1. Troubleshooting monitors

To undertake component level repairs, test equipment, circuit diagrams and access to spare parts are all needed. However, the following points will allow a "rule-of-thumb" approach to be followed.

  • The absence of one or more of the primary colours is usually caused by broken pin(s) in the data connector. A makeshift repair is to replace the connector, but the whole cable is better replaced.
  • With a "dead" monitor, the on/off switch should first be checked (a surprisingly common fault), followed by the switched mode power supply and the fuse(s) therein.
  • If the screen is blank but the indicator light (if any) is illuminated, the likely cause is a faulty line output transformer.
  • A single horizontal line in the middle of the screen denotes collapse of the vertical oscillator/amplifier or a defect in the deflection yoke.
  • A single vertical line can also be indicative of a deflection yoke fault. Distorted images can easily be caused by interference from nearby electro-magnetic sources such as another monitor.
  • Wavy images are due to one or more inadequate supply voltages.
  • Too bright or dim displays are again due to supply voltage abnormalities (within the monitor), assuming the brightness/contrast controls are correctly adjusted.
  • Visible raster lines on the display should be eliminated by the brightness/contrast controls. If they are not, the CRT's control voltages should be checked along with the line output transformer and the horizontal output transistor.
  • A rolling or an unsynchronized display could also indicate the existence of a broken pin in the connector.
  • An imbalance of colours indicates lack of gain in one of the colour video circuits. The appropriate transistor should be replaced.
  • Raster present but no images could be due to a faulty CRT. Other CRT related problems include an out of focus display, a dim image lacking in contrast and an image saturated with green or magenta.

An oscilloscope is very useful for making many of the above checks, but great care is needed in positioning the earth return of the probe. It should not be clipped to the metalwork where the possible presence of a potential up to several hundred volts may well be sufficient to damage the oscilloscope. It should instead, be connected to the power supply's zero volt line.

As ever with component level repairs, a thorough visual inspection to uncover dry solder joints can pay dividends in monitor repairs.

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