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Old 01-25-2012, 03:02 PM   #25
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Default Re: Dell P1110 21" CRT repair thread

Haven't kept you up to speed with the news, have i? A while back i got a P1130. It only had the excessive brightness issue, after adjusting with WinDAS everything is fine. As opposed to my P1110, it didn't do the whole "start up bright then slowly drift back to normal" thing. And by slowly i mean slowly - the darn thing took over 2 hours for brightness to stabilize, and if the computer went idle and turned the monitor off, then it would do the same thing over again. The P1130 worked normally after i adjusted the G2. Interestingly, the P1130 got a G2 voltage of almost 100 volts above the P1110 for the same brightness range, and i started to dig further.

Turns out there was a little difference in the video circuitry of the P1130 (built in 2003) as opposed to the P1110 (built in 2000). Attached you will find the relevant portion of the schematic of the P1110. Take note of pin 17 of the cutoff amp, with the funky self-biasing transistor circuit. Someone had analyzed the circuit before and it turns out it was some kind of thermal compensation, but noted that it does affect brightness. Well, compensation or not, the P1130 didn't have this, and instead connected pin 17 directly to ground. It also didn't have the brightness drift issue.

So, connecting the dots, i bypassed the drift correct thingy on the P1110. The brightness got significantly lower, i had to adjust G2 value with WinDAS again. The new G2 voltage for the P1110 was almost identical to the one the P1130 wanted. And... NO MORE BRIGHTNESS DRIFT!!!! I can finally have a proper monitor. Well, in fact, now i have two proper monitors.

Now, some might wonder what was the circuit doing there in the first place. Well, i think i got an answer for that. As you know, vacuum tubes take a while to warm up, and CRTs are no exception. A CRT monitor starts up dark and takes a few minutes to warm up and get to normal brightness. What Sony wanted to do was to eliminate this warmup period, so they put that little transistor that shifts the cutoff amp's bias point during this warmup of the tube, so the brightness starts up at nominal levels from the get go.

At least that's the theory. And it probably even worked when the monitor was brand new. But apparently the bloke who put that little transistor in there forgot to factor in the aging of the tube... and thus the circuit ends up overcompensating, causing the brightness drift i mentioned above. A monitor that takes 5 minutes to brighten up is lots better than one that starts up over bright and takes two hours to get to normal. Plus given its position on the board, i cannot possibly understand how that transistor could have ever sensed the temperature of the tube, or even of the video amp for that matter... No wonder it got removed in the following version.

Anyway, there you go. If you ever see one of those, now you know how to fix it. IIRC lti had a Sony Trinitron with the same brightness drift issue, get to work already man!
Attached Images
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Originally Posted by PeteS in CA View Post
Remember that by the time consequences of a short-sighted decision are experienced, the idiot who made the bad decision may have already been promoted or moved on to a better job at another company.
A working TV? How boring!
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