Intermittent fault in Eizo 19" T766 CRT

Ah, that explains it.

The question about the line capacitor was unrelated. It had to do with another minor issue of this monitor (it slightly getting brighter over time, ever since I got it), of which was suggested it could be this cap which was degrading. While we were on the subject, I just thought I'd ask.

Taking what bz said into account, that the Trinitron tube has a single electron gun, doesn't this mean that if the screen turns fully blue (and not red and green as well, meaning white), including retrace lines, that the short can exist nowhere except inside the tube?

Wiebe Cazemier wrote in news:e631c$4814fa58$d4cc82be$ snipped-for-privacy@cache2.tilbu.nb.home.nl:

Three guns means three filaments, three cathodes, and three sets of focusing electrodes. One gun means that the three cathodes all use the same focusing electrodes.

Ah. I see.

I would expect a cathode bypass cap to be near the cathode. On the other hand a cap involved in driving a cathode with a signal might be near the video output. Sorry I can't help more.

I remember quite a long discussion somewhere about 'monitor too bright', google may find it for you.

If I remember correctly, it was a panasonic monitor being discussed and two fixes were mentioned. One involved going through the setup menu and the other putting a resistor in parallel with another resisitor. This may or may not be of any help to you.

You were also involved in the discussion about my monitor getting brighter. But, it's no issue at all. A while ago, I turned down the G2 voltage and ran the automatic color calibration and everything was perfect again. Except for this intermittant fault issue, that is.

I suppose it's got nothing to do with it, but still I think it's best to ask whether turning down the G2 voltage could have induced the failure I'm dealing with now? I turned it down only a very tiny bit. So little that it was hard to make such a subtle adjustment.

The Trinitron tube has a "unified" electron gun, it's more marketing than anything else, but in a nutshell it uses the same focus and accelerating electrodes for all three cathodes. There are still three separate guns in effect, just bundled into one tidy assembly. Electrically it is no different than the three separate gun assemblies used in other CRTs.

You will not damage anything by disconnecting a cathode, the accelerating anodes, grids, focus, etc will still be active.

OK, thanks. I will have a look soon if I can desolder the blue gun and see if the screen still flashes blue. But first, I will look up all the parts after the (blue) cathode driver transistor and use a strong magnifier glass to look for broken solder joins and such.

I have the monitor running now without the blue gun connected. The fault persists. When I turned it on, the screen flashed blue on and off like a stroboscope. I was able to reproduce it by tapping the neckboard. Now that it's warm, the fault is gone again.

There is one difference. Before, the screen would contract and expand and eventually turn off when it did this, probably because the CRT driver (an LM2413 BTW) was shorted to ground through the blue cathode.

Anyway, the fault seems to be in the tube, in the blue gun. I will try the shaking method a bit more, and hopefully I don't have to take any drastic measures. BTW, I read somewhere in one of the sci.repair faqs that you have to tap the neck quite hard: "not enough to break it, but close". I don't really dare to do that...

Whoops, I meant, with the blue gun disconnected...

Yep, sounds like it really is the tube. If it's a heater-cathode short, you could isolate the heater, powering it with 2 or 3 turns of wire around the flyback core. Careful not to overdo it, it really only takes a couple of turns on these high frequency transformers.

Indeed, I read about that. The downside is, that image quality will suffer, because the cathode will/can be in contact with those wires, which have stray capacitance, and noise pickup, etc.

Additionally, I don't know if the Trinitrons feature of automatic color calibration touches the filament current, but if so, that will render that feature useless, and it's a feature I love about this monitor. Do you know if it does so?

But I will try the shake/tap method a bit more first. The short is every so slight, that even after 20 seconds of warming up, the fault is almost completely gone and hardly reproduceable. I wonder if it also does it when it's upside-down...

I also read about those, but didn't investigate. It's basically the same idea as winding an extra turn on the flyback, right? Do you simply place it between the original filament power supply+gnd and filament? How does it affect image quality, also taking into account the eventual possible situation that the blue cathode will be in contact with the heater permanently?

why not use a isolation xformer for the heaters?

One more question about them: the cathode signal is not DC, so won't current from the cathode driver still find it's way to ground through the transformer?

No, it doesn't work like that, the transformer isolates it. If the primary was dead shorted, it might have some effect on it, but the video signal is in the MHz range, the transformer will be around 30KHz.

As a last(distasteful)resort you could discharge a cap (~1000mfd 10-50volt)between pins with an intermittent short in the blue gun.

I've decided not to try this. It's dangerous as it is, but especially for heater-cathode shorts, because you often also blow out the filament.

Indeed. I just measured, and it has 5 V DC on it. No AC component exists.

How sensitive are the filaments to voltage difference? I just measured that they are powered by 5V DC. Will those two windings also give me 5V RMS?

Additionally, how do I wind around the flyback; there's a plastic assembly around it and all.

I was thinking. Can't I tap some power from some AC source with a small isolation transformer, and construct a small circuit to give me 5V DC? Or is that over engineering?

I tried the tapping a bit more, and now the red gun also has this problem... I didn't tap that hard I think (from the description "not enough to break it, but close" I would think I need to tap a lot harder).

Now my question is: are those cathodes/filaments so fragile that I could have damaged them, or is it possible I dislodged dirt which now also affects the red gun?

The filaments were cold BTW, when I tried this. I suppose warm filaments are more easily damaged.