Focus drift with warmup

Focus is controlled by a voltage of 2-8 kV DC usually derived from the flyback transformer and includes some resistors and capacitors. One of these could be changing value as it warms up. (assuming nothing else changes significantly as the unit warms up - e.g., the brightness does not decrease.)

Focus voltage is derived from a subset of the high voltage winding on the flyback using a resistive voltage divider which includes the focus pot. These are extremely high value resistors - 200 M ohm is common - and so leakage of any kind can reduce or increase the focus voltage. All other things being ok - i.e., the picture is otherwise fine - I would suspect this type of failure rather than the CRT.

The connection to the CRT is usually a separate wire running from the flyback or its neighborhood to the CRT neck board. Look for components in this general area. Use cold spray or a heat gun to isolate the one that is drifting. If you have access to a high voltage meter, you should be able to see the voltage change as the TV or monitor warms up - and when you cool the faulty part. If it is in the flyback, then sometimes the part with the adjustments clips off and can be repaired or cleaned. Most often, you will need to replace the flyback as a unit.

• If the optimal adjustment point of the focus control doesn't change that much but the best focus is simply not as good as it should be, the CRT is probably the problem. However, if the optimal point produces acceptable focus but it changes (and possibly moves off of one end of the adjustment knob range) as the unit warms up, the flyback or one of the components on the CRT neck board are likely drifting.

• If you have a high voltage meter, you can measure the focus voltage to determine if it is being changed by the focus pot and if it is in the ball park (2-8 kV typical). Sometimes, the part of the flyback with the focus pot can be snapped off and cleaned or parts replaced but usually you need to replace the whole unit. There may a capacitor or two on the PCB on the neck of the CRT that could have increased leakage as well thus reducing the focus voltage.

• To determine if the CRT is the problem, for sharp focus after the unit has warmed up. Power-off for an hour or so and carefully pull the CRT neck board off of the CRT. Then, power up the unit. Let it run long enough such that there would have been a detectable focus drift. Now, power-down, plug the CRT neck board back in, and power-up. Watch the image as it appears on the screen:
  • If the focus starts out fuzzy and sharpens up as the image appears and gradually becomes sharper as the CRT warms up the CRT is likely tired.

    The only catch here is that plugging the CRT neck board into the CRT results in an additional load on the flyback due to the picture beam current which heats it more as well. Thus, if the problem takes a few minutes to appear, keep the brightness turned down except to check the appearance of the picture from time to time.

    You can set the focus control for optimum when warmed up and just turn the TV on in well in advance of your favorite shows or add a user focus adjustment by drilling a hole in the plastic case for an INSULATED screwdriver or flyback focus knob extender :-). The CRT may continue to function for quite a while so this is not impending doom.

  • If the focus is relatively stable as the image appears and increases in brightness AND is about as sharp as it would be with the TV warmed up, the problem is most likely in the flyback. However, also check for bad components or decayed (tan or brown) glue on the CRT neck board. A drifting flyback will need to be replaced as it will probably get worse and fail completely. Clean the surface of the circuit board and CRT socket in the vicinity of the focus and screen terminals and traces. Contamination or just dirt and grime can easily cause problems especially on humid days since the resistance of these circuits is extremely high (100s of M ohms).

  • If the focus is relatively stable as the image appears and increases in brightness AND is similar to what it would be with the monitor cold, you have a very strange situation where some load on the high voltage power supply, perhaps, is causing a thermal problem. This would be rare.