Got the Blues fixing a CRT monitor (HP A4576A)

Good thing used to be totally flawless but a couple of weeks ago the color Blue started fritzing out. When I switch the monitor on all colors are alright but after a minute or so the blue goes away. Somtimes coming back later for a couple of seconds and leaving again.

Since Ive had the same problem before with another monitor which I could repair by finding a few broken solder joints and resoldering them I also tried to repair this one. No luck so far.

On the back side of the monitor is a circuit board where obviously the colors are processed since the leads from the vga connector end up here. Tried to find the bad guy on this board using a wooden stick pushing here and there, tried to apply coolant spray, carefully first, then emptying the hole bottle, but none of this could cause the slightest flickering of the blue.

I tried to guess which way the blue signal goes and randomly resoldered lots of joints, still no luck.

Last thing I did was to compare the voltages (DC) at certain points on the ways of the color signals. First at the connectors where the fresh VGA signals come in:

Red - Red Gnd = 192mV Green - Green Gnd = 191mV Blue - Blue Gnd = 232mV

Then at the rear end of the signal processing where all the final stuff enters the tube.

Red - Gnd = 91V Green - Gnd = 92V Blue - Gnd = 123V

Im no electronics pro and so I havent the slightest idea if these numbers are of any use.

Any hints would be appreciated! It is a great monitor and other than the missing blue everything else is still in perfect condition, not bad for a 8 year old monitor which has been intensely used.

Reply to
Skandalos
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You might have to buckle down with a soldering iron & DMM and check all the resistors to their marked value - start with any high value ones first! If you have an ESR meter it wouldn't hurt to go over the caps, but fault due to dried electrolytics tend to improve as the components warm up.

Reply to
ian field

Would it make sense to swap the blue with one of the other colors to at the gun to make sure it isn't the tube that is at fault?

WT

Reply to
Wayne Tiffany

That is a valid diagnostic method - but the voltage readings given by the OP show the blue cathode isn't being driven!

Reply to
ian field

Speaking from some ignorance here, but could the voltage reading be affected by the load? And the load being controlled by what's inside the tube? I learned long ago to look for the obvious clues first - sometimes they are the hardest to see. :-)

WT

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Reply to
Wayne Tiffany

I'd check a few resistors looking for ones that have gone high in value before cross linking the CRT guns in case the CRT has internal shorts that damage the O/P circuit.

Reply to
ian field

ian field schrieb:

Thanks for the hints. Sounds like a hell of delicate work with those tiny SMD resistors. But shouldnt they actually react to coolant spray?

DMM = digital multi meter?

ESR = ?

So the error could be inside the tube as well? Sounds like Im going to rule that out first, if I find a way to swap the colors. The gun has its own little circuit board and there is a ribbon cable that transports the color signals from the larger circuit board that's mounted around the gun board. Just behind the connector of that ribbon cable on the gun board there are three big wire mounted (no idea if this is the correct term) resistors. Should be fairly easy to cross two of these.

As for checking the resistor values those 3 big ones are probably a good start as well.

Reply to
Skandalos

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