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Main => Monitor/Video Forum => Topic started by: castlesteve on August 14, 2008, 10:31:06 am

Title: How to bypass overvoltage protection - U2000/U5000/k7400/k7500
Post by: castlesteve on August 14, 2008, 10:31:06 am

Can anyone give me a hint on how to temporarily bypass the overvoltage protection circuit on these boards?  This circuit is identical for all of U2000, U5000, k7400 and k7500 boards.  I just need to test a theory as I believe it is stuck on and preventing the following circuit from turning on (oscillating correctly) Q703 -> transformer -> HOT -> flyback

I searched and searched and couldnt find a link to it... but i know ive read about it before somewhere.

Thanks,
Steve

Title: Re: How to bypass overvoltage protection - U2000/U5000/k7400/k7500
Post by: sarge on August 14, 2008, 06:36:14 pm

What makes you think that it's not shutting down for a reason?  Can you give a little more specific information as to what you are repairing and/or what you have already done?

Title: Re: How to bypass overvoltage protection - U2000/U5000/k7400/k7500
Post by: castlesteve on August 14, 2008, 07:03:09 pm

I've repaired dozens of these boards and have never seen this kind of failure.   Without going into detail about my history or etc, assume I've covered all the standard tests.   I have 15 years of EE experience... But would like to know the proper way to disable this feature.

Title: Re: How to bypass overvoltage protection - U2000/U5000/k7400/k7500
Post by: smartbomb2084 on August 16, 2008, 12:32:30 am

Can this even be done?  Isn't the overvoltage protection circuit part of the power supply controller UC3842A?  Pin 3 is the SENSE input and pin 2 is connected to the power supply output.  Small elctrolytics ( like C117 on a U5000 ) can cause the sensing circuit and feedback circuit to malfunction making the controller think there is a problem.