There are some pretty knowledgeable people on here with regards to electronics, so hopefully someone can answer my question. I'm making an oscilliscope out of an old tv - however, the TV appears to have fault detection, where if the vert/hori connections from the coil are removed, the tv turns itself off after a few seconds. Does anyone know what the best way to bypass this is? I've tried bridging the pins on the board with tin foil, but this has no effect if both pairs are done individually or in tandem. If all four are connected together at once, the tv emits a loud buzzing and nothing is displayed. I'm no expert with electronics, so may be doing it completely wrong. Its a colour TV, does that make a difference?
A lot of TVs will have frame colapse detection so if something fails it'll shut down so it doesn't burn a line or dot on your screen (it can happen quite quickly). Bypassing it would require some electronics knowledge which you appear not to have so I'd recommend just finding another TV. Don't short out anything on the neck board as there is still high voltage aound there. I WOULD ALSO RECOMMEND YOU DISCHARGE THE TUBE FIRST BEFORE MUCKING AROUND.
This. Do this. Seriously. The tube is VERY dangerous. I think Assembler himself saw someone get electrocuted with one of these.
Yeah I've been using rubber coated pliers to handle wires etc and only doing it while its been unplugged. From what I've read theres not much chance of being able to disable this fault detector, seems to be on the gun itself from what I can tell. Looks like I'll have to wait until I can find an old black & white TV. Which is annoying, because I got some pretty cool effects out of it while plugged into my hifi:
TV tubes and certain components still carry high voltages even when off and will give you a shock to remember if you tocuh them. Just wrap some mains flex wire (the copper part) around a long handle screwdriver and the other end of the flex to the metal chassis, put your other arm around your back and jam the screwdriver under the anode cap and it'll be discharged. TVs from the mid 80s and older are less likely to have protection circuitry so they are your best bet, try second hand TV shops.
TV Scope You'd need a dummy load on the deflection circuit to keep the protection circuits happy. As well as the crt holding a charge, some of the caps in the power supply should be discharged by shorting them with a 1M resistor until they're safe.
I'lll bear that in mind for the next attempt, cheers. I've been browsing the local freecycle, hopefully something suitable will pop up shortly I've seen that guide - trouble is I don't know what a 'dummy load' or a 'deflection circuit' is - could you enlighten me?
At the very back of the colour crt, there are electron guns which fire electrons towards the inside of the crt face, which has a coating of phosphor paints. When the electrons hit the phosphors, they emit light which give us a picture. The guns are all in a fixed position towards the centre of the crt, so in order to steer the electrons to the outer edges and corners of the crt, the tv uses coils to generate a magnetic field which steers each electron up/down/left/right. This is done by the deflection yoke, fitted to the neck of the crt. You should see the colied wires wrapped around it, and a 4-pin connector which plugs into the mainboard. If you unplugged the deflection yoke while the tv was in action, the electrons would no longer be steered, instead the picture would collapse into a concentrated dot in the centre of the screen. The heat generated by this would burn the phosphor paint which isn't good. Most tv's can sense if the deflection coils lose power, and will cut the power to the electron guns to prevent this from happening. So you'd need too fool the tv into thinking the deflection coils are still functioning correctly by wiring up a 'dummy load'. The best thing to do would be to remove the deflection yoke you currently have from the crt neck, while keeping it plugged into the main board, insulate it and put it as far from the crt as the wires will allow. Then connect a spare deflection yoke from another tv to the crt neck and use this one for connecting up to your hifi or whatever.
I just want to say that this whole project seems needlessly suicidal. You can buy a real oscilloscope for what, a couple hundred bucks with no risk of death? Because make no mistake, CRT TVs are concentrated, lethal death-dealing devices once you take off the cover.
From what you've said, your first step should be to STOP! Get some books on CRTs and televisions. Read them before you go any further. When messing around with televisions, you MUST know what you're doing, and what's what. When you say oscilloscope, do you mean a piece of test gear? Or are you just trying to get funky patterns? I agree, if it's the test gear, you're much better off looking for an old one on eBay - you'll find that the ex-military types go for next to nothing now, and are usually fine as a beginner's 'scope.
I would recommend you have someone with more knowledge in this in regards to how they work and safety measures than risk your life or at the very least risk loosing the motor skills you take for granted now to make something you can purchase. My local electronics surplus store tends to have many oscilloscopes for sale, some labeled for parts, some fully working.
I've been shocked by a CRT tube and believe me it really fucking hurts. Try to imagine sticking your arm out in front of a speeding car and having it hit your hand. that's what it felt like to me. The force was so strong I was blown on to my back. The strange thing is that even though the blow back on to the floor was very hard I didn't feel a thing due to the shock on my hand. It's something I NEVER want to experience again. NEVER mess with a CRT. I was a teenager back then and knew no better but now I do after learning the hard way. Yakumo
You haven't lived unless you've been shocked by a crt! But seriously, they can cause a whole lot of pain. I've been shocked a few times from discharged crt's, as the charge immediately starts to build up again. Worst injury was when my arm recoiled and caused my hand to smack off a heatsink. Looked like i'd been assaulted with a cheese grater.
