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


"ian field" <dai.ode@xxxxxxxxxxxx> wrote in message
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"Wayne Tiffany" <wayne.tiffanyRMVJUNK@xxxxxxx> wrote in message
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"ian field" <dai.ode@xxxxxxxxxxxx> wrote in message
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"Skandalos" <uberllama@xxxxxxxx> wrote in message
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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.

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.

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



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

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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