Occasionally, small conductive flakes or whiskers present since the day of
manufacture manage to make their way into a location where they short out
adjacent elements in the CRT electron guns. Symptoms may be intermittent or
only show up when the TV or monitor is cold or warm or in-between. Some
possible locations are listed below:
|NotTaR of Television Sets : Shorts in a CRT
1994-2007, Samuel M. Goldwasser. All Rights Reserved. Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied: 1. This notice is included in its entirety at the beginning. 2. There is no charge except to cover the costs of copying.
I may be contacted via the Sci.Electronics.Repair FAQ (www.repairfaq.org) Email Links Page.
<< Red, green, or blue full .. |
| Providing isolation for a.. >>
- Heater to cathode (H-K). The cathode for the affected gun will be pulled
to the heater (filament) bias voltage - most often 0 V (signal ground). In
this case, one color will be full on with retrace lines. Where the heater
is biased at some other voltage, other symptoms are possible like reduced
brightness and/or contrast for that color. This is probably the most
common location for a short to occur.
- Cathode to control grid (K-G1). Since the G1 electrodes for all the guns
are connected together, this will affect not only the color of the guilty
cathode but the others as well. The result may be a very bright overloaded
NEGATIVE picture with little, none, or messed up colors.
- Control grid to screen (G1-G2). Depending on circuitry can result in any
degree of washed out or dark picture.
- Screen to focus (G2-F). Screen (G2) and focus voltage will be the same and
the controls on the flyback will interact. Result will be a fuzzy white
raster with retrace lines and little or very low contrast picture. Symptoms
will be similar to those of a flyback with breakdown in the focus/screen
- Focus to high voltage (F-HV). High voltage will be pulled down - probably
arcing at the focus spark gaps/other protective devices. Line fuse and/or
HOT may blow. A high impedance short may only result in increased focus
voltage but this is probably unusual.
- Other locations between electron gun elements as feed wires.
Except for the high voltage to other places, the short may actually be located
in the CRT SOCKET or even on the CRT neck board, probably in the spark
gap(s) for the problem pins. Remove the socket and test between the suspect
pins on the CRT itself. If the CRT itself is fine, the spark gaps should be
inspected and cleaned/repaired and/or components replaced. At this point, the
cause may still be present - a short inside the flyback for example resulting
in excessive voltage on one or more pins.
Assuming this is not the case, replacing the CRT may be the best solution
but there are a variety of 'techniques' that can often be used to salvage a
TV that would otherwise end up in the dump since replacing a CRT is
rarely cost effective:
- Isolation - this will usually work for H-K shorts as long as only one gun
- Blowing out the short with a capacitor - depending on what is causing the
short, this may be successful but will require some experimentation.
- Placing the CRT (TV or monitor) face down on a soft blanket and GENTLY tapping the neck to dislodge the contamination. Depending on the location
of the short, one side or the other might be better as well. Sometimes,
this can be done in-place while watching the picture.
A combination of (2) and (3) may be required for intermittent shorts which
don't appear until under power. See the sections below for additional
details. However, for shorts involving the focus and high voltage elements,
even a sharp edge can result in arcing even if there is no actual short.
There is no remedy for these types of faults.
<<Red, green, or blue full .. |
| Providing isolation for a..>>