the MAX4315 chip, and for the HEF4051
You will want to remove the HEF4051 chip from the pcb, and a few other components (see the red circle in the pic to see the components removed)
Dont forget to also remove the 4.7uf capacitor on the underside of the board.
You will now wire up the MAX chip. Use the datasheets to match the pins.
Take note of this:
- 'VDD' on the HEF is NOT connected to VCC on the MAX, you need to take 5v from
the proper place (see the DSC03734 image - 1st one) and connect that to VCC on the MAX chip.
- The "Y" inputs on the HEF chip map to the "IN" inputs on the MAX (ie. Y0 -> IN0)
- VSS on the HEF is GND. The pad on the pcb corresponding to this connects to GND on the MAC chip.
- SHDN on the MAX is connected to 5V (VCC) via a 10k resistor (resistor is not super important, but it prevents the MAX from getting damaged by accidental mis connection)
- You will also want to connect a 0.1uf capacitor (ceramic is fine, non polarized) between VCC and GND on the max chip (as close to the max as possible). Again this is not required, but if the voltage regulator is not stable, this will make sure the MAX is running properly.
- You will need a 75ohm resistor connected between 'OUT' pin on the MAX and to the location indicated by the blue arrow on the picture.
On the second picture you will see a brown board and parts below the MAX chip, IGNORE it. Its there for some tests that I did at first, and are not required for the fix.
One more thing, try and make the wires as short as possible. This prevents interference.
Hopefully this is clear enough for the technically inclined to try out.
Im busy this week, and probably wont be able to draw up more formal information, next week Ill get on it (if people want it)
(Maybe ill make a small pcb that will make life easier)
Im running it for about 5 days now. Working beautifully.
This fix also works for any other switcher based on the 4051 ic, such as some of the low end 'XBOX 360' switchers.
PS. If any more experienced AV designers wish to add to this mod please do so. Im by no means an expert in this field, I simply saw a need and found out how to fix it.