So, adding to my growing list of projects where I add features to things that should have had them from the start, I bring you: composite input on a pocket TV. This particular TV (Casio TV-890) was made useless by the Digital broadcast television switchover, it has no jack to put in an auxiliary signal, and thus can’t be used at all. So, since we all know the world runs on some sort of composite video signal (or can be made to) let’s crack this thing open and find us some signal paths. Maybe we’ll check the pads coming off of the RF can first, maybe look up part numbers for a video chip, maybe…..oh, well that was anticlimactic
So, a similar model has a three conductor 3.5mm jack with spdt switch built in. Presumably this is for video/audio and, yup, it is. The next question is where am I going to find one of those jacks? how do you even source a part like this? Well, I just so happen to have one, no idea where it’s from but I have one close enough.
Now, some of you may notice mine is a dpdt switch, yup, and I only have to re-locate one capacitor to make it fit.
But, there’s another thing. On the underside of the board there are two zero ohm resistors jumping the contacts that would normally be connected on the jack that isn’t present (for reliable operation they should be removed, otherwise you’re back feeding the RF stuff, or getting signal mixing). What I mean by that is: these types of jacks sometimes have, say, 5 pins when you might think they only need 3. Obviously you need audio, video, and ground (or left, right, and ground in the case of an audio application) but what are the other 2 pins for? They’re pass through audio lines. Think back to a set of speakers that stops playing when you plug headphones in, that’s what’s going on. When you don’t have headphones in, the audio gets routed through the pass through pins into the speakers, but the physical act of plugging in headphones breaks the connection and the audio is routed to your headphones. This is used in the opposite way, in the sense that you are routing a signal into the TV, rather than out. The use of a separate switch may be to cut power to the RF components so power isn’t wasted while they are not in use.
Well, what can I say? It’s not much of a hack. It works, or, it did. I think I was slimming it down for use as a portable screen for something or other and I broke it, so there isn’t a picture of it working, but I swear it did. Oh well, at least I got to tell people how to upgrade theirs.