Greg's Homemade Oscilloscope Page

Time to show your TV who's boss

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Ok idiots, here's the deal. The base of this project is the guts of your every-day stinky, dirty, poor quality television that you don't want any more. Since the tv has most of the stuff already inside it to function as an oscilloscope, it requires only a few modifications. Beyond the TV, a variable frequency saw-tooth generator circuit and a few amplifier circuits are all that is required. This is a fairly simple project, although I have to say the results I got aren't especially outstanding if you actually plan to use this thing for more than decoration. But for someone with any sort of intelligence, it might just work..

How to Get Shocked.. I mean started

Well the first thing you'll need is a stinky dirty old TV from the garbage pile. I managed to score a real *gem* over on the Stanford campus. This thing is from the 70's for sure! It's made by MITSUBISHI and it has the old-school dials for 'UHF' and 'VHF' and doesn't seem to get in a single channel. It was raining the Saturday night I found this guy, and it was covered in mud, dirt and snails. I was going to let the water-soaked circuit board dry out for a few days before firing it up, but na!

First out you need to remove the back cover from the TV to expose all its guts. You prolly want to unplug the thing before you take the cover off to avoid being shocked too badly. And don't be sticking yer fingers all over the place in there either, some of the components in there can hold a charge of several thousand volts for hours. Next locate the horizontal and vertical deflection yolk coils attached to the CRT. These are going to be coils of copper magnet wire wrapped around the skinny part of the picture tube. Trace these two coils (four wires) to their end, usually a connector plugged into the main board. Yank out the connector from the board, mine looked like this:

These deflection coils are what 'steers' the electrons on their way from the gun to the phosphor screen. By passing currents through these coils, their field changes and interacts with those little electron guys, changing the trajectory. The rest of this project interfaces with these coils.

Since these cables are pretty short and are around the back side of the TV, I cut off the connector and spliced in a new 4 conductor cable about 8 feet long so I would have easy access to the wires from my work-bench. At this time you could drill a hole in the back of the TV cabinet and poke the wires through so you can put the back cover back on, but I left mine off in case I felt the need for a good high voltage wake-me-up shock later in the night.

Mapping Out the Deflection Coils

At this point I power up the TV to make sure it still works and there is no 'fault' or protection circuit that disables the electron gun when the deflection coils are detached. If every thing is cool, there should be a bright dot in the center of the TV screen indicating the gun is still firing (along with the several thousand volts of acceleration voltage from the flyback transformer). This bright dot is the columnated beam of electrons striking the screen. If there is no dot in the center there may be a fault detection circuit in the TV. You can fool this protection circuit by putting 'dummy' coils across the board contacts where the deflection coils were removed.

The two pairs of deflection coil wires can now be tested to see which coil controls horizontal and vertical deflection of the electrons. Apply a low DC voltage (don't plug it into the wall unless you want a fireworks show) across a pair of the deflection coil wires. If the dot shifts to the left, you've got the horizontal circuit but the polarity is backwards. If the dot goes up, then you've the vertical deflection with the correct polarity. For my TV, the red and white wires were the horizontal coil and it took +-3 Volts for 1 Amp to span the entire tv set. The black and green wires turned out to be vertical with +-7.5 Volts corresponding to 140 mA for complete span of the screen. If you just want to fool about with funny lines and patterns, you can just hook these coils up to various function generators or low voltage AC (like from a VARIAC) and see what patterns you can make. The picture on the right is what mine looked like with a couple volts at 60 hertz hooked up to each deflection coil. This would be a circle pattern if each of the deflection coils had the same deflection distance for the same voltage applied. It is also especially amusing to reverse the deflection coil polarity and hook them back up to the board. This causes the regular TV broadcast to be displayed backwards, like viewing through a mirror (fun for movies with sub-titles). Backwards, upside-down, sideways, you can do them all just by switching around the deflection coils. Also great to see how long your roommate can tolerate playing the PS2 with backwards controls.

The Hard Part

So for this thing to work as an oscilloscope a couple things need to happen here. First thing is we need a saw-tooth generator to sweep the horizontal deflection circuit back and forth. The saw-tooth generator needs to be variable frequency (in order to 'match' the periodicity of what ever wave-form you are wanting to look at), have bi-polar swing (goes positive AND negative) and it also needs to be able to source at least an AMP without distorting a whole lot. The first two parts can be accomplished by using an op amp configuration and a push-pull amplifier with a PNP and a NPN transistor pair. My set-up was not so hot at doing this without distortion, but if you're not a moron like me you should be able to figure something out for this part. That takes care of the horizontal deflection and time base circuitry for the scope. The second part we'll need is just an amplifier to run the vertical deflection circuit. This amplifier just needs to take the input signal (the one you want to look at with the scope) and make it fat enough to drive the deflection coil. Here's the photo of my saw-tooth sweep circuit (on my $35 ebay scope) and the TV with a 60 cycle AC signal from a VARIAC. At best with my circuits I would hardly call it a tool, but kind of cool anyways. Couple more photos of various signals and patterns that I saw Number 7 Number 8


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Last updated: 02/20/05
Copyright 2005, Greg Miller