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NotTaR of Television Sets : Typical TV power supply front end       
 Copyright © 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.
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Typical TV power supply front end

The partial schematic below is similar to those found in the majority of TVs sold in countries with 110 to 120 VAC power. Many parts are not shown including the power switch or relay, RFI bypass capacitors across the rectifier diodes, and RFI line filter. 


                                         Bypass resistor
     Line fuse   Main bridge  Fusable    +----/\/\-----+
           _      rectifier   resistor   |   +-----+   |
     H o--_ --+------|>|---+---/\/\--+---+---| REG |---+---+---o B+
              |            |         |       +-----+       |
              |  +---|>|---+     C1 _|_ Main    |         _|_ Regulator
   115 VAC    |  |           400 uF --- filter  |         --- output
              +--|---|<|---+  200 V  |  cap     |          |  capacitor
                 |         |         |          |          |
 +-> N o---------+---|<|---+---------+----------+----------+---o Non-isolated
 |                                                                 return
 +-> G - Power line earth ground via building wiring

  • The line fuse is typically 2 to 4 A, usually a normal fast blow type. Even so, it may not blow as a result of faults down the line - the fusable resistor or regulator may fail first.

  • The main bridge rectifier is often composed of 4 discrete diodes (similar to 1N400Xs) but may also be a single unit. Failures - usually shorted diodes - are common.

  • The main filter capacitor can range in size from 200 to 800 uF or more at 200 to 250 V. THIS CAN BE LETHAL! A typical TV may continue to work at normal line voltage without any noticeable degradation in performance (hum bars, hum in sound, or shutdown) even if this capacitor is reduced in value by 75%. Its uF value is therefore not critical.

  • The regulator is often an IC or hybrid module. Failures resulting in no or reduced output, or no regulation are common.

  • The regulator output capacitor is needed for the B+ regulator to function properly. If this capacitor is reduced in value or develops a high ESR, regulation may fail resulting in instability, oscillation, or excessive B+ and shutdown.

  • The regulator bypass resistor reduces the amount of current control needed of the regulator. Caution: even if the regulator has been pulled, the B+ line will have substantial voltage as a result of this resistor.
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