Diagnosing a CRT fault, uneven brightness, Hurco Ultimax II
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  1. #1
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    Default Diagnosing a CRT fault, uneven brightness, Hurco Ultimax II

    Hopefully someone knows the cause of this, as LCD retrofits are uncommon (single vendor, expensive) for this control.

    Gradually the brightness of the screen has become extremely uneven, highly overbright on the left side, fading to almost invisible on the right side. I believe it to be tube or yolk related, as I have replaced the driver board with a known good one to no avail. I might be imagining it, but I think the image was better immediately after changing the board, but very quickly (overnight) went back to uneven brightness.

    The entire screen lights up bright by cranking the brightness, so it's not like the tube is just dead on the right side, although the brightness bias is still visible when it's cranked up like this.

    Normal brightness, text on right side barely visible:



    Cranked brightness:



    Thanks, Gregor

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    Looks like a really old CRT. Do you need to pull a string to start the hard drive? Could the screen be burned in on one side more than another? Do you sleep with your head pointed to Magnetic North?
    Sorry, I am not too serious tonight.

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    It's not an external mag field, that would screw up the image.

    A decrease in emission on the CRT would be more even, I think.

    A bad capacitor in the circuit feeding the brightness input to the CRT could do that. So could a problem in what is called the "DC restorer" circuit in the CRT control board.

    Bad CRT maybe.... not sure exactly what fault would do that, potentially pieces getting loose from the cathode and partly shorting the grid for the electron beam might do that, I think. I have not worked on a TV for at least 30 years, so,,,,,

    Long shot.... crap that got in and may be in the plug going to the CRT, or a loose connection in the plug or associated wiring.

    I do not know how much was replaced when you put in the mew board. If that was the whole of the circuitry, then the CRT becomes more of a suspect.

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    I have almost the same issue on my Ultimax II. My fading brightness is on the right-hand graphics screen though, if that helps any with troubleshooting what the issue might be. I don't use conversational programming, so, doesn't annoy me like your left-hand screen would.

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    The most likely culprit is a defective high voltage aka flyback transformer though it could also be another problem. It looks screen voltage related.

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    Thanks for the replies.

    I am by no means an expert of anything CRT, but my initial thoughts were also that it looked far more like a control board than a CRT tube fault, hence why I changed that out first of all.

    The board contains everything, including the flyback transformer, and is hardwired with the smaller board that plugs into the end of the crt. There is no other circuitry driving the crt. The board I used was known good, working correctly in another machine that I recently scrapped. It's possible that it's developed some fault in the time that it's been on the shelf, but it seems too much of a coincidence that it would be the exact same fault as the original board.

    All voltage generation is done on board, the only supply to the board is a single 12v supply, very low current, that comes through the same little ribbon cable as the TTL video signal.

    There is a second screen right next to this one with an identical crt and almost identical, although not interchangeable (different resolution) board. That is all powered by the same 12v supply, and is working fine.

    I don't know if it helps diagnose the fault, but even with the brightness turned all the way down, such that all text is invisible, the left margin of the screen still glows visibly.

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    analog video signal contains the brightness information, the CRT portion is just interpreting and adjusting the overall level (via the brightness knob), so the problem might be in the source of the signal, some dried up filtering cap, something like that, in the portion where the signal is being generated/combined, not the CRT itself

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    Quote Originally Posted by jz79 View Post
    analog video signal contains the brightness information, the CRT portion is just interpreting and adjusting the overall level (via the brightness knob), so the problem might be in the source of the signal, some dried up filtering cap, something like that, in the portion where the signal is being generated/combined, not the CRT itself
    Video source on this system is all digital, TTL level. Video, hSync, vSync. Everything else is on the crt board.

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    SO it is like an old PC monitor?

    MGA [monochrome graphics adapter]

    I have looked into this for the old Heidenhain monitors, I think I still have spares for mine so have not gotten serious. heidenhain is inverted MGA, if you hook the wires up to a MGA monitor you get the display bright background dark letters visible retrace[your retrace is visible on super bright pic]

    It should be worth prowling the computer bone yards for an old MGA to see if it works

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    Quote Originally Posted by gustafson View Post
    SO it is like an old PC monitor?

    MGA [monochrome graphics adapter]

    I have looked into this for the old Heidenhain monitors, I think I still have spares for mine so have not gotten serious. heidenhain is inverted MGA, if you hook the wires up to a MGA monitor you get the display bright background dark letters visible retrace[your retrace is visible on super bright pic]

    It should be worth prowling the computer bone yards for an old MGA to see if it works
    Nah it's not that simple unfortunately. Way back when on here there is my thread about the saga of trying to get the signal to work with various MDA/MGA converters. It's very similar to MDA but the resolution/timings are not compatible.

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    Hmm, too bad

    From my own experience I know that converting an old signal to a new monitor is very difficult, but have you actually tried wiring an old monitor to your machine?

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    Quote Originally Posted by gregormarwick View Post
    Video source on this system is all digital, TTL level. Video, hSync, vSync. Everything else is on the crt board.
    there must be a circuit mixing together the TTL signal on top of the overall brightness, if you say all the inputs are TTL, then it is done on the CRT board, so I'd look at the filtering stuff on there, usually the beam is "turned off" at end of each line to prevent the diagonal scan line showing up, but it does show in your pics

    here is some random ewd from a regular monochrome crt, ignore the stuff on the left, look at the horizontal signal path, and then work from the WC4 of the tube to back to horizontal output transformer, there are few filtering caps there (from signal to ground), it should look similar on your CRT board as well, and just replace all the caps in the corresponding signal path with new ones, they are cheap and the board is probably one layer, so very easy to desolder

    https://www.aescomputers.net/Schemat...unknown%29.gif

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    Looks like the phosphor has been eaten away on the inside face of the tube. Over the decades the electron beam scans over the exact same spot and eats away at the phosphor. Since you can read the display with high brightness, do it or replace the Cathode Ray Tube.

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    Try moving it, moving what is near it, or slipping a sheet of Iron or steel along each side in turn and observe what changes.

    Sometimes it is simply an external magnetic or electromagnet field near one side of it.

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    The mag field idea is good, I mentioned that slao, and can look just about like that, but the text is not distorted or moved, which the mag field would generally do, so that is less likely. Not entirely impossible, I suppose.... Worth degaussing the thing if you have the means to do it without going to a lot of trouble.

    Since the entire electronic assembly has been replaced, a straight up electronic problem is not very likely. That leaves connections and the CRT.

    The way it seems to just grade off to the right, no lines burned into the tube, no ghost images of the text, etc, suggests it is not as likely to be a phosphor problem.

    As I look at this again, I notice that when the text is less bright, it seems to get "holes in it". Look at the word "Spindle" where it says "tool in spindle". That could be a sign of some sort of interference that causes the brightness information to have noise in it. Then when brightness turned up, it might just over-ride the noise. That CAN BE a CRT issue, if the (AC powered) filament gets partly shorted to the cathode or grid, for instance.

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    Had exactly the same artifacts on the old Heidenhain TNC CRT, replaced all the electrolytic caps on the CRT board and it's been working for 5 years. A couple of the caps were hard to find, non-polarized, high voltage, and medium capacity.

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    My Hurco does the same thing, I have found some light percussive maintenance with my knuckles helps get its act together.

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    So, I've finally had some time to look at this.

    I replaced the crt with another, that was working perfectly the last time it was under power, and the problem remains.

    The crt that I have in the machine right now was last used with the board that is also in the machine right now. And they both worked fine about a year ago when that machine was put to pasture.

    Which really can only mean that the board that I thought was known good, somehow by horrible coincidence, has contracted the exact same fault as the original one.

    Right?

    I intend to just brute force replace all the electrolytic caps on the board and hope that will make it right. If I should be looking elsewhere, where should I look?

    Thanks, Gregor

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    Electrolytic capacitors are always a good candidate because they don't last forever. The electrolyte dries up over time.

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    If anyone stumbles across this in the future, and has an Elston 32-95198279 CRT board in their Hurco, the culprit for this specific fault is C103, a 10µF 100V electrolytic.

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