Dell P992 19" CRT Issues

[Sleddriver]

Hello! First post here.

I've owned this monitor 9 or so years and it's performed well till recently. When cold, the display would oscillate in tilted-ovals quite rapidly when first lit, then gradually slow down as it warmed up. It would finally stabilize so I could read it after ~ 10 min. Then it began doing it after 'waking up'.

Another symptom was that it would defocus for awhile, sometimes focusing again. Sometimes not.

It would also tilt green at times intermittenly.

I replaced the Dell with another monitor to check the video card and it looked fine, so I suspect it's the Dell monitor.

I removed the case, discharged the tube and had a look around. I noticed where the HV cable had been arcing (due to dust) on the 'cage', so I dusted everything with compressed air.

This monitor is a Sony Trinitron 19CRV chassis, equivalent to a CPD-G410R, I think. Man date is Nov. 2001. So far I've seen no bulging caps. I plan on hooking the o'scope to the power rails to check for ripple.

Anyone had any similiar experiences/symptoms with a Dell-Sony like this before?

[Krankshaft]

Re: Dell P992 19" CRT Issues

Are your sure the degaussing coil isn't staying on? That can cause the display to distort but it's only supposed to fire on power up.

The current flowing through the coil also flows through the NTC thermistor heating it up as it's temperature rises so does it's resistance until the current draw from the coil is very small (not zero but certainly not enough to effect the CRTs deflection).

The NTC acts as a temperature controlled variable resistor limiting the current to the coil. Most of these are inside a little plastic case about the size of a fusible link on a car. Inside this case these devices have two thin wires welded to a metal disc if one of these cracks it can cause the coil to come on and off at any time. I have seen some NTCs without this case but most of them have cases.

If the CRT is defocusing check the focus wire from the flyback it should be soldered to the neck board it's a bit smaller than the anode lead. If the solder isn't cracked remove the neck board and check the socket for any signs of arcing.

If there are no signs of problems on the outside I'd power the monitor up and lightly tap the flyback near the focus pot with something insulated like a screwdriver to see if there is an intermittent connection on the resistive divider.

[Sleddriver]

Re: Dell P992 19" CRT Issues

At present, I'm not sure about anything. Haven't really dug into it yet. Though I have read of many problems with these monitors on Dell's site. I'm surprised mine has lasted as long as it has.

When it's acting up, it's as though all the desktop icons are doing tight 'back-flips'! They start out really fast, the gradually slow down and stop as it heats up. Very, very weird. I've seen a whole screen roll before, but this rolls in a small, tight loop. Now it will never calm down or stop. So the monitor is no longer usable.

Clearly something is oscillating. . . .

Thanks for the reply. I'll cut out some time and investigate further.

[133MHz]

Re: Dell P992 19" CRT Issues

I've fixed this one in the past, it's caused by a dried up electrolytic capacitor. I can't remember the exact details, but it's right in front of the fins of a big heatsink near one of the corners of the main board.

The heat dissipated by the heatsink slowly destroys the capacitor over the monitor's lifetime creating the thermal failure that you're experiencing. When I fixed this problem on my sister's P992, I moved the cap away from the heat source by extending its leads with a few inches of wire. That should prevent the problem from happening again in the future.

Unfortunately the monitor has now developed the infamous excessive brightness problem. Other than lowering the G2 voltage either through software or by increasing the value of a resistor, nobody seems to know the cause.

[Th3_uN1Qu3]

Re: Dell P992 19" CRT Issues

Unfortunately the monitor has now developed the infamous excessive brightness problem. Other than lowering the G2 voltage either through software or by increasing the value of a resistor, nobody seems to know the cause.

It's best to do it thru the WINDAS software, the resistor method makes the monitor unable to perform automatic calibration ("color return"). I believe that the brightness problem is caused by this very function. Every time you run it, it tends to bump the G2 a little bit.

[Sleddriver]

Re: Dell P992 19" CRT Issues

Thanks for the replies! I've been searching for an isolation transformer to protect myself & my o'scope. I found the Sony schematic and printed it out on 11 x 17 sheets. Much easier to read that way.

133Mhz, I too experienced excessive brightness problem. Then I'd hear a loud ZAP and it would be normal. I found the HV wire was arching to the alum. cage. Also cleaning out the dust and black dirt on the back of the crt helped with this.

I too suspect a bad cap. Will check all power rails for AC ripple with the o'scope.