NotTaR of Television Sets : Channel tuning drifts as set warms up   
 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.

     << Loss of Channel after War.. |  Index  | Noise in picture and soun.. >>

Channel tuning drifts as set warms up

This may be a slight drift - like someone is messing with the fine tuning or such a substantial change in tuning frequency that the channels go by as though you are surfing.

Possible causes depend on tuner type:

  1. Quartz tuner (10 button direct access digital synthesizer) - For a slight drift, a component is probably changing value, possibly the crystal in the reference oscillator. For gross changes - flipping through channels - it is more likely to be a digital control problem - the microcontroller is misdirecting the synthesizer to change frequency.

  2. Varactor tuner (buttons but not direct channel access) - If only a single pushbutton selection is the problem, the the varactor tuning diode for that button is probably changing capacitance. If all channels in a band (Vl, Vh, U) are having a problem, it is more likely to be a drifting D/A or faulty AFT (Automatic Fine Tuning) circuit or power supply.

  3. Turret or switch tuner (Knobs) - A component like a capacitor is changing value.

You will have to get in there with a heat gun or cold spray and track it down the old fashioned way. At least, the problem is almost certainly localized to the tuner box (and possibly the controller if applicable).

As noted, gradual slight changes in tuning are likely due to frequency determining components drifting.

Uncontrolled channel surfing is probably a logic problem. For the quartz tuner, this could still be marginal connections causing the microprocessor to misdirect the synthesizer to change channels.

For the latter case, particularly, the cause may still be bad connections resulting in loss of channel memory and/or erratic behavior.

 <<Loss of Channel after War.. | ToC | Noise in picture and soun..>>