Vertical Hold

So whatever happened to the vertical hold control on televisions? It’s been many a year since I’ve seen one, and I have pangs of nostalgia for the scrolling screen.

And since modern TV’s don’t have vertical control problems, what the heck ever made them necessary in the first place?

They’re still “necessary,” it’s just that modern TVs do the adjusting automagically.

In order for the picture to be vertically stable (no rolling) and horizontally stable (no tearing), circuits inside the set called oscillators had to be set to a particular frequence depending on the station you were trying to receive. That meant every time you changed the station, the set’s oscillator had to change along with it. This was usually accomplished by mechanically “ganging” the oscillator setting together with the channel setting. By rotating the channel sdelector, you were simultaneously selecting the desired station as well as setting the local oscillator to the correct frequency necessary to keep the picture steady.

This is now all done with magic. Chips inside the set know what the correct local oscillator frequency is supposed to be for every channel. They set & monitor the frequency to prevent it from drifting away. Drift was a problem with old analog oscillators- you would set the correct frequency (by adjustments to that wheel that was beneath the channel selector (remember that funky dial you had to simultaneously push in while turning?) and then a moment later (usually when you got back to your chair) the picture would slowly begin to roll/tear again.

The general term for a self-correcting circuit that holds oscillators at a certain frequency is phase locked loop, or PLL for short. Just about anything that can be said to have any kind of tuner in it makes use of a PLL circuit.

Thanks, Attrayant. Mind if I call you Magnavox from now on?

Hehehe. We used to call it Maggot Box when I worked in repairs.

“Professor” will do nicely.

Damn Attryant - you sure have some good gouge on old TV. Back in B&W days I was able to adjust the tuning by hitting the set, or even stamping on the floor. Whay can’t I do that any more? I miss being able to hit something with effect.

Part of the problem was that the line and frame oscillators were made from discrete components, that is plain old resistors, transistors(or valves in older sets), capacitors.These components especially as they age are affected, their design values change and the oscillation frequency changes.
Old valve tv sets were worse because there was a lot of heat generated which ages components and the valves themselves degrade over time.

The idea was that they provided the frequencies needed for those functions but were also given a correcting ‘kick’ by components of the recieved signal.

Think of it like a weight on a long spring, you pull the weight downwards and let go, it bobs up and down at a particular speed, but it won’t keep going for ever so you(the oscillator) give it a little push at the end of each bounce.
Now imagine that over time the spring suffers a little bit of metal fatigue so that it bounces at a differant speed, you keep putting some energy into the system and it keeps it correct.You get your timing for this from the recieved signal.Eventually the springs properties change so much that you cannot keep up the same speed, so you adjust the length of the spring.

If the input of energy is not great enough to correct the the oscillation it will just free run at some other speed.Hence the rolling and tearing of the image.

That control is adjusting the circuit to compensate for changes in circuit conditions.

Modern tv sets have oscillators that use small pieces of quartz crystal in them. These are incredibly accurate and stable, their properties to all intents and purposes just do not change.

The local oscillator is not responsible for line and frame control.Not in a direct sense anyway.

Your aerial picks up every tv signal at once and passes it down the cable to the tv tuner.

Each station has a differant frequency and when the whole bunch is recieved it is mixed with another signal, the local oscillator, to produce a harmonic signal which is further processed.

This harmonic is fixed in frequency, what you do is to change the frequency of the local oscillator every time you change the station.This produces what is called an I.F stage, intermediate frequency.This is what allows the tv to separate one station from another.

This process is called heterodyning, or super heterodyning as there is usually more than one stage of frequency mixing.

This is GREAT ! I’ll see you folks at The Big SolderFest :smiley:

Thank you for this info, I get asked this stuff too on set sometimes. I used to more, before I moved to A TFT Monitor


Because back the B&W days there were a whole lot more mechaical contacts that could oxidize, and because of he heat generated by the tubes, cold solder joints were common (heating & cooling causes solder joints to expand & contract, eventually “fracturing” them). Tubes usually plugged into sockets, and the contacts would eventually oxidize causing intermittent problems that a swift kick on the chassis might cure if the vibration was strong enough to work the tube’s pins in the socket, possibly scraping through the oxidation.

The same thing happened with the monterous mechanical tuning assemblies, which were essentially a massive collection of contact switches.

And Dave has cleaned up my mess again as usual. When I looked at my post half way through typing it I decided that I had covered PLL operation well enough that I didn’t need to go all the way to the sync seperator where the actual frame control is handled. “Who would know?”, I figured.

[sub]:Makes mental note to keep an eye on casdave:[/sub]


Way, way long ago, before I had any EE degrees, I used to wonder whether each channel’s “funky dial” setting was independent, or if adjusting the “funky dial” was a global adjustment. (Of course, I din’t think “global adjustment”, I thought “does it change all the channels, or just the one?”. I always thought it probably adjusts them all, but it seemed that it adjusted them each separately. There might be only one channel that wasn’t set right, and I never messed up another channel by fixing it. Here I am now, and I still don’t know, and haven’t even wondered about this for decades. So which is it?

It really depends how old the TV was. In the really old days the tuners had coils wrapped for each channel right in the tuner itself. They had what was called turret type tuners that came next. The turrets snapped out for cleaning etc.They tuned with a slug in a coil.
They were infamous for being noisy. Sometimes walking across the floor would be enough to cause them to act up.

Then came the wafer switch type. No adjustable coils. IIRC they had tunable capacitors. Not quite as noisy.

Ah yes the old days. Some guys would buy a book,a bunch of tubes,and go into TV repair buisness.

Heat was the problem with the old sets. It took heat to make a tube operate and that heat would cause electronic components to break down.

Goodness Gracious. Next you’ll be asking me for part numbers for 5AU3 amplifier tubes or something.

Not having any schematics to refer to right at hand, I would have to say that I believe the fine tuning “funky dial” adjustment was a single trimming capacitor that stayed in one place. Even though as you turned the dial to a different channel & the switch contacts would be made to a different set of tuning elements, the fine tuning capacitor worked for whichever set of tuning elements happened to be in place at the time.

With the setting of the trimmer capacitor being correct for, say, channel 5, it would probably not be right as soon as you moved up to channel 7. That means you had to fiddle with it each time you changed channels if the signal was particularly weak. Once you got the tuning right (or at least close), you were supposed to press in the AFC button and the PLL circuit (primitive as it was in those days) would kick in to keep the setting locked.

Maybe casdave will stop by and put in his 2 Hz. :slight_smile:

Yeah, we used to call it “percussive maintenance.” Here’s a true story: My honorary uncle, Stan Lebar, who I met when I was an adult living in Baltimore, Maryland, had an odd trophy on top of his TV set in the family room. (Uncle Stan went to electrical engineering school with my dad at the University of Missouri.) While visiting one day in the early 70’s, I realized with a shock that the trophy was an honest-to-God technical Emmy. Naturally I asked how he got it. The answer: he was working for Westinghouse when the Apollo missions were in progress. The first color TV cameras to be taken to the moon were Westinghouses (weird; about THREE other companies made better cameras). One of the astronauts inadvertently panned across the sun, causing the tubes inside the camera to “flare out.” In the Space Center, in Houston, Chris Craft sends for the TV camera expert on call–Uncle Stan, right–and asks “How should they repair it?”

On the MOON?!? In a vacuum?!? A couple hundred thousand miles from the earth? With no repair tools? Uncle Stan shrugged, said “Tell 'em to hit it on the nameplate!” Which is what they did (and for reasons having to do with how easy it is to break a weak static charge), it worked! So he got the Emmy…

I may have mis-remembered the name of the manufacturer, but the rest of the details are accurate.

I just want to say you people rock! Of course, you’re all unemployed now, but at least that now-useless 1970’s TV repair experience has reduced my ignorance. Thanks! And if anyone else has stupid old-TV questions, keep 'em coming, because I’m oddly fascinated.

Eeeh! when I were a wee lad.

T.R.F sets blimey not seen any of those around for a while, not since the ‘Ally Pally’ days (see if anyone gets that referance)

One reason might be that the ratio of local oscillator to incoming rf signal is not linear so that the I.F varies slightly.

Usually the local oscillator was set to be bang on at the mid range tuning position andadjustable padding cpacitors were used to bring the local oscillator more in line.

If you looked at the response curve of the first IF stage it would resemble a very flat S laid on its side.

In days of yore when things were rather more hit and miss the first component of a signal to be lost was the vertical synchronisation pulses and you could tweak the hold controls to almost get it right with a very slow rolling.

If the vert synch pulses varied, in recieved strength, from station to station then its possible that you would need a differant setting on the hold controls for each.

Also going back that far you have to remember we were using a much lower frequency band for tv broadcasts and transmission phase changes affected these signals much more.The differances in physical distances and weather conditions between transmitter and user would be more noticeable.

Anyone around when they used to use tiny batteries too derive the bias on the old valve sets ?