Now the boss has gone home, I can spend a bit more time with some advice!
Dajgoro wrote:
Quote:
You would then need to amplify your video signal to aproximately 50-60V pk-pk and feed it to the CRT grid via a 0.01uF capacitor rated for 2500V!
Why 2500V if the signal is 50-60V?
Your signal amplifier will probably have its output at or near ground potential. The grid will be the most negative pin on the CRT at aproximately 1500 - 2000 volts negative of ground. The coupling capacitor between the amplifier output and the grid needs to withstand this voltage, hence a 2500V rating.
Dajgoro wrote:
Quote:
Pin 2 is the cathode. This should be at 1500-2000 volts negative of pin 9.
So it should be -1500V?
On which potential is then pin 9?
Pin 9 should be at ground potential. (0V) The cathode should then be at aproximately 1500V negative of ground.
Dajgoro wrote:
And how many voltage sources do I need in the end?
I build a 1500V DC source out of a laptop backlight inverter, and I got 200-250V form another inverter. Also I got 6.3V for heating. Anything missing?
These should do you.
The 1500V supply can supply all the voltages needed for the CRT. You connect a potential divider made of a chain of resistors and potentiometers across it and tap off the appropriate voltages.
A suggested chain would be
from 1500V supply negative
100k potentiometer - slider connects to CRT grid via a 100k resistor. This is the brilliance control.
68k resistor
- - - CRT cathode connects to junction of 68k and 220k resistors
220k resistor
500k potentiometer - slider connects to CRT pin 5. This is the focus control.
510k resistor
510k resistor
510k resistor
510k resistor
To Ground - 1500V supply positive - CRT pin 9
4 510k resistors are used instead of a single 2M resistor because these resistors will have about 1000V across them and most resistors are not rated for this high a voltage.
If the CRT won't come to a sharp focus, note which end of the focus control gives the best focus. Reduce the resistor connected to this end of the focus pot by 100k and increase the resistor connected to the other end by 100k. This shifts the focus pot up or down the resistor chain whilst maintaining the total resistance, and thus the other voltages, at the same values.
Decreasing the value of the 68k resistor will allow you to increase the brightness of the image. Don't reduce it below 22k however or there is the danger that the grid will be driven positive of the cathode by the image signal - this will damage the CRT very quickly!
The 250V supply can be used to power your deflection circuits.
The 6.3V supply is needed to supply the heater. Note this could pull as much as an 1.5 amps! Connect one heater connection to the cathode via a 10k resistor. IMPORTANT - the entire 6.3V supply will be at 1500V with respect to ground. Ensure your mains transformer can take this - many can't.
The deflection plates are connected to ground by 1M resistors. The deflection signals can then be coupled to the deflection plates via 0.01uF capacitors. For a monitor application, you shouldn't need X and Y shift controls.
I suggest you look up schematics of valve oscilloscopes. These will show you the sort of circuit you need to drive the CRT
With a simple design such as this, the overall brightness of the image will probably change as the white / black ratio of the image changes. I've not found an easy way to rectify this.
One final point - unless you have a magnetic shield for the CRT, be sure to keep all your transformers as far away from it as possible or their magnetic fields will cause the image to wobble at mains frequency. When I played around with CRTs, I found a distance of 3 feet (1 metre) was needed to avoid magnetic fields affecting the CRT.