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Roving dark spot on CRT

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    Roving dark spot on CRT

    Anyone ever seen anything like this? Seems to change when the monitor is tapped or moved around.

    The fact that the spot remains in the same place regardless of sync or lack of it tells me that something has flaked off the innards and is occluding the electron beam. Perhaps cathode detritus? It clearly moves a bit when you tap the side of the monitor. I think the best you can hope for is for the schmutz, whatever it is, to drop into an area where it doesn't get in the way. Have you tried running the monitor upside down, on its face, on one side or the other, etc.? Maybe you can dislodge it.


      Yeah, that's strange, LOL. The first thing I would do is make sure the board connected to the yoke is connected well (loose, cold solder joints, etc..) I'm no monitor expert though. Someone else may know the exact cause.

      See, Chuck above posted just before I did. That explanation does make sense.


        Adding on chuck's suggestion, have you tried picking up the monitor and shaking/jumping with it in your hands? :P
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          That's a pretty bizarre fault, but the fact that it changes when you tap the monitor likely means there's a bad solder joint somewhere that is at least contributing to the fault.


            Thanks guys. Strange thing is that spot wasn't there when I first fired it up. I then moved it about to look at the broken feet from the stand and that's when it appeared. My concern here is that even though the monitor looks cosmetically great, it clearly was abused by FedEx and I wonder if something may have broken internally. But what Chuck mentioned is interesting. I can certainly try turning it upside down and see what happens. It's a nice little monitor.. I'd love to have at least one working RGB monitor in addition to my 5154.


              Ok I just reviewed my earlier video of when I first fired it up and I was wrong.. it had that dark spot and it was huge.. all the way down almost to the bottom. So if anything it got better in my later video that I posted here.


                That is a very weird looking issue. The way it moves when you tilt the monitor does make me think it might be in the tube. But it might be worth opening the unit up and just tapping around on the electronics with a plastic tool to see if anything else in there triggers movement.


                  Flake-off from the cathode coating used to be one of the failures in the old TV days. The problem was that the flakes are somewhat conductive and if they fall between two elements, the CRT is inoperable. CRT "Rejuvinators" often had a "Remove Shorts" function--but don't try it on a Trinitron--you'll destroy the CRT. It was a gamble--if you could clear a short by zapping it, you'd save the cost of replacing an otherwise non-functioning CRT. In your case, however, you don't have a short; just some crud in the way of the beam.


                    There is likely nothing at all wrong with the CRT. And if there was anything in the way of the beam the defect would remain a fixed shadow.

                    There is likely a defect in the blanking pulse circuitry. The beam blanking pulses, both H & V, should be a clean rectangular wave. But after flyback the H pulses have a large tail on them continuing to blank the beam which recovers after about 1/4 of the V scan. The fact it is changing suggests capacitor leakage issues in the blanking sub-circuits.

                    If you post the schematic I can advise where to test with the scope.


                      How do you explain that the dark spot remains stationary after sync is lost (tail end of the video), Hugo? That's what caught my eye.

                      Not saying that you're wrong, Hugo--my eyes could be playing tricks on me. It wouldn't be the first time.
                      Last edited by Chuck(G); May 4, 2021, 01:41 PM.


                        Originally posted by Chuck(G) View Post
                        How do you explain that the dark spot remains stationary after sync is lost (tail end of the video), Hugo? That's what caught my eye.

                        Not saying that you're wrong, Hugo--my eyes could be playing tricks on me. It wouldn't be the first time.
                        The blanking pulses are not derived from sync directly, they are derived mainly from the H & V scan output stages, so it does not matter if there is a video input to the monitor or not, if there is a H blanking pulse defect it will still be there in the scanning raster in the same place, why it is not there for all H scanning lines I am not sure yet.

                        This sort of thing relates to the age old question, when a defect is seen in the image, is the defect in the Picture Content or is it in the Scanning Raster. In this case, the fact it remains there, with no signal, means it is in the scanning raster.Which also means something could be physically obstructing the beam, but when I have see that its always a fixed unchanging shadow.

                        Looking at the form of it, it looks like the H blanking pulses are abnormal just after the start of scan to give that tear drop like effect where the beam is being cut off in that zone. Probably, where the H & V blanking pulses are mixed, there will be a faulty capacitor. It depends on the design, some sets use a dedicated blanking amplifier. So we need the schematic.

                        Edit : and pondering it more, if the vertical blanking signal was modulating the H blanking, then even de-synchronized with no video input, the defect would stay in the same place, starting just after H and V retrace in the upper left of the image.

                        This is just an initial hunch of course.
                        Last edited by Hugo Holden; May 4, 2021, 10:20 PM.


                          I watched the video here are my observations.

                          1) the raster is square, correct size and properly aligned.
                          The yoke and alignment magnets must be in the correct place

                          2) focus and brightness are good
                          tube voltages are nominal

                          3) color purity is good
                          gun is undamaged and not dislocated

                          4) "the blob" moves smoothly as the monitor is turned.
                          if this were a bad contact it would jump not move smoothly so not electrical.

                          whats left is mechanical. two things come to mind:

                          The supports for the shadow mask/aperture-grille are damaged displacing it, blocking the beam and shifting due to gravity
                          The coating (anode) on the inside of the tube is separating from the glass envelope and blocking the beam.

                          neither is good news.

                          What I would do if it were mine.... set it on its side and whack it like a bongo.
                          It will do nothing or make it better or make worse. But right now it's E-waste ;^(



                            4) "the blob" moves smoothly as the monitor is turned.
                            if this were a bad contact it would jump not move smoothly so not electrical.

                            Yes, if that is the case, I agree it cannot likely be electrical. Possibly a flap of internal aquadag as you suggest.

                            Though looking at the early part video again, there is something interesting that goes against that theory, in that when the monitor is not being bumped or moved there is a sudden rapid change in geometry of the area, and it is hard to understand how that could happen with internal an internal loose flap of aquadag or debris occluding the beam. (though I guess electron impact might explain it) Even though its also difficult to not think that if the problem is vibration/movement sensitive , that it must be a defect inside the CRT.

                            If it were my monitor, before I condemned the CRT, I would look at the grid-cathode signals on the CRT with the scope, just to make 100% certain the beam was not being cut off electrically by defective blanking signals.
                            Last edited by Hugo Holden; May 7, 2021, 01:48 PM.


                              I would loosen the deflection yoke, and try rotating it to see if the dark spot moves with the image. If it doesn't, then it has to be something inside the CRT.