Understanding the CRT television regulation control circuit

Sometimes I get question from my customers where they complain the B+ of the CRT television is higher than expected.

In those cases I always refer them to the feedback circuits. In this article I am going to highlight a few things about this circuit.

First let me point out that all switch mode supply (S.M.P.S) tend to increase more and more after startup.

Therefore engineers decided to come up with a solution and that is when they introduced the regulation circuit.

Below you can see components involved in regulation in the circuit diagram below.

From the above circuit diagram of a common china made television you can see the feedback circuit marked with blue.

The components connected with the blue lines are actually the components involved with this work and therefore you should concentrate to check them whenever you experience B+ is high.

In most cases in regulating the voltage they monitor the B+ voltage whereby a sample is taken back to input thanks to opto-isolator which accomplish this without forming an electrical connection.

Basically the output voltage is controlled by controlling the on/off time of the switching transistor (S.O.T) V513.

As you can see V512 is the driver transistor and as the name implies it exactly does that, it drive the switching transistor fast or slow depending on the out voltage by taking the base of the switching transistor to the ground and vice versa.

On the secondary side we have R552, RP 551, R553 connected in series. The sample voltage is taken from RP551 (usually a 2k preset) which is used to drive the opto- isolator.

If the internal transistor of the opto-isolator short, the power supply will stop working, same to internal diode

But if any faulty occurs on this circuit which can cause the internal diode not to receive any voltage, this will cause the power supply to increase a lot and this will in turn cause some components on the primary power supply to get damaged due to high voltages generated or the set will go into shutdown depending on the model.

Note: Never turn ON CRT television without the optical coupler or one leg out of the circuit as this can cause serious damage to the television.

Thank you guys

See you in the next class.