MAG DX715T Problem

[Bombastik]

Greets

I own a now bit-old MAG Innovision DX715T (17", Mitsubishi DiamondTron tube) CRT.

I'm quite content with it, but I'm having a very enerving problem with it.

Quite frenquently (rather more always), after 1 minute it is on, I hear a "click" on the backside of the CRT neck and the image shrinks to a thin vertical line.

IF I turn it off then on, most of the times (90%) it immediatly does the same, but the other 10% of the times it turns on OK.

It remains OK for a random period of time (very short actually) or until shake it or pound the table were it sits; it will again 'click' and shrink to a vertical-line.

If I keep doing this for, oh, about 15 minutes, it will eventually STOP having this behaviour and work fine (even if I pound hard on screen face).

So there is this 1 to 16 minutes period were I'm allways strugling with it and turning it on and off.

Obviously it seems there something "kaput" in the Horizontal scanning circutry that is malfunctionin while it is not properly "warmed-up" (I doubt a valve tube is the problem ).

Any of you CRT techies have a clue?

Thanx.


-- I did use to have a Signature line, but it was to lame, so I removed it and replaced by this crap --

 

[HammerBot]

Horizontal deflection faults are the most common CRT and TV problems. Most of the time it is caused by a defective drive transistor for the horizontal deflection coil. The periodic shutdown you experience is probably due to some internal current limiting and restart circuit.
Although you monitor appers to respond to physical abbuse, I doubt your problem is related to a loose connection. However, I you have the guts, you can try disassembling the unit. Dust it off. One by one disconnect wire connectors, give them some cleaning spray and reinsert them.

Also, the CRT (valve tube) PCB (the circuit board mounted directly on the CRT) may have some spark gaps (many modern monitors I have disassembled don't have those). These are designed to allow excessive voltages to discharge to ground. Sometimes the spark gap looks like a small light bulb. Those are pretty reliable. However, often the spark gaps are simply a slot in the PCB. If these slots gets very dusty, sparks may jump prematurely.

<i><b>Engineering is the fine art of making what you want from things you can get</b></i>

 

[Bombastik]

Thanx for your input.

The problem seems definitly be something in a transitor or a relay (since I hear the click sound when the image "closes"... could be the tube though, since I think monitors don't use relays nowadays) on the horizontal deflection coil circuitry.

It's not exactly phisical abuse, but more the shanking or impact (on the table for example) trepidation. For instances when I hit the table or type a bit harder on the keyboard during those 15 minutes the image is constantly shaking... more link "reducing"... and most of the time it simples "closes".

After this period if still type hard it will have the same beahvior (shaking) but it will not close anymore (unless I really hit the monitor, that will do it). But after another 5 minutes it's solid as a rock (even if I hit the monitor).

I have a friend of mine that had a VERY similar problem with a second-hand 17" AOC monitor. He is more technicly expertised and he found the problem was one of the "tracks" of the PCB was lifting (i.e: coming out of the PCB). He applied a streak/path of solder on that part of the track and the problem was solved.

If it is the same problem, it could explain the "impact" trepidation simpthom.. but I can't clear out why after 15-20 minutes the problem is completely gone.

I've already disassembled the casing of the CRT and I couldn't find any defect on the PCB (altough there is a lot of dust on it); but I did noticed that without the casing (and the monitor foot stand) when I turned on the monitor the problem didn't happen.
But then again there is a metalic structure holding the entire assembly (around the PCB) and the monitor was sitting on it on the table, so it would be a lot less "trepidation" prone since the casing plust the monitor swivel stand was gone.


-- I did use to have a Signature line, but it was to lame, so I removed it and replaced by this crap --