Shorts in a CRT

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:

• 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 divider network.
• 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:

1. Isolation - this will usually work for H-K shorts as long as only one gun is involved.
2. Blowing out the short with a capacitor - depending on what is causing the short, this may be successful but will require some experimentation.
3. 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.