TV Tech Basics: Oscillators vs. Multivibrators
 

I hope I'm posting this in the right place.

We have a lot of really experienced people on this forum, many of whom have probably an enormous amount of knowledge about the circuits and technology inside these old TVs. I am not one of these people. While I worked as a kid in a TV repair shop, most of my experience was practical and I didn't really learn any theory or much troubleshooting beyond "if there's half a picture, replace one of the output transistors on the vertical module" type stuff.

Now that I have picked up the soldering iron again and have started collecting some vintage sets that I find interesting, I want to learn more about what's inside. I've got several of the books recommended here on VK and been reading them, and learning quite a bit, but there are still some gaps and questions.

In particular I'm curious about the difference between the use of an oscillator or multivibrator circuit in different sets (talking about the vertical section in particular, since that's what I'm stuck on right now). Why choose one over the other? Why is there sometimes a blocking transformer and sometimes not? And how do you get from a sine wave produced by an oscillator to the sawtooth-ish thing needed to drive vertical sweep?

Would one of you smart guys mind doing a brief discourse?

Hi Alan,

An oscillator is normally a gain element (amplifier) with some positive feedback that only feeds the frequency required. The output is normally a sine wave but it can provide other wave shapes.

A multivibrator is normally two inverting elements ( transistors or valves) that are cross coupled so that a state change on one element forces an opposite state change in the other element. The cross couplings provide a little delay that determines the frequency of operation. The output is normally a square or pulse wave.

HTH

Peter

"And how do you get from a sine wave produced by an oscillator to the sawtooth-ish thing needed to drive vertical sweep?"

I stand ready to be corrected, but I believe all sweep oscillators were pulsed. The RCA horizontal synchroguide was a blocking oscillator with a tuned circuit added to produce a sine-wave component for better reaction to sync pulses.

Oscillators in general do not produce pure sine waves naturally since their amplitude increases until something in the circuit limits it, so at the limiting element, the waveform is not a pure sine wave. An exception: HP linear oscillators used a light bulb as a slow-acting feedback element to set the amplitude below the point of non-linearity. http://www.hpl.hp.com/hpjournal/pdfs...Fs/1960-04.pdf

Very true! Oscillators cover a range of different configurations. Not just sine wave types. Scanning waveforms are basically generated by charging a capacitor and rapidly discharging it once a certain voltage is reached. This process can free run but in a television the discharge is triggered by the synch pulses taken from the received signal.

Peter :yippy:

One may ask, why not do away with the oscillator and just trigger the generation of a sawtooth on each sync pulse.
Two reasons:

1) An oscillator with a limited frequency range provides a great amount of noise rejection compared to a pulse-by-pulse trigger

2) In the horizontal sweep / high voltage circuits, the oscillator does not provide the sawtooth waveform. It is generated by switching a constant voltage across an inductor. If the current is not shut off in a reasonable time, it continues to increase until limited by the flyback primary winding resistance, a sure burn-out scenario. You may recall that the first IBM PC monitors could be destroyed if powered up without sync connected - the result of design by noobs who did not think of this problem, which had been solved in TV sets for decades.

Thank you to everyone who took the time to answer, this is really useful. I'm sure I'll have more questions as I get into this growing pile of old TVs in my garage.