I don't have an EE background, but I would like to understand some aspects of old TVs.

  1. When we tuned channel what exactly changed inside the tv. Was there a circuit card in the tv where something changed? What exactly changed in the circuit card? On some tvs you tuned physically, but on newer models(still old) you did it through the remote. Did you in one case change something physical?, and what did you change in the other case?

  2. How did the tv save each channel? I assume there wasn't a hard drive there, so how did it retain its tuning? And how was for instance the channel number 5 saved as channel number 5 without a hard-drive?

  • 2
    \$\begingroup\$ Depends on how old! The old ones were mechanically tuned. As the 70’s progressed, electronic tuning was introduced and phase locked loop circuits were used. Thus each channel was a number that was stored in early tech eeproms. \$\endgroup\$
    – Kartman
    Feb 18, 2021 at 2:38
  • \$\begingroup\$ @Kartman Can you please explain what changed in those mechanisms when we tuned electroniccally tuned it? \$\endgroup\$
    – user394334
    Feb 18, 2021 at 2:58
  • \$\begingroup\$ The keywords are tv and phase locked loop. Google can do a much better job than me. \$\endgroup\$
    – Kartman
    Feb 18, 2021 at 3:01
  • 1
    \$\begingroup\$ A search term turret tuner should yield images of the really old mechanical tuners that scanned through VHF channels 2-13 (North America). Inductors and capacitors tuned to different frequencies were fixed on each of the turret's members. \$\endgroup\$
    – glen_geek
    Feb 18, 2021 at 3:07

3 Answers 3

  1. Tuning with physical knobs generally physically changed the resonant tank circuit by changing the value of either the capacitor or the inductor used in it.

  2. They didn't have to save a channel, the frequencies of all the channels in a region are set by regulatory authorities (e.g. the FCC in the US).

2b. Anything that was saved would likely be saved to an eeprom, not a hard drive. Hard drives are too expensive when you only need a tiny bit of storage.

  • \$\begingroup\$ Since the TV is always plugged into the mains, SRAM also makes sense as a storage medium. \$\endgroup\$ Feb 18, 2021 at 9:56

Turret tuners were the first to store channels - they came loaded with a card and a trim screw for each channel, turning the knob swapped out the card and engaged the apporiate trim screw. each card was designed for its specific channel.

Radio-button (latching button) tuners came next they had an bank of multi-turn potentiometers one for each button. This bank of potentiometers setup was also used with some early all-electronic tuners.

After this came voltage storing chips, a type of analogue memory. a voltage representing the channel is remembered as a gate charge on a mosfet. with these where tuning was done using with up/down buttons or a single potentiometer, and channel change was momentay buttons.

After this came computerised sets where the tuning is a number stored in an NVRAM.


In old analog TVs the channel does not saved like our days but there is a variable resistor or potentiometers changed manualy with a scrow

this is a resonance circuit for the oscillator which can be a VHF or UHF enter image description here

To change the frequency we should change the inductor or the capacitor or by varieng the voltage trough the varicap circuit like this simplified example enter image description here

the channel can be viewed by adjusting the resistors of the desired frequency:

close the swithe of p1=30% for channel 1

close the swithe of p2=50% for channel 2

close the swithe of p3=80% for channel 3

the capacitor of 200pf is connected with the oscillator of the tuner

this is a real circuit diagram of philips tunet TV enter image description here


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