Component video switching problem

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I recently purchased two remote controlled AV switchboxes for my brother in law as a gift, to replace the two switches he had which were manually switched. He had one switchbox for audio and one for video (component video).

There were no problems with the el cheapo manual switchboxes. They had 3 RCAs - one for composite video, the other two for audio. It was not a problem that these were used for component video.

The problem is that the component video image is oversaturated. It is to the point that the colour settings on the projector need to be turned down so much that normally it would nearly be black and white!

I'm baffled as to how this could happen.

We tested all three (mine included which works fine on the TV on composite video). They all have the same problem.

1) Does anyone know what the problem is? Or what it could be ...

2) Why would this switch box be different to the el cheapo unit with a simple manual slide switch?

3) Can this box be tweaked to fix the problem?

4) If none of this can work, can anyone suggest other options here?

Pictures of the inside coming ...
 
I opened up the box ...

Surely this box would just have a relay in the signal path, and some PCBs with some brains to learn the signals to activate the switches (it learns switching signal from your existing remotes).

No relays found!

The front panel has tiny switches which work when the buttons are pressed in. But when you use the remote, it obviously has some other form of switching. All I can see is the ICs that must do it.

An externally hosted image should be here but it was not working when we last tested it.


An externally hosted image should be here but it was not working when we last tested it.


Another lesser problem I noticed is once I had the tuner sending a signal to one input, and I switched to an input with nothing going to it. I could still faintly hear the signal from the tuner - it was leaking through. Not very re-assuring that I'm getting good SQ!

Any ideas on how this could happen?

Can I tweak/upgrade this thing?

I had hoped I might be able to put in some better quality shielded signal cable, but I see lots of wire links not easily altered!
 
No relays and leakage between channels: Sounds like they used an analog MUX chip to do the source switching. (Cheaper than relays) Those are usually driven from a microprocessor with a logic level binary value to select the inputs. They probably amplify the signal that comes out of the MUX, which is causing your overdrive problem. You could insert a trimpot in the MUX output path, so you could attenuate the signal before it gets amplified.

If you want to solve the level and leakage problems, then I suppose you could find that input source select signal, power amplify it with an NPN transistor, and use that to drive a set of three SPDT relays. Then you would cut the signal traces to/from all the connectors and wire up the relay I/Os instead.
 
Why not? Each video input connector is probably (or at least should be) terminated with a 75 Ohm resistor to ground. I think you could remove the resistor and replace it with a 75 Ohm pot. The line would still see 75 Ohms, and the wiper would take an adjusted level of the signal to feed into the much higher impedance MUX & amplifier circuit.

It would only be a problem if the pot introduced a lot of non-resistive impedance. A high capacitive or inductive load could do nasty things to the image sharpness by rolling off the highest frequencies. If that was a problem, then you could just use the pot to determine the right combination of resistors to make a 75 Ohm end-to-end voltage divider.

Of course, using a pot with an end-to-end resistance much different from 75 Ohms would lower the energy transfer through the input transmission line. Don't do that!
 
You probably have something like this now:

input----------------------> to MUX chip
|
|
\
/ 75 Ohm resistor
\
|
|
gnd------------------


You would replace the resistor with the pot like this:

input
|
|
\ 75 Ohm pot
/ <-------------------> to MUX chip
\
|
|
gnd------------------

You can tell which is the wiper pin of the pot (connects to MUX chip) because the resistance to either end pin changes when you turn the pot control. The two end pins always will have 75 Ohms between them.

Of course, you can just try an experiment with a pair of resistors like this to see if it works:

input
|
/ 27 Ohm
\
| <-------------------> to MUX chip
\
/ 47 Ohm
|
gnd------------------

This would cut the color level by 47/74. Or try other combinations where R1 + R2 = about 75 Ohms.
 
Can I do this without making any changes inside the switch box?

Say putting a pot in line with the output leads going from the switch box. Then I could replace it with resistors inside the actual cables. I have done this with audio before to attenuate sources to get them to match. Is it the same with video? And if so, what pot value would I choose if I did it this way?
 
inline resistor

You could try it. That would be a simple way to do the attenuation, but the results are unpredictable. The video cables are balanced impedance transmission lines. That means that every point along the cable "sees" the impedance of the rest of the cable as 75 Ohms. This lets you transfer signals through the cables with minimal loss. If you add a resistor in line with the center wire, it will no longer work as well as a transmission line.

But really, that may be just what you need! You could try adding a 1/4 watt 10 Ohm resistor in line with each of the cable center wires to see the effect. Or you could add a 75 Ohm pot (wiper to one end) instead to make it adjustable. Try it and see. I don't think you could damage anything unless you short the signal to ground.
 
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