Spent several hours reviewing old posts, many similiar probs but didn't find this exact problem - hoping one of the knowledgeable folks here can advise the cause and whether I (with limited electronics experience, decent soldering skills, and an auto ranging multi-meter)can do something to fix it.
(Sorry for the length of this post - I'm trying to include all info that might bear on the prob.)

SYMPTOMS: After 3+ yrs of use, my son started having this problem - either when monitor powered on from AC disconnected, or when comes out of standby when computer turned on, there is a brief crackling noise (like a static discharge but louder), coming from the glass face of the CRT, accompanied by a smattering of tiny (maybe 1/32 - 1/16" diameter) white "sparks" or electrical arcing in an apparently random pattern - over time, this has left something resembling white Etch-a-sketch markings (I assume where the coating on the inside face of the glass has been burned/evaporated off) - the tracks range in width from the width of the aperture grill shadows to perhaps 2 or 3 times that, and are gently curved, typically extending almost the width (or in some cases, the height or the diagonal) of the visible part of the CRT.

SPECULATION: From reading many other threads here, I'm wondering if this might be dirt particles inside the tube or a short in the degausing wire (though none is visibly evident, and the menu triggered degauss works normally).

OBSERVATION: These marks appear to be in front of the actual phosphor dots - if I look at the text displayed from an acute angle, I can see it clearly, whereas viewed from directly in front, it is slightly obscured by the tracks. He says occasionally, the image flickers very briefly at random times (I haven't observed that at 800x600@85 Hz) - otherwise it works fine.

INVESTIGATION: I removed the outer case, blew the dust out, removed the main PCB from its metal tray, and pulled the PCB off of the yoke (where it connects to the pins coming out of the back of the tube) - there were the typical few solder joints where excess flux bridged the gap between adjacent areas, some of which were conceivably carboned up a bit - tho probably not. I always scrape the excess flux off w/ a thumb nail - probably useless but it bugs me. No evidence of any arcing anywhere on any of the PCBs or components soldered to them, nor on any of the wires - and (although I know this doesn't rule out a cold solder joint), no obviously bad (dull or cupped under at the board) solder points - all appeared to be shiny and reasonably well done (didn't unsolder the metal shielding from the board that plugs into the yoke - so it only got a partial inspection. Peeked under the rubber cap where the (anode?) big red wire goes into the CRT at the top using a wooden letter opener - clean, no evidence of arcing (I haven't ever disconnected any of the high voltage stuff on any monitor for fear I'll mess up the discharge and cook an IC or my heart - no one else to stand around with a wood bat to knock me loose if I mess up) - but I did push that connection to make sure it was tight (ditto the board attached to the yoke. The ground strapping and numerous ground wires to the chassis all were tight & looked ok.

NEC level 1 tech support (I finessed the purchase date to get to the "in warranty" phone support) was mystified; she had me hold for a minute or three to consult someone else, who didn't volunteer an explanation of the cause but did answer my second question - is it safe to use the monitor - saying yes, but the problem will continue to worsen unless repaired.

My guess is that nothing short of replacing the CRT will cure the visible and annoying etching or erosion of the inner glass/coating, but if there is an obvious cause with a cure that might be within the competence of someone like me (I can solder/unsolder from PCBs, decode resistor color stripes, know what a capacitor, transformer, IC look like, and generally how to use my multi-meter). Any and all help and comments welcomed (unsolved mysteries, particularly involving computers, which I build and can usually fix, bug me - even if I can't fix it, my curiosity eggs me on).

Thanks in advance, and thanks for hours of interesting reading about others' problems and their solution - an excellent forum),

Kendall Layne aexdri@aol.com

This may be caused by the reduction in capacitance of the flyback transformers internal EHT capacitor, this capacitor is very often used in the frequency compensation of the EHT regulator.

When this capacitor looses value the EHT voltage on start up overshoots the correct value by a huge amount sometimes by 100%, this maybe the cause of the marks in the CRT.

I personally would not continue to use it as sooner or later it is going to fail, also the unit maybe producing X-Rays during this short time during start up.

Rich B

Rich,
thank you for your clear and concise explanation of the likely cause, and also for the caution re X-rays - NEC's assurance that it was safe to continue using the monitor seems rather suspect in light of your diagnosis.

Assuming I refreshed my understanding of how to safely discharge the HV capacitors and had a second person available as a safety, is it likely that a replacement part could be had for under $60, and that other parts would not also need replacing to fix this (assuming we could live with the etching that has already occurred) - or would we just be throwing good money after bad at this point?

Kendall

Hi if you know someone who has a meter capable of mesuring capacitance you can verify the diagnosis by measuring the capacitance of the FBT cap from the high voltage lead to pin 13 of the FBT connected to C310.

FBT price's vary widely with model, model age and where you get them.

Regards

Richard