Trinitron computer monitor fault?

[steve30]

I have a Dell P790 monitor, with a 17" Trinitron tube, dating from 1999. I've haven't used it much sine 2008 when I changed to a bigger monitor, but it has always been quite reliable.

However, I went to fix some problems with a computer yesterday, and required the use of this monitor, and it started behaving weird. The image was shrinking horizontally, the brightness was changing, and it was flashing on and off. I made a 3 minute video of the problem here:

Does anyone have any idea what the problem could be? I haven't had chance to take the monitor apart and inspect it yet. Might do that later.

Many thanks.
Stephen Coates

[T3sl4co1l]

Weeeeird!

Your problem may be similar to what I had; http://seventransistorlabs.com/Monitor/index.html

Take off the back cover and see what's inside.  Also, try to find the service manual if you can.  Likely, you'll have a main deflection board, or something like that.  The board contains voltages from 100V to 1500V, so... probe very carefully, with as insulating a stick as you can find!

The pincushion changing makes me think it's possibly a broken solder joint or SMT component near the horizontal drive (as was my case) or pincushion or S-correction circuits.  The failing intensity may be the same (i.e., video cutoff under weak horizontal conditions, to protect the tube).  Or it may be something completely unrelated (e.g., ~100V supply to the video amps going intermittent?).

Good luck!

Tim

[steve30]

Thanks for the link. I'll have a detailed look inside the monitor when I get chance.

What is the best way to discharge the capacitors in a monitor?

[T3sl4co1l]

A 10-100 ohm resistor will do fine, or a screwdriver if you're lazy... the HV you should probably leave alone, since you don't need to remove it.

Tim

[steve30]

Well, I took the monitor apart, and cleaned out 15 years worth of dust  .

Unfortunately I couldn't see anything bad inside it.

The fault still seems a little bit intermittent. And today, the tube kept turning off (with the power light blinking fast). So I switched it off and left it to cool down, and now it is back to all the flickering as shown in the video.