What was the purpose of the small button on tube television receivers?

[Woodology]

Summary: I had an old dial tune TV set. Obsolete now.

It had a small button that I think was suppose to improve reception. What did that button do physically?

it was used to set the brightness

 

[Charlie Cheap]

From 1968 to 1988 I was a TV repairman...now called a Home Entertainment Electronics Technician...Big Deal! The button was most likely a built-in degauser to remove any magnetic distortion of color on the screen. Just moving the TV from one wall to another could change the magnetic field requiring degausing (I forget proper spelling). We had a magnetic loop about 12" diameter we moved around in front of the screen as we slowly backed away. Color TV made Black & White images by mixing proper amounts of Red - Blue- Green for what was called a proper grey scale. Any thing with an electric motor could magnetize the screen and most had a built-in degauser that worked each time the TV was turned on. Shutting off a vacuum cleaner in front of the TV could magnetize the shadow mask just behind the CRT face. It was fun when the TV had rabbit ears and I moved my hand like magic around the antenna to block the sync-pulse...causing the picture to roll or flip. Then moving my hand away caused things to lock back in sync. To kids it was magic. I graduated from Elkins Institute in Dallas where Rush Limbaugh went. He studied Radio Broadcasting "D J" while I studied repair. He is worth many millions and I have a few bucks in the bank. Hind sight is definitely 20/20.

 

[dlgoff]

Paging @davenn and @dlgoff

What you said:

As long as the TV was still turned on, that would help to keep the TV tuned right on the frequency of that chosen channel. If you wanted to change channels or if you turned the TV off and later turned it back on, you would need to do the fine-tune knob (with AFC off) again first, and then re-engage AFC. But that's just my educated guess, unless somebody can remember better or find an old TV manual...

 

[Charlie Cheap]

There was an AFC (Automatic Frequency Control) button that locked the channel to a pre-set frequency. There was also a pre-set 'picture' button that locked brightness, contrast, and color intensity to a pre-set level. Before PLL (Phase Lock Loop) things tended to drift, then very compact IC's (Multi-pin Integrated Circuits) came along and we had to adjust to making our body stay at the same electrical potential as the circuit. That required clipping a ground to our wrist and the chassis...which went contrary to previous safety precautions for electrical work. Body voltage, YES...ever walk across a carpet and get a shock when touching something metal? That could knock out an IC. Some could bias on with a 10th of a volt or even less. Some came plugged into a foam block that kept every pin separate from the others to avoid cross-pin voltage before installation. You electronics guys already know this and I got out of the business in 1988. So cut me a little slack. I know how fast things change these days.

 

[homerwho]

What you said:

This seems to be the why I never used the button. I never heard it mentioned in the manual about tuning and engaging. And tune another while disengaged and reengage.
Thank you

 

[George Jones]

I have another question. If I tuned that to channel 11 but l don’t have signal. What is displayed. I realize it’s static. But is the static the same channel to all without a broadcast?

Some the static was quite old, i.e., from 14 billion years ago.

 

[alan123hk]

To the best of my recollection, there is a pure analog automatic frequency tuning (AFT) circuit that started to be widely used in many TV sets in the around 1970s until it has been replaced by digital AFT/AFC circuit in around 2000s.

It is certainly the principle of this pure analog automatic frequency tuning circuit is also based on the feedback theory.

 

[homerwho]

"Well, which is it young feller?" (Quiz Question -- what movie is that from?)

The "Improve Reception" control was a fine-tune dial or a single push-button?

I don’t know but it’s the Duke

Some the static was quite old, i.e., from 14 billion years ago.

it appears so

 

[homerwho]

I don’t know but it’s the Duke

it appears so

Summary:: I had an old dial tune TV set. Obsolete now.

It had a small button that I think was suppose to improve reception. What did that button do physically?

It is whatever electromagnetic noise there is at the frequency of the channel. That noise will vary from channel to channel and from one location to another.

This evening I cruised YouTube and watched a show about the Big Bang and it’s plateau after 20 some days. According to the video Cosmic Mictowave Background had begun to appear. In the same show they say that artifacts in the static on those TV sets were sporadic CMB. I was watching on the tv and a did not think to figure out how to share it here. They discussed the noise with artifacts as CMB. Interesting that back then that CMB was included in a BBT discussion. The button had three positions. Thanks.

 

[homerwho]

Here we go I found the show.

 

[Tom.G]

Interesting video! Too bad it leaves so many things unanswered, even the successful experiments.

Now back to that 'rectangular button' you were asking about. Below is a short video that seems to describe it. But first an overall explanation.

The function indeed appears to be switching the Automatic Frequency Control (AFC) On and Off. The AFC functions by controlling the frequency of the local oscillator in the tuner by applying a variable voltage.

If I recall correctly, the varying voltage was obtained from the Ratio Detector used for the FM audio demodulation. A Ratio Detector has the characteristic of having a plus or minus DC offset output depending on whether the incoming IF signal is above or below its resonant frequency. The audio subcarrier is at 4.5MHz, and if the the tuner is on-frequency the IF will be at 4.5MHz.

If the tuner drifts off-frequency the audio IF will differ from the nominal 4.5MHz, the Ratio Detector will then generate a voltage, which is fed back to the tuner to shift the local oscillator frequency.

I suspect the On-Off button was needed because the tuning could only be shifted a small amount, requiring the user to get close to the correct frequency (correct tuning).



Hope this helps!

Cheers,
Tom

 

[tech99]

This is an early B/W receiver, so in the USA the sound is on a separate FM carrier, so w can obtain an AFC voltage from the ratio detector. On later receivers the sound is obtained by the inter-carrier beat, and so the sound frequency does not vary with tuning and AFC cannot be obtained in this way.

 

[homerwho]

I don’t know but it’s the Duke

it appears so

Summary:: I had an old dial tune TV set. Obsolete now.

It had a small button that I think was suppose to improve reception. What did that button do physically?

It is whatever electromagnetic noise there is at the frequency of the channel. That noise will vary from channel to channel and from one location to another.

This evening I cruised YouTube and watched a show about the Big Bang and plateau after 20 some days. According to the video Cosmic Mictowave Background had begun to appear. In the same show they say that artifacts in the static were sporadic CMB. I was watching on the tv and a did not think to figure out how to share it here. They discussed the noise with artifacts as CMB. Interesting that back then that CMB was included

Interesting video! Too bad it leaves so many things unanswered, even the successful experiments.

Now back to that 'rectangular button' you were asking about. Below is a short video that seems to describe it. But first an overall explanation.

The function indeed appears to be switching the Automatic Frequency Control (AFC) On and Off. The AFC functions by controlling the frequency of the local oscillator in the tuner by applying a variable voltage.

If I recall correctly, the varying voltage was obtained from the Ratio Detector used for the FM audio demodulation. A Ratio Detector has the characteristic of having a plus or minus DC offset output depending on whether the incoming IF signal is above or below its resonant frequency. The audio subcarrier is at 4.5MHz, and if the the tuner is on-frequency the IF will be at 4.5MHz.

If the tuner drifts off-frequency the audio IF will differ from the nominal 4.5MHz, the Ratio Detector will then generate a voltage, which is fed back to the tuner to shift the local oscillator frequency.

I suspect the On-Off button was needed because the tuning could only be shifted a small amount, requiring the user to get close to the correct frequency (correct tuning).



Hope this helps!

Cheers,
Tom

thanks

 

[tech99]

This link describes a typical colour TV receiver which uses an AFC discriminator operating at the picture IF frequency of 45.75 MHz (USA). https://www.eeeguide.com/color-television-receiver-block-diagram/