How to Discharge Cathode Ray Tube to Remove Shorts Between the Screen (G2) and Focus Plates

How To Discharge Cathode Ray Tube To Remove Shorts Between The Screen (G2) And Focus Plates In Electron Gun

Among all the electronic repair articles that I have sent to you, this method is consider a little bit dangerous if you do not know what you are doing. For those who are new in this field I suggest that you only apply it with the help of a senior tech. Study this article carefully and the necessary precautions that one should take before starting to use this trick. However, if you think that you have the courage to try, then go ahead to perform the test.

A 15” Philips 105s7 Monitor came in with the complaint of display blur. Normally the first thing that a technician will do is to tune the focus adjustment knob at the flyback transformer to get a sharp picture back. However, in this case while tuning the focus instead of expecting a sharp picture, you get a brighter raster (white background) with flyback retrace lines across the screen. Usually when you turn the focus knob, only the character get sharper without affecting the raster and this is also true when you tune the screen (G2) knob adjustment, only the raster will have effect but not the focus.

Since adjusting the focus control has effect on the raster, this clearly tells us that there must be something that had gone wrong in the monitor. With this problem, I could only think of 3 areas that I have to check i.e; a shorted flyback transformer divider network, a defective CRT socket and a faulty picture tube. In order to confirm if the flyback transformer divider is shorted or not is very simple. Just use a Monitor blur buster (MBB) and defeat the original focus and screen cable by connecting the focus and screen cable from the Monitor blur buster. Now tune the focus or screen control at the MBB to see if the picture gets sharp or not. If you get a sharp picture we can conclude that is the flyback transformer divider network that is faulty and if the picture stills blur then it can only be the CRT socket faults or a bad picture tube. In order to confirm if the CRT socket is the cause of the problem, one just has to replace with another working CRT socket to see if there is any improvement in the picture. If it still blur then I believe you are now sure as which one is the true cause of the picture blur problem-it is the CRT or picture tube! If it is picture tube problem, then you may ask, can it be repair or not since a picture tube is a vacuum tube that can’t be disassemble? Yes, it can be done but with some precautions that you need to take.

To repair such problem, one must first understand how a picture tube was constructed especially the electron gun. I won’t go too details about electron gun but I will let you know that the focus and the screen G2 plate is the nearest and if there are any dirt’s or particles that had happened to be between the two plates, then it would surely affect the display (picture becomes blur, dim or too bright with flyback lines).

Once you have certain that the cause of the problem is picture tube, now remove the CRT board from the neck of the picture tube and locate the pins of screen (G2) and focus. If you look at the photo, most of the CRT pin outs is almost the same except to some Sony Trinitron tubes. Connect an alligator clip between the focus pin and cold ground and the G2 pin to the shaft of the flat type screw driver. The reason for connecting the alligator clips in such a way so that the generated Monitor high voltage (about 24 kvdc) can be use to spark off the internal shorts in the electron gun between the focus and G2 plate.

Before you switch on the Monitor, please make sure you disconnect or unplug the VGA signal cable from the CPU otherwise when the discharge begins it may destroy the CPU VGA card. Not only that, please check any surrounding in the Monitor as no extra wires or cables touches the power section, if not it may blow the power supply. As mentioned in the above first paragraph that this method really need you to be extremely careful and alert so that nothing would happen when you start to discharge the high voltage through the anode cap.

If you are ready now, hold the handle of the screw driver (remember, the handle have to be thick otherwise certain of the high voltage may jump across and discharge through your hand) with your left hand and power “ON” the Monitor. Once the high voltage have present (with the rushing sound you’ve heard), now switch off the Monitor by pressing the front panel on/off button, you then quickly poke in the screw driver shaft to the anode before the high voltage being discharge by the bleeder resistor in the flyback transformer. In other words, we want to utilize that short moment of the high voltage to spark off the shorts in the electron gun before the high voltage being fully discharged.

Normally you will hear a “pop” sound when the high voltage arc through the screw driver shaft at the anode cap. Repeating this for few times and the shorts definetely will go away. Connect the CRT board back and test the Monitor to see if the problem has been solved or not. Assuming the Monitor still blur after few of the discharged, you can always reverse the alligator clips where now you are connecting the G2 pin to cold ground and using the focus point to poke through the anode cap. This will usually clear the shorts away.

Remember, the blur problem in this article is totally different from the blur problem (blooming) caused by a defective flyback transformer divider network where you need a Monitor blur buster to solve it. Using the method above I’ve solved lots of this Monitor blur problem whether it is a 14” or 21” Monitor!