While switching power supplies provide the conventional DC voltage levels needed for PC and peripheral operation, they are not well suited for high-voltage applications. In order to power specialized devices such as CRTs and LCD backlights, the ordinary power supply is supplemented by high-voltage supply circuits which can turn relatively low voltages into high voltages that range anywhere from several hundred volts to tens of thousands of volts - depending on the particular need. This chapter illustrates the operation and troubleshooting approaches two important high-voltage circuits; the backlight supply, and the CRT flyback supply.

Today's notebook and sub-notebook LCDs are almost always based on a transmissive light design - that is, the light you see from the display is generated entirely from behind the LCD by a backlight assembly. Whatever light emanates from the display is interpreted as being transparent (or colored). Light that is absorbed by energized liquid crystal material appears opaque. In order to run a CCFT (cold cathode fluorescent tube) or EL (electroluminescent) backlight, a source of several hundred volts is needed (often 200 volts or more). Since the battery pack in a mobile computer is certainly not capable of sourcing that much voltage, it must be created "on-the-fly". If you remove the front housing from an LCD panel, you can locate the backlight supply right next to the LCD as shown in Fig. 43-1.

The key to a backlight power supply is the principle of inversion - converting ("chopping") DC into an AC signal. A simple inverter circuit is shown in the illustration of Fig. 43-2. DC from the battery pack is fed to an oscillator. The oscillator chops the DC into low-voltage pulsating DC. In turn, the pulsating DC is applied across a small, high-ratio step-up transformer which multiplies the pulsating DC into a rough AC signal. This high-voltage AC signal can then be used to run a cold-cathode fluorescent tube (CCFT) or electroluminescent (EL) backlight. As you might notice, the conversion of DC into AC is virtually opposite of the process used in linear or switching power supplies (thus the term "inverter") where a relatively high AC voltage is transformed into a low DC voltage(s). If DC is required from the inverter rather than AC, there will be a subsequent rectifier and filter after the transformer output.

Backlight problems usually manifest themselves in the LCD itself. Without proper backlighting, the contrast and brightness of a display will be extremely poor. The display may appear clearly in strong daylight, but may disappear in low light or darkness. When backlight problems occur, you should investigate your inverter supply as well as the particular mechanism (i.e. CCFT or EL panel) producing the light.

Symptom 43-1. The backlight appears inoperative. The LCD may seem washed out or invisible in low light. Remember that virtually all notebook and sub-notebook computers are designed to shut down the backlight after some period of inactivity regardless of whether the system is being powered by battery or line voltage. Backlights such as CCFTs and EL panels do not last forever, so disabling the backlight not only saves power during battery operation, but saves the backlight itself. If the backlight cuts out suddenly, it may simply have timed out. Try pressing a key or moving a mouse to restore backlight power. You can usually select the backlight timeout period through the system setup software.

Disassemble the display portion of your display to expose the inverter board (typically located behind or next to the LCD). Apply power to the system, then use your multimeter to measure the inverter's DC input voltage. Input voltage usually runs anywhere from 6 to 32Vdc depending on your particular system and backlight type. In any case, you would expect to measure a strong, steady DC voltage. If input voltage is low or absent, there may be a faulty connection to the system motherboard.

Next, use your multimeter to measure the inverter's AC output voltage. Fluorescent tubes and electroluminescent panels typically require 200 to 600Vac for starting and running illumination. WARNING: the insulation of ordinary test leads may break down measuring voltages over 600 volts. If you will be measuring voltages over 500 or 600 volts, be sure to use better-insulated test probes. If output voltage is low or absent, the inverter circuit is probably defective. You may simply replace the inverter circuit outright, or attempt to troubleshoot the inverter to the component level. If output voltage measures an acceptable level, your inverter board is probably working correctly - the trouble may exist in the light source itself. For example, a CCFT may have failed, or an EL panel may be damaged. Try replacing the suspect light source.

If you elect to try troubleshooting the inverter board itself, you may see from Fig. 43-2 that there is little to fail. Remove all power from the computer and check the oscillator transistors. A faulty transistor can stop your inverter from oscillating, so no AC voltage will be produced. Replace any defective transistors. Beyond faulty transistors, inspect any electrolytic capacitors on the inverter board. A shorted or open tantalum or aluminum electrolytic capacitor may prevent the oscillator from functioning. The transformer may also fail, but they are often specialized components that are difficult to find replacements for. If you are unable to locate any obvious component failures, go ahead and replace the inverter board.

High-voltage is perhaps the most critical (and dangerous) aspect of any computer monitor. A CRT requires an extremely high potential to accelerate an electron beam from the cathode to its phosphor-coated face (easily a distance of 12" to 14" or more). To accomplish this feat of physics, a positive voltage of 15000 to 30000Vdc is applied to the CRT’s face through a connection known as the anode. It is easy to identify the anode connection - it is underneath the thick red rubber cap on the upper right corner of the CRT (with the neck toward you). Fortunately, the high-voltage system is relatively easy to understand.

A typical high-voltage system is illustrated in the schematic fragment of Fig. 43-3. As you probably see, there is really only one critical part - the flyback transformer (or FBT) marked T302. The horizontal output transistor (Q302) generate high-current pulses that control the horizontal deflection yoke (H-DY). Horizontal output signals are also fed to the FBT which boosts the signal to its final high-voltage level. The lower primary winding is connected back to the video circuit and acts as an "error amplifier". This allows the high-voltage level to vary as contrast and brightness are adjusted. The FBT assembly produces three outputs. The high-voltage output is connected directly to the CRT anode through a well-insulated, high-voltage cable. A supplemental output of several hundred volts feeds a small voltage divider network consisting of two potentiometers and a fixed resistor. The top potentiometer controls the higher voltages and is used to drive the CRT's focus electrode(s). The second potentiometer drives the CRT's screen electrode(s).

The voltages generated by a FBT are all pulsating DC signals. Since all transformers work with AC rather than DC, there is always a question of how DC is produced by an AC device. The answer is in the small diode located between the top and middle coils of the FBT secondary. This diode forms a half-wave rectifier built right into the FBT. Pulsating high-voltage is smoothed by the characteristic capacitance of the CRT. The pulsating focus and screen voltages are smoothed by filtering components located on the video board attached to the CRT.

The loss of high-voltage can manifest itself in several ways depending on exactly where the fault occurs, but in virtually all cases, the screen image and raster will be disturbed or disappear completely. Whenever a screen image disappears, you should first suspect a fault in the conventional power supply. By measuring each available output with a multimeter, you can often determine whether the problem is inside or outside of the power supply. When one or more conventional power output levels appear low or absent, concentrate your troubleshooting on the conventional supply. When all outputs appear normal, the problem is likely in the high-voltage system. Refer to Fig. 43-3.

Symptom 43-2. Anode high-voltage measures very low - there is no raster and no picture, or there is a vertical line against the raster. The power LED appears steadily lit, and all conventional power supply outputs measure correctly. Make sure that the display contrast and brightness controls are set to acceptable levels. Also check that the video signal cable from the video adapter is connected properly. Use your oscilloscope and measure the horizontal pulse at the collector of the horizontal output transistor (Q302). If the pulse is present, the fault is likely in the flyback transformer assembly. Try replacing the FBT. Keep in mind that the FBT is a critical part and must be replaced with an identical part. If the problem persists, suspect the CRT. Use a CRT analyzer/rejuvenator (if possible) to check the CRT. If the CRT checks bad or the equipment is not available, try replacing the CRT. Note that the CRT is often the most expensive part of a monitor. Before replacing the CRT, you should carefully weigh the cost of another CRT against the cost of another monitor.

If the horizontal pulse is missing from the collector, use your oscilloscope to check the input at the base of Q302. You should find approximately a 5 volt pulse. If the pulse is present, Q302 has failed and should be replaced with an exact replacement part. If the pulse is missing from the base, check the collector of the horizontal switching transistor (Q301). If the pulse is present at the collector of Q301 but missing from the base of Q302, the fault is likely in the horizontal output transformer (T303). Try replacing T303. If you find the pulse signal missing from the collector of Q301, check for the pulse at the base of Q301. If the pulse is present, Q301 is defective and should be replaced. If the pulse is still missing from the base of Q301, the problem is likely in the horizontal output controller. Try replacing the horizontal controller IC. If you are unable to follow trace the circuit or locate the defect, you can simply replace the main monitor PC board.

That’s all for Chapter 43. Be sure to review the glossary and chapter questions on the accompanying CD (there are no Internet references for this chapter).