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Troubleshoot Shorted CRT Neckboard, now darker picture.

 
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Joachim



Joined: 04 Sep 2006
Posts: 2

PostPosted: Mon Sep 04, 2006 1:12 pm    Post subject: Troubleshoot Shorted CRT Neckboard, now darker picture. Reply with quote

Hi!

This is a rather long post, but I'm simply trying to give as much information as possible to you. Please bear with me Smile

I have an old CRT Projector, (Nordmende TVP-100). I recently adjusted the G2 voltage of the different cannons (R G and B) to adjust whitebalance (minor adjustments). After this adjustment the unit ran fine for a few days, until it suddenly shut off, and when I tried to turn it on again I saw blue sparks inside it. Something had shortened. It must have been quite a large voltage, because it was more of a hissing sound for about half a second, not a "POP", and it illuminated the inside of the Projector in a quite remarkable way.

As I opened the back cover I could see that a ribbon cable leading to the user controls that are on the back cover, actually had been jammed in between the red CRT neckboard and the back cover, and some soldered pins on the CRT neckboard had pierced the plastic cover of the ribbon to the controls, causing a short. This short was, I think, over a resistor on the neckboard. I hadn't checked for proper cable clearance when I closed the back cover after i had adjusted the white balance Sad

At this time the Projector is hanging in the ceiling, and until I'm able to get it down I can't examine it further. From what I could see there was no apparent scarring or burn of the ribbon cable or the crt neckboard. The sparks seem to have originated from somewhere deeper in the chassis, and looked to come from just beneath the red CRT. I haven't located the flyback. Could it perhaps have been the flyback sparking?

The short had also killed some segments of the control's 7-segment leds, since too much current had run through to the Control Leds.

Now, I plugged the unit back in and it powered up, but the image was darker than it used to be. I thought this had to be because the short had blown some components, affecting G2 or G1 voltages.

I raised G2 Voltage at each of the CRT neckboards to compensate, but this didn't work that well, because now they are cut off when too much white is in the picture, just as they would if they are gained too much. Which they are. The thing is the CRT's act as if they have a very high gain, but the image output is much lower. All CRT's are affected in the same way, though it was only the red crt that got shorted.

I can increase the user brightness but that doesn't seem to be the problem. that only results in a more washed out picture. It is as if the unit all of a sudden has lost maybe 50% of its span between black and white, and now produces much less contrast.

Unfortunately I haven't got a service manual for this unit, but I think the issue can at least be explained by someone more familiar with troubleshooting CRT's and voltages than I am. I just have basic electronics education and soldering skills, but combined with patience and common sense I've managed to repair and tweak a few things in my lifetime so far Smile

Please, if you have any thoughts or more questions
I would like to know what is the probable cause of the more dim picture, and where to start troubleshooting. I will assist you in any way i can with pictures and measurements if you need more information.

Any help much appreciated,
Thank You!

Edit: I have now taken down the projector from the ceiling. When I trun off all but one color (there are switches for that) i get good brightness from the single crt, but when I turn on the 2nd and 3rd color brightness goes down, as if there wasn't enough voltage to run them all. This did not happen prior to the short, then all CRT's were equally bright, no matter if i turned one of them on or off.

Edit2: The low voltage happens even with the Red CRT neckboard removed. Exactly the same symptoms.

Perhaps this information can help.
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