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NotTaR of Television Sets : Fuse blows or TV blows up when sync is d..  
 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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Fuse blows or TV blows up when sync is disrupted

This is a problem which is not going to be easy to identify. One possibility is a drive problem. The messed up sync resulting from swtiching channels, or changing input connections might be resulting in an excessively long scan time for just one scan line. However, this may be enough to cause a current spike in the horizontal output circuit or an excessive voltage spike on the collector of the horizontal output transistor. Normally, the HOT current ramps up during scan. During flyback, the current is turned off. This current is normally limited and the voltage spike on the collector of the HOT is also limited by the snubber capacitors to a safe value. If scan time is too long, current continues to increase. At some point, the flyback core saturates and current goes way up. In addition, the voltage spike will be much higher - perhaps destructively so.

Troubleshooting these sorts of problems is going to be tough. However, a likely area to investigate would be:

  • Drive circuitry for the HOT including the coupling components.

  • The chip that generates takes the sync input and generates the horizontal drive signal.

  • A bad low voltage regulator might permit the B+ to rise to excessive levels during black scenes (i.e., video mute during channel changing).