Got the Blues fixing a CRT monitor (HP A4576A)

[Skandalos]

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.

 

[ian field]

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.

 

[Wayne Tiffany]

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

 

[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

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

 

[Wayne Tiffany]

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

 

[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

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.

 

[Skandalos]

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.

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.

 

[ian field]

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.

Some people hold the opinion that SMD resistors are more likely to crack
than go high in value, I'm not sure if that's true but melting the solder at
one end of each resistor and applying a slight rocking motion to the iron
tip will quickly show any cracked resistors, only slight pressure mind - if
the solderings iffy at the other end the resistor will drop off!

Coolant spray probably won't have any effect on chip resistors, what you
could try is check some of the resistors in situ with a digital multimeter,
other components in circuit will probably cause a reading lower than the
resistor's marked value - but if one reads higher than the marking you know
it has to be a dud! Don't forget to reverse the probes and take the highest
of the 2 readings.

ESR is electrical series resistance - all electrolytic capacitors have it
but as they dry out it rises dramatically and interferes with normal circuit
function. If you are going to get into servicing (especially if you want to
work on PSUs) an ESR meter is a must have, over the years I've built a
couple of home brew ESR meters of my own design, they both worked after a
fashion but in the end I had to stump up for the Dick Smith ESR meter kit
(about £65!) - guess I should have bought one when they first came out, they
were only about £30 back then! It doesn't matter if your board has SMD
electrolytics because most ESR meters on the market are designed to test in
situ.

Its up to you and the amount of difficulty involved whether you try swapping
the gun drives, the voltage readings you quoted do not suggest a tube fault,
but I've been caught out a few times - and those times it was on the bench
in front of me so I could see all that was going on!

 

[Skandalos]

Some people hold the opinion that SMD resistors are more likely to crack
than go high in value, I'm not sure if that's true but melting the solder at
one end of each resistor and applying a slight rocking motion to the iron
tip will quickly show any cracked resistors, only slight pressure mind - if
the solderings iffy at the other end the resistor will drop off!

Ok, I'll try that.

Coolant spray probably won't have any effect on chip resistors, what you
could try is check some of the resistors in situ with a digital multimeter,
other components in circuit will probably cause a reading lower than the
resistor's marked value - but if one reads higher than the marking you know
it has to be a dud! Don't forget to reverse the probes and take the highest
of the 2 readings.

One more great hint!

ESR is electrical series resistance - all electrolytic capacitors have it

Found a wikipedia article about "equivalent series resistance". Must be
the same.

but as they dry out it rises dramatically and interferes with normal circuit
function. If you are going to get into servicing (especially if you want to
work on PSUs) an ESR meter is a must have, over the years I've built a
couple of home brew ESR meters of my own design, they both worked after a
fashion but in the end I had to stump up for the Dick Smith ESR meter kit
(about £65!) - guess I should have bought one when they first came out, they
were only about £30 back then! It doesn't matter if your board has SMD
electrolytics because most ESR meters on the market are designed to test in
situ.

Im not going to put any money into it. These old 21" CRT monitors are
worth about 100$. If I can fix it with a little effort, very well, if
not, no big problem. I already bought a pretty good Eizo T965 as a
replacement which has even better display quality.

Main reason for fixing the old HP thing isnt even the money. I just cant
think of putting a piece that's nearly like new (except for the fault)
to the garbage.

I wont get into things before next weekend, but I'll report about it then.

 

[Ohmster]

Ok, I'll try that.

It is possible, but you are likely to find that problem by gently
tapping the board while it is running, gently flexing it, or even
spraying it with freeze spray, all while it is running, to see if the
color cuts out or goes back in again.

Yeah that is correct, but the cooling might have an effect if it is due
to a bad solder or the end of the chip resistor is loose. The rapid
cooling can shrink tiny cracks enough so that the circuit will begin to
operate again, even for a moment.

One more great hint!


Found a wikipedia article about "equivalent series resistance". Must be
the same.

Yes, that is what ESR means, "equivalent series resistance", but you
will not have that issue until the thing is several years old and has
had a chance to "bake" the capacitors for several years of operation.

Im not going to put any money into it. These old 21" CRT monitors are
worth about 100$. If I can fix it with a little effort, very well, if
not, no big problem. I already bought a pretty good Eizo T965 as a
replacement which has even better display quality.

Yeah that is right. I found a nice ViewSonic 21" monitor on the side of
the road and decided to fix it at home to get a nice "free monitor" out
of the deal. Now at work, I could fix this thing, what with a bench,
proper lighting and magnification, test equipment, service literature
and computer, and parts, but at home, with only the kitchen table, some
hand tools, and a flashlight, the chances of fixing it go way down. I
found several dried and top open electrolytic capacitors, they dried
out and blew open, a burned resistor and coil on the CRT board, and the
CRT driver chip blown and cracked in half. I replaced the caps,
estimated the value of the resistors and coil as taken from another CRT
board, kept my fingers crossed, and plugged it in. It did not work, the
resistors burned again, and the CRT IC blew and cracked again. Not
worth any more time or money (Spent $10 on the IC and $2 on some caps.)
on it any more. Back to the side of the road it went, as I found it the
first time. A very big part of being an electronic technician is
knowing how to make good decisions, even if it means bailing out, like
it was prudent to do in my case.

Main reason for fixing the old HP thing isnt even the money. I just cant
think of putting a piece that's nearly like new (except for the fault)
to the garbage.

Sometimes you have to do just that.

I wont get into things before next weekend, but I'll report about it then.

Look forward to seeing the next installment on your adventures.

Paul