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

[homerwho]

TL;DR 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?

 

[HallsofIvy]

"Tube"? How old was it? The button is probably a "degausser". Theoretically it removed any secondary magnetic field that could build up on the screen. I don't think it was ever clear how useful that was,

 

[homerwho]

It was from the late eighties

 

[homerwho]

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?

 

[berkeman]

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?

Button or dial? It was common to have a "fine tune" dial control on those early sets, since they did not have frequency synthesizers and other ways of making very accurate frequency adjustments.

 

[homerwho]

Button or dial? It was common to have a "fine tune" dial control on those early sets, since they did not have frequency synthesizers and other ways of making very accurate frequency adjustments.

Dial. It had a button that when pressed it would latch. It did have the fine tune knobs

 

[berkeman]

Dial. It had a button that when pressed it would latch. It did have the fine tune knobs

"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?

 

[homerwho]

I don’t recognize the quote. But it is humorous.
It had the push to fine tune wheel around each dial. But it had a three position push button. Well old timer what do you think the change was. It never did didly swat. I’m interested in what it was. The Degause was a one press and done. This set had intermediate position on the button

 

[berkeman]

I don’t recognize the quote. But it is humorous.

Well old timer what do you think the change was. It never did didly swat. I’m interested in what it was.

My guess would be either a fine tune on the frequency, or a change in the AGC range.

 

[Averagesupernova]

Possibly Automatic Fine Tuning, AKA: AFT.

 

[phinds]

It never did didly swat.

I assume that's a typo, yes? Or is there some part of the country where "diddly squat" is "didly swat" ?

 

[256bits]

I never knew a television that did have a de-gausser button on the unit, and I have known a few.

 

[DrClaude]

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?

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.

 

[256bits]

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?

So you were receiving VHF signals, channels 2 -13.
Any UHF channels 14 -69?

 

[sandy stone]

I never knew a television that did have a de-gausser button on the unit, and I have known a few.

Actually, our first color TV back in the 70's had one. It was a momentary contact button, and when you pushed it, the picture would jump and wriggle, as if it was a reflection in a pond you had dropped a pebble into. It would settle down in a couple of seconds, with possibly some change in the color. IIRC, it was supposed to counteract accumulated magnetization of the chassis and picture tube.

 

[homerwho]

Actually, our first color TV back in the 70's had one. It was a momentary contact button, and when you pushed it, the picture would jump and wriggle, as if it was a reflection in a pond you had dropped a pebble into. It would settle down in a couple of seconds, with possibly some change in the color. IIRC, it was supposed to counteract accumulated magnetization of the chassis and picture tube.

As did my first trinitron computer monitor. Not sure its the switch in question

 

[Rive]

But it had a three position push button.

A picture would help.
My humble bet is that it was the VHF/UHF selector.
I can't recall what was the usual third position. Maybe the composite input?

 

[homerwho]

I'm sorry but the set went to the landfill for recycling. it had a UHF dial and fine tuner and a VHF dial with fine tuner that was much slower at adjustment. It had a volume knob. And below that was this rectangular small button. It was a Panasonic from the 80's. The difference to the picture was negligible however the physics of the reception would be interesting. on /off

 

[berkeman]

It was a Panasonic from the 80's.

A quick Google Images search on Panasonic TV 1980s turns up some possibilities. Here is the CT9010, which does indeed have an AFT button...

http://vintageelectronics.betamaxcollectors.com/panasonictelevisionsetmodelct9010.html

https://en.wikipedia.org/wiki/Automatic_frequency_control

 

[homerwho]

Yeah that's basically it. Is there a digital complement?

 

[berkeman]

Did you read the Wikipedia link? It does mention what happened when digital frequency synthesis was introduced...

 

[homerwho]

Sorry if I got ahead of myself. I didn't see the Wiki link. Thank you. Is what I needed to read. Thanks

 

[berkeman]

I didn't see the Wiki link.

That's my bad. The link was kind of hidden at the end of my post after the picture.

 

[homerwho]

If you can I seem to have narrowed it down a lot. on topic now. Is it a tracking function of some kind? Thanks for the kind correction on the wiki

 

[berkeman]

Is it a tracking function of some kind?

That is my impression, although I've never designed that particular kind of circuit (I've designed PLLs, which are a related type of circuit though).

My impression from the Wikipedia article is that the feature is used to help stabilize the tuner's lock-in to a channel once is is tuned well. It helps to prevent drift of those old tuner circuits with temperature and time as you stay tuned to the same channel. So I'm guessing (somebody find the old manual please?) that you would change the channel knob to the channel you wanted, turn the Fine Tuning knob to get centered on the broadcast signal, and then turn on the AFT/AFC.

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...

In radio equipment, Automatic Frequency Control (AFC), also called Automatic Fine Tuning (AFT), is a method or circuit to automatically keep a resonant circuit tuned to the frequency of an incoming radio signal. It is primarily used in radio receivers to keep the receiver tuned to the frequency of the desired station.

 

[berkeman]

It never did didly swat

And maybe this is why it didn't seem to do much. If you try to use it to improve the picture right away, that's not what it was for. It was to keep the picture tuned well as you watched the same channel for a while (especially right after you turned the TV on and the tuner circuit wasn't warmed up yet).

 

[homerwho]

That is my impression, although I've never designed that particular kind of circuit (I've designed PLLs, which are a related type of circuit though).

My impression from the Wikipedia article is that the feature is used to help stabilize the tuner's lock-in to a channel once is is tuned well. It helps to prevent drift of those old tuner circuits with temperature and time as you stay tuned to the same channel. So I'm guessing (somebody find the old manual please?) that you would change the channel knob to the channel you wanted, turn the Fine Tuning knob to get centered on the broadcast signal, and then turn on the AFT/AFC.

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...

 

[homerwho]

I really appreciate the dialog. It wasn't;t explained to enable after a fine tune. It's a bit nostalgic to ask what did that 30 year old tv button do. But shoot the people that knew maybe thinning.. Thank you

 

[Averagesupernova]

I have to dig in some old television/video books but I suspect the AFT was a form of PLL. Probably compared the IF signal against a 'standard' xtal oscillator in the TV set. I won't say any more until I can verify.

 

[berkeman]

But that's just my educated guess, unless somebody can remember better or find an old TV manual...

Paging @davenn and @dlgoff

 

[Averagesupernova]

I didn't find what I wanted in my TV systems/video book but I will mention a few other things.
-
I didn't think the day would come when I would have to explain how a manual TV tuner worked but here goes:
Suppose I turn the selector to channel 4 and dial around the fine tuning control to make the picture come in nice and clear but then decide channel 4 is broadcasting crap so I turn to channel 10. Now again I need to adjust the fine tuning but alas, it turns out that channel 10 is spewing out even more crap so, back to channel 4. I will NOT need to readjust the fine tuning. Each channel has it's own tuned circuit with an adjustable element. Fine tuning one channel does not affect the fine tuning on a different channel. You are actually tuning different components.
-
Where AFT comes in is correcting for drift over the temperature range while the receiver warms up. If it is turned on it will take care of frequency drift within reasonable specs. I know that cable companies in the past have shifted the carrier frequency from 'standard' on some channels to avoid some interference. AFT will take care of this.
-
Here is a link to a tutorial in PDF form for piece of test equipment used to service older TV sets. It includes a quick test for the AFT function of a TV receiver.
http://www.google.com/url?sa=t&rct=...-%205283.pdf&usg=AOvVaw0zKLgs4OK1t5TpXx9MaTl6

 

[ParagustIS]

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?

My guess, is the button.. bypass or applies the fine tune-if it be auto or knob adjusted...

 

[berkeman]

My guess, is the button.. bypass or applies the fine tune-if it be auto or knob adjusted...

Welcome to the PF.

Did you read any of the posts in the rest of this thread? Or just the first one...

 

[ParagustIS]

Welcome to the PF.

Did you read any of the posts in the rest of this thread? Or just the first one...

Sorry i just replied to the first question. I scrolled through the posts. Didnt see anything lf bypass /engage button.. sorry if i missed it. I registered just now as i recognise a gold source. I am still to fill oit my profile.. apologies, i don't get offended.. if I am ever in the wrong. Just tell me... i like to be wrong, it the only possible condition to deliver true learning. No game of ego confirming for me. Shoot me Down at every sight... there's nothing greater. Nice to meet yall, sorry. I know this isn't the right place.. ill go intro in rhe correct section.

 

[berkeman]

No worries, we're glad to have you here. Enjoy the PF!

 

[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/