The first part of the article mentioned about how you can tap from the secondary diode to get the desire output through the 7808 voltage regulator. The weakness of this circuit is that it can’t power up the 17″ Monitor because the power supply have some limitation in delivering current to the bigger CRT tube. The power supply will either shut itself off or power blink. In order to counter this problem, we have to tap voltage from some other places in the Monitor. Unfortunately there were no more places (point) in the Monitor where you can tap the voltage to power up the bigger CRT tube. Due to we could not locate the point then we have to create (design) our own circuit. The best is to use the Monitor flyback transformer ferrite core and with wire loop around the core will produce high pulse ac. This high pulse ac can’t be measure by your normal meter.
With few turning at the core (external loop), you can get a higher ac output voltage to power up the CRT heater. This method only applies if you want to brighten up a Television picture tube. You can’t use this way on Monitor CRT because as mentioned earlier in the first part of the article, Monitor runs on different resolution. The higher the resolution, the higher the B+ voltage supply to the primary winding of the flyback transformer. The higher the B+ voltage, the greater is the voltage output at the secondary side making the output voltage (ac) produced by the external loop even higher. Thus this will make a sudden increase in the filament voltage causing an instant bright in the Monitor raster. Some monitor designed will shutdown if it detect a sudden brightness in the raster.
Even if the monitor didn’t shutdown and if the user decide to lower the Monitor resolution, the raster will suddenly become too dim due to lower B+ voltage. You may ask yourself why computer Monitor has its own 6.3 vdc output and why not like TV, the heater voltage comes from the secondary winding of TV flyback transformer? The answer is because the Monitor designer wants a steady 6.3 volt for filament and don’t want the raster to be affected by a changing in Monitor resolution. In order to solve this problem, after making about 7 to 10 loops around the flyback transformer core, you have to convert this high pulse ac into DC voltage. You can’t simply use any diodes for this purpose. A general purpose diode like the famous 1N5401-1N5408 would not be able to work in this circuit. The high pulse ac voltage will immediately kill the diode once the Monitor is switched “on”. You have to use an ultra fast recovery diode to do the job.
For your information, not all ultra fast recovery diode can efficiently run on this circuit. You have to select the diode type that has a faster speed time like a 50 nanosecond and below. I had tried a 75 nanosecond ultra fast recovery diode and it blown at 800 by 600 resolutions but work well in the 640 by 480 resolution. After done some R & D, I found that the best part number to use is the UF5404. This diode has the specification of 400V, 3A and 50ns. Once you have connected the ac voltage to the diode, you now have to use a filter capacitor to filter off the ripple. You can use a value of 220 to 1000 microfarad 50V. Measure the output DC voltage and make sure it must have about at least 15 to 20 over volts before you solder to pin 1 of 7808 voltage regulatorYou can also try to use a 7809 if the display is not bright enough. Don’t use any voltage regulator that is higher than 7809 otherwise the heater filament will get too hot and burn.
Pin 3 of the voltage regulator is the output and it must joint to the heater pin at the CRT board as shown in the photo. Carefully place all the wires in a strategic location and attach the voltage regulator at the heat sink beside the flyback transformer. You are now ready to switch on the Monitor and check if there is improvement in the display. If it okay then burn in test the Monitor for couples of days to check if the display is still bright and clear. If you are not satisfied with the sharpness, you can try to increase another volt by changing the voltage regulator part number to 7809. Some technicians prefer to use a variable voltage regulator such as the LM317T with a variable resistor so that they can adjust to the desire voltage. If the picture tube is still dim and no way that you can improve the display, I guess the time is up for the CRT tube. You either replace a used tube or return the Monitor back to the customer. I hope you enjoy reading this article. If so, please feel free to share this article with your friends or simply forward this article to anyone you wish! There’s no limit on the number of people you can share this with! If you have a question or comment about anything in this article, drop me an email. Your feedback is welcome!