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NO Tuning (Snow on screen or Blanking Color)

I normally work with a mirror in front of the TV so I can stand behind the set and see the screen.
One of the simple tests I do is the "I'm an aerial test", which means I find the IF out pin of the tuner unit, and touch it with a screwdriver tip whilst holding the blade, keeping my my other hand off the TV, (so I act like an aerial) if the IF stage is ok, then each time I tap the IF pin the screen snow changes slightly.

Most Varicap tuners need 12v supply for power and 0-30v for tuning through channels, also AGC voltage and AFC voltage and some use Band selction voltages
First check along the pins of the tuner unit looking for +12v if not there trace back along the pcb tracks back to PSU or regulator till you find it.
If +12 ok then you'll need to find the VT (tuning) pin on the tuner unit, and test for a variable voltage as the tuning bar moves across the screen.
If the voltage tracks across the range 0 to 30v ish, with an aerial connected and no sound or picture shows, then we are probably going to end up with a replacement tuner unit

If the voltage stays still then we have to look at how the tuner supply circuit works
The usual system works by using a high value resistor (12k-47k)to drop the voltage from the HT line (100v-150v) down to 30v stabilizer
This 30v is applied to the collector (via its resistor) of a transistor (low noise) and the micro proccesor sends a regular chain of pulses which can be fixed at a variable on_time , say the MicroPro sends a regular short ontime train of pulses to the base of the transistor, then the collector will be pulled down regularly to zero but only for a very short time, whilst it is mostly still up at 30v
If the collector voltage is smoothed out the average voltage would be say 28v, and if the ontime were longer then the smoothed voltage would be say 15v, and so on. So as the tuning bar moves across the screen , the micropro is sending out an increasing (some decrease) ontime pulse . This smoothed variable voltage is wired to the varicap tuner unit, via a set of resistor/ capacitors (smoothing) this alters the tuner, and so the station/program.
When a station is found the pulse ontime is stored as a digital number by the micropro , so it can remember to send out the right ontime pulses for say channel 1.

One of my tricks is to get a 100k potentiomter and connect it from the 30v line to earth, and the slider pin to the tuner TU pin (disconnected from the pcb) and tune the pot gently till a station appears

ACG Automatic gain control
This will adjust the tuner gain if the signal is too strong, but if faulty there is usually a picture of sorts, This AGC voltage is usually about 2.5v or about 9v depending on the type).
AFC/AFT Automatic Frequency Control or Automatic Fine Tuning
This again should not stop the picture but it may lock in OFF tune slightly

Band Switching NOT UK we only use bands 4/5 which are within the single range of a varicap tuner.

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