I found an old CRT TV set in the garbage. It didn't work, always showing the "snow effect", even when I connected it to another device via the SCART port.

So I followed an online tutorial to make it into an oscilloscope. However, I encountered some problems. I'd like to know if they are likely to be resolvable or not, since I'm not an expert in this field.

There are four connections for the CRT that should control horizontal and vertical deflection. I disconnected the wires that control horizontal deflection and connected in their place those that previously controlled vertical deflection, in order to have a horizontal line at a scanning frequency of 60 Hz. Then I connected two wires for controlling the vertical deflection (at the other end of these wires I can put an audio output or just a battery). When these two wires are not connected to anything I should see a horizontal line.

What happens is that the horizontal line that should appear like a solid bright white line appears instead as a weak reddish line, as you can see in this picture:

horizontal line http://img259.imageshack.us/img259/6536/foto1yjz.jpg

The strange thing is that I tried with a friend to connect and disconnect the four wires to the CRT when it was powered on and, apart from the sparks, the process of connecting and disconnecting those wires resulted in a white bright line as it should be.

Even when all the wires for horizontal and vertical deflection are disconnected a weak red point appears and only with active manipulation performed when the device is on I manage to see a bright white dot instead.

I'd like to know if this behavior is due to the TV set being defective and is not easy to repair, or if it is somehow my fault and it is easily solvable.

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    \$\begingroup\$ Red showing up is probably caused by the fact that the yoke of the CRT is in fact very precisely configured to make three separate beams (R, G, B) point into one single spot on the shadow mask, with the CRT construction being far from ideal for simple sawtooth and simple coils. The canons are aligned in a horizontal row at the base of the CRT (you can see them at the far end of the tube). When you exchange vertical and horizontal control voltages from the coils, the electron beams will no longer find their way to the correct place on the shadow mask and ultimately hit the wrong color fosfor. \$\endgroup\$ – jippie Jul 26 '12 at 18:13
  • \$\begingroup\$ Thanks @jippie for your comment! However I did not understand well what you mean by "the CRT construction being far from ideal for simple sawtooth and simple coils". Moreover the problem is not only that the line is red, but also that is short, irregular, displaced on the left and not bright enough \$\endgroup\$ – Fiat Lux Jul 26 '12 at 21:55
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    \$\begingroup\$ I used to work at a Philips factory where these deflection coils were made. They are far from straight forward, to compensate for the the tubes that in turn have a non-ideal curvature. An ideal CRT-front would be a part of a sphere, but nobody wants to watch TV on a globe. Both directions horizontal and vertical are different. Ideal electron guns would originate three beams from exactly the same point, but that is impossible. Indeed I noticed the beam intensity being low, that is strange. Don't know why that happens as I expect a beam like that to be extremely bright. \$\endgroup\$ – jippie Jul 27 '12 at 21:10

This kind of surgery on a CRT is not a good idea unless you know what you're doing. There are high voltages involved, and they can really hurt or worse. You also can't count on just disconnecting the deflection coils and using them any way you want. Some monitors, particularly those in older TVs used the horizontal deflection coil as part of the circuit that made the high voltage. Old TVs are amazing feats of electrical engineering in that every part did multiple things to the extent possible. The volumes were high enough to pay for engineers cleverly reducing the circuit by pennies at a time.

Another issue you seem to have ignored is the brightness. The individual red, green, and blue drive signals originally came from somewhere. Without controlling these properly you don't know what beam current, and therefore what color and brightness you end up with. It's also quite possible to burn a hole in the phosphor by leaving the beam current on while the beam is not being deflected. That probably happens even at a small fraction of the normal beam current when the beam is being swept in a raster pattern.

Again, messing with this stuff at your apparent skill level is not a good idea. That isn't meant as a insult, but you have to know what you can do and what you can't without a good chance of getting into trouble.

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    \$\begingroup\$ +1, mostly because of: "Old TVs are amazing feats of electrical engineering in that every part did multiple things to the extent possible." If you want to read about only some of the great ideas coming together in a CRT TV, a good start is chapter 4.1.1, An Introduction to Horizontal Deflection, in this document by Philips, now NXP: nxp.com/documents/application_note/APPCHP4.pdf It is a work of beauty, especially the part that shows how everything comes together before Fig. 24 is introduced. \$\endgroup\$ – zebonaut Jul 26 '12 at 16:17
  • \$\begingroup\$ Thanks for your answer. I admit I thought that a CRT color television was simpler in terms of electrical engineering. I would like to ask you if messing up with old black-and-white CRT televisions or alternatively with CRT monitors (without TV) would be different in terms of easiness and possible hazards. \$\endgroup\$ – Fiat Lux Jul 26 '12 at 17:42
  • \$\begingroup\$ Also thanks to @zebonaut for the amazing reference you linked \$\endgroup\$ – Fiat Lux Jul 26 '12 at 17:42
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    \$\begingroup\$ B/W CRT's are a lot simpler because you don't require three colors, a large part of a color monitor is built in threefold (Red, Green, Blue). Also don't have to worry about shadow mask related stuff (essentially for clear colors mostly integrated in the yoke configuration) and convergence (three beams moving in exactly the same way on screen and at the same moment). Where a color CRT carries about 16kV, a B/W CRT can still easily carry 4-6kV. A B/W monitor is much simpler, but it still requires extreme care when working on it. \$\endgroup\$ – jippie Jul 26 '12 at 18:05

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