Standby power supplies

The standby supply may consist of:

• A low voltage power transformer feeding one or more sets of rectifiers, filter capacitors, and possibly regulators.

  A power surge could cause the primary of the transformer to open up. There may also be a thermal fuse under the outer layers of insulation which blew either due to overheating or a power surge. However, if the primary is open, it is best to replace the transformer rather than attempting repair it.

• One or more voltage dividers connected directly to the AC line feeding rectifiers, filter capacitors, and possibly regulators.

  Open resistors and dried up capacitors are common failures since the resistors are often not rated adequately and run hot, in close proximity to the capacitors.

• A portion of the main (switchmode) power supply that runs all the time.

  Failures could be almost anything that would affect normal operation of the power supply as well as problems with the control circuitry.