Got the Blues fixing a CRT monitor (HP A4576A)

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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.

Re: Got the Blues fixing a CRT monitor (HP A4576A)

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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.



Re: Got the Blues fixing a CRT monitor (HP A4576A)

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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



Re: Got the Blues fixing a CRT monitor (HP A4576A)

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



Re: Got the Blues fixing a CRT monitor (HP A4576A)

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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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Re: Got the Blues fixing a CRT monitor (HP A4576A)

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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.



Re: Got the Blues fixing a CRT monitor (HP A4576A)
ian field schrieb:
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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.

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