Old TV Picture Tubes Lights Again

When we tore apart an old rear projection television a few weeks ago, I did not expect those picture tubes would ever light up again. We took everything apart quickly within the narrow time window, so we didn’t have time to be careful to keep the electronics driving those CRTs intact. Those electronics are in pieces now, and in that writeup, I said the tubes were beautiful glass work and I hoped to display them as such in the future.

Well, there has been a change in plans.

On the same day as that teardown, [Emily] was gifted an old non-functioning camcorder. She has since taken that apart, too. The first component to see reuse was its tiny black and white viewfinder CRT. And as she dug deeper into the world of old CRTs, [Emily] came across this YouTube video by [Keystone Science] going over the basics of a cathode-ray tube and shared it with me. We were inspired to try lighting these tubes up again (without their original electronics) at yesterday’s SGVTech meetup.

The first step was to straighten out the pins at the rear end of our salvaged CRTs – they got a bit banged up in transport. A quick web search failed to find details on how to drive these tubes but probing with a meter gave us a few candidates for exploration.

Probing CRT pins

  • A pair of wires had around 8 ohms of resistance, highest of all wire pairs that gave a reading. This is likely the heating filament.
  • A few other wire pairs gave readings we didn’t understand, but several of them had some relation to a common pin. The common pin was thus our best candidate for cathode pin.

We knew the anode is connected to the side of the CRT, so now we have all the basics necessary to put a blurry dot on screen. A bench power supply was connected to the eight ohm load, and a few seconds later we can see a dull glow. Then a high voltage transformer was powered up across our anode and candidate cathode.

RPTV picture tube and transformer

After a bit of debugging, we have our blurry green dot! We proceeded to power up the other two tubes, which gave us a blue dot and a red dot. The colors look good to us, but apparently they’re not quite the right colors because during our TV disassembly we saw some color filters on the red and green tubes. (The blue tube had no color filter.)

During the course of the evening, the quality of our dot varied. Most of the time it was a blur approximately 5mm in diameter. On one occasion it bloomed out to 3cm diameter and we didn’t know what had caused it. Likewise, we had a pinpoint bright dot for a few seconds not correlating to any activity we could recall. As far as driving a CRT, we know enough to be respectful of the high voltage involved, but obviously we still have a lot more to learn. It’s just as well we don’t know how to focus the dot, because in the absence of sweep, a constant bright focused dot would likely burn a hole in the center of the screen’s phosphor layer.

A first step towards moving the beam was to put some power on the magnetic deflection yokes. These coils of wire were hooked up to a function generator, and we were able to get movement along one axis. Its maximum output of +/- 20V could only deflect a small fraction of the screen size, but it was something.

We didn’t have a second function generator on hand, but we got movement along another axis using magnets. They were taped to a shaft that was then put into a cordless drill. Holding the spinning drill near the control yoke induced movement along the other axis. Combined with the function generator, it allowed us to make a few curves on screen.

RPTV Red curves

Tinkering projects with visual results are always rewarding. With this success, there might yet be life ahead for these tubes as something other than pretty glass. A search found a hobbyist’s project to drive a CRT for an XY vector arcade monitor. That project page also linked to an excellent description of vector CRTs as used in old Atari arcade machines. Lots to learn!

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