How to fix monitor screen dimming issue

This OEM Relisys CRT 17″ monitor came up with a complaint of a dark screen (can’t see the back frame). Even though on the front panel via the on screen display (OSD) you have set the frame to max back still not displaying and the whole display looked pretty dim.Normally whenever we see this type of dim display problem we will go straight to the flyback transformer and adjust the display voltage control (G2) to increase the voltage to the tube image so that the screen becomes brighter again.

Somehow, on certain CRT monitor designs, there is no display control (knob) (G2) that you can adjust. It contains only the two knobs for you to adjust the focus (F1 and F2 knobs). Since we can’t find any G2 controls on the flyback transformer, one would always look for a preset internal brightness control, but unfortunately this model doesn’t have one. It only has the B+ control and the high voltage control preset. The high voltage preset has been glued with silicone and we can’t just adjust this preset, otherwise the monitor will go into power off mode intermittently. The only preset you have left to adjust is the B+ control. Yes, it is true that sometimes on certain monitors you can adjust the B+ control preset to increase brightness, but this is not recommended as increasing the B+ voltage will increase the HOT output transistor voltage. The HEAT may not last very long and sometimes it will explode within a couple of hours.

On this monitor, I tried adjusting the B+ preset to see if the brightness would show up or not, but the result made the screen wider and still darker. If I keep adjusting the B+ control, I’m afraid that the HOT may explode in a very short time! This is just an experiment, remember don’t tune the B+ control without a good reason.

After having used the monitor for a couple of years, the CRT cathodes (emission) may have weakened and this caused the screen to go dark. To make the screen brighter again, you can increase the heater voltage from 6.3 volts to about 8 to 9 volts. Or you can modify the G2 circuit to increase the voltage without adjusting any B+ or high voltage preset on the monitor. In this article I am referring only to the modification of the G2 circuit.

Since the flyback transformer does not have the G2 control, we cannot trace from the flyback to find the G2 wire. In fact, we can trace back from the CRT board to see where the G2 cable came from. Yes, it came from the main board and not from the flyback transformer. Flipping back the main board you can easily see the small G2 circuit which contains a diode, high voltage capacitor and very high ohm resistor.

This is how the G2 circuit works. The high-pulse AC supply came from one of the flyback transformer’s secondary pins and passed through an ultra-fast recovery diode. The function of the diode is to convert the high AC pulse into DC voltage and the high voltage capacitor (102 types 2kv resin ceramic) is used to filter the high frequency ripple and then the voltage flow through the diodes. resistors and the resistors determine (set) the output voltage. Once you understand how this circuit works, you can easily modify the circuit or change the value of the resistors to increase the G2 voltage.

In the picture you can clearly see that by removing the jumper and adding 4.4 Mega Ohm (2.2M in series with another 2.2M gave the result 4.4M Ohm) I was able to easily increase the G2 voltage. You may ask “How do you get the value of 4.4M?” I actually tried some high ohm resistor values ​​such as starting from 100K then 1M then 2.2M before concluding that 4.4M ohm has the best display (brightness). If you remove the jumper (link), the screen will become too bright and not good for the user.

In some monitor designs, the G2 voltage can be at the highest value and if this happens, no matter what resistor value you change or modify the circuit, the G2 output voltage remains the same because the G2 voltage is already at the max. You can only reduce the G2 voltage, and there is no way to increase the G2 voltage! If this happens, you need to modify the voltage of the heater to increase the brightness.

Conclusion: In fact, in any electronic circuit, if you really understand how a particular circuit works, you can do some modifications to get the electronic equipment working again. If the electronic equipment is deemed beyond repair, there is nothing wrong with trying whatever modification method you feel is best to get the electronic equipment working again. To do that, you must first equip yourself to have a basic understanding of electronic theory. For your information, the above problem was a common fault and I have already solved many and made a good income with a simple 10 minute job. Have a nice day my friend!