CRT Repair Question
marmotking
about? I have a couple of nice Model III machines where a bright spot
remains in the middle of the screen for quite some time after turning
off the power. I'm sure this is a fairly common ailment. Any cool
web sites out there describing general CRT trouble shooting
procedures? Thanks!
marmotking
winston19842005
0619cbdb-8100-4ba7...@p10g2000prm.googlegroups.com,
"marmotking" <tr...@marmotking.com> wrote:
Ailment? This is how the old TVs worked back then!
Of course I suppose you need to define "quite some time"...
marmotking
that, and I have seen monitors with a burned in spot in the middle
because of it. So, it's not normal. I've never been good at
repairing CRT issues, but I'd like to be able to with more fluency. I
figured this might be a good place to start.
Rainer Fredrich
http://www.repairfaq.org/sam/crtfaq.htm#crtagl
"Why is there a splotch of colored light at the center of the CRT
after I kill power to my TV? Why does this not happen if the plug is
pulled instead? It seems to last for hours (well maybe minutes at
least).
(Portions of the following from a video engineer at Philips.)
A broad diffused glow (not a distinct spot in the middle of the
screen) that lasts for a few seconds to minutes is called 'afterglow'
and may be considered 'normal' for your model. The warm CRT cathodes
continue to emit electrons due to the high voltage that is still
present even though the signal circuits may have ceased to operate.
For more sharply defined spots there are two phenomena:
1. Thermal emission from a cathode that has not yet cooled off (and
this could take several minutes) gives a more or less circular spot
near the centre. It is actually 3 spots from the 3 cathodes, we at
Philips call them 'Christmas balls'.
2. Field emission from sharp whiskers on any electron gun part
gives a much sharper spot, sometimes with a moon-shaped halo around
it. Even with the filament off, there may be some electron emission
from these sharp points on the cold cathode(s) due to the strong high
voltage (HV) electric fields in the electron gun. I do not know how
likely this is or why this is so.
The shape of the spot is an inverted image of the shape of the
emitting area(s) on the electron guns cathodes.
The visibility of both effects depends in the same way on the decay
time of the high voltage (HV/EHT) on the anode.
When turned off with the remote or front panel button, you are not
actually killing AC power but are probably switching off the
deflection and signal circuits. This leaves the HV to decay over a few
minutes or longer as it is drained by the current needed to feed the
phantom spot or blob.
When you pull the plug, however, you are killing AC input and all the
voltages decay together and in particular, the video signal may be
present for long enough to keep the brightness (and beam current) up
and drain the HV quickly. Whether this actually happens depends on
many factors - often not dealt with by the designers of the set.
A proper design (who knows, yours may simply have been broken from day
1 or simply be typical of your model) would ensure that the HV is
drained quickly or that the other bias voltages on the CRT are clamped
to values that would blank the CRT once the set is off. If the problem
developed suddenly, then this circuitry may have failed. On the other
hand, if it has been gradually getting more pronounced, then the
characteristics of the CRT or other circuitry may have changed with
age.
In most sets it is left to chance whether the picture tube capacitance
will be discharged by beam current at switch-off. It may simply be due
to the behaviour of the video control IC when its supply voltage drops
that causes the cathodes to be driven to white and this may not be
formally specified by the manufacturer of the IC. Some of of the
latest sets have an explicit circuit to discharge the EHT at shutdown.
As noted in the section: "Safety guidelines" the HV charge on the CRT
capacitance can be present for a long time. A service technician
should be very aware of that before touching HV parts!
Interestingly, most sets for the Asian Pacific market have a bleeder
resistor built in that will discharge the EHT without the need for a
white flash at switch-off. These will in fact drive the beam to black
at switch-off via a negative voltage to the CRT G1 electrode. The AP
market is very sensitive to proper set behaviour, they don't like a
white flash.
In short, it all depends on the demands of the particular market, the
chance of the picture tube producing a spot/blob, and the mood of the
designer.
So, it may not be worth doing anything to 'fix' this unless the
splotch is so bright (more so than normal video and for an extended
time) that CRT phosphor damage could result. This is usually not a
problem with direct view TVs but would definitely be a concern with
high intensity projection tubes. "
Rainer Fredrich
http://www.repairfaq.org/samnew/tvfaq/tvcbspo.htm
"With really old TVs, it was almost expected that when shut off, the
raster would collapse to a spot which would then gradually fade away.
If severe enough, the result after awhile would be a permanent dark
spot in the center of the screen. Modern sets usually avoid this by
forcing the CRT to be blanked for a few seconds after power-off while
the high voltage decays or unblanked as the raster collapses to
discharge the high voltage quickly with a white flash. However,
claiming they all do this by design may be optimistic! :)
The following was in response to questions about a Sony TV but should
apply in general.
(From: JURB6006 .)
I think in this case the filter for the supply to the final video outs
has gotten to a point where is discharges faster than the tube. Sony
designs usually use fast blanking at power down, but it can only blank
while the +220 VDC line is alive. There might also be a cap between
the 220V line and the G1 biasing circuit. Also Sony's almost always
incorporate some sort of HV bleeder but if it opens up usually there
is a convergence problem. If it's not any of the above, the plot
thickens. Look for a bad diode, or even an active spot killer circuit
that has failed.
You might want to look into this, as I tell customers, when it
collapses to a line it is 480 times as bright, it won't look 480 times
as bright because the phosphor simply cant put out that much light.
I've also made comments in here on how fast a CRT will burn if the
yoke is unplugged. It's then the original 480X multiplied by 640. If
that beam is in focus that means the "drive" to the phosphor is over
300,000 X what it should be. Even if the spot is defocused to 100X
it's normal size, that is still 3,000X the current on that part of the
screen. Even if yours doesn't burn the screen, in time you might
expect a minor purity problem in that area on a fine pitch color CRT.
This is an effect known as "doming", and it happened even on some low
resolution NTSC TVs! It usually happened more at the sides, and if it
became permanent you could fix it up if you used enough stick-on
magnets, but you can't do that in the middle.
I bet the thing is a real pain to work on too, so I might consider
just never shutting it off, or a simple modification. If G1 is
grounded, it is easy. One resistor, a cap and a diode, ba-da-bing ba-
da-boom. Two caps if you like to be fastidious. There are some designs
in which this will not work. It does work 99% though, if the vidouts
are driven they will discharge the +220 VDC supply, what I do is to AC
couple the supply to the G1. It can be done with three component, but
there are enhancements I recommend to protect the CRT from excessive K-
G1 voltage. Don't want to cause a short there. Five or six components
will do it on almost anything. "
Rainer Fredrich
coil with no electrical transit. I replaced it with a good one* and
since then the video is working well. Maybe a starting point for
troubleshooting your Model III.
The designated coil was L301 as described in the Model III Service
Manual ("Video Monitor Assembly Schematic Diagram", Page 78, Schematic
Quadrant A-5).
You will find the schematic (exerpt of the manual) quickly here:
http://s8.directupload.net/file/d/1914/f4gtlo5h_pdf.htm
* my replacement air-coil was self made from coated copper wire and
molded in a little case to protect it:
Inductance = 39µH (as L301 is specified)
Inner Diameter = 4mm
Length of Coil = 5mm (approx.)
Wire Diameter = 0.09mm
Number of Turns = 110
Regards,
Rainer Fredrich
Germany
N Morrison
A search for (spotkiller tv circuit) brings up mostly patents, however
TV repair books may have more info.