NotTaR of Television Sets : Bypassing the Startup Circuit           
 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.

      << Dead TV with periodic twe.. |  Index  | Shorted Components >>

Bypassing the Startup Circuit

Where the TV is dead and a startup problem is suspected, a TV service person would test for startup problems by injecting a signal into the base of the HOT of about 15.75 kHz. If the TV then starts and runs once this signal is removed, the diagnosis is confirmed. This is risky - you can blow things up if not careful (including yourself). A 555 timer based circuit will work fine as a signal source for this.

WARNING: be careful if you do this. The HOT circuit may be line-connected and it is possible to destroy the HOT and related components if this is not done properly. I once managed to kill not only the HOT but the chopper transistor as well while working in this area. An expensive lesson.

You can reduce the risk somewhat (to the TV at least) by using a series light bulb load and/or running on reduced line voltage. The most important thing to avoid is putting in an excessively long drive pulse which will result in the flyback transformer saturating, huge amounts of current, and likely a dead HOT and possibly other parts if there is nothing to limit the current. For NTSC/PAL, it is fairly safe to assume that a 50 percent duty cycle 15 to 16 kHz drive signal will not result in fireworks as long as there aren't other problems (like a shorted flyback/LOPT).

If after a second or so, the TV fires up (not literally!) and stays happy until it is turned off, a startup problem is almost certain. It could be the standby supply (if used) or a dedicated startup circuit that has failed.

But, don't push your luck - if the TV starts after a second or so of your drive signal but doesn't continue to run when it is removed, don't be tempted to leave your circuit connected - it could still be stressing other parts. Find out why the normal horizontal drive is not being generated - possibly a power supply or horizontal oscillator problem.

If nothing happens, either startup is not the problem or there are other components that have failed preventing the HOT drive signal from having any effect.

 <<Dead TV with periodic twe.. | ToC | Shorted Components>>