The addition of keyed agc to early TVs has been a topic here from time to time, with folks planning to build their own add-on circuits. I just noticed that there's a contemporary keyed-AGC kit on ebay just now - https://www.ebay.com/itm/224068451223. I guess it would allow someone to modify their restored TV for better performance while still keeping it period-correct.

TransVision made 630-type chassis for "custom installations", and plenty of accessories, for many years after RCA stopped using it. Was the '630 the model T of television, with plenty of other companies riding its coattails?

I've got no skin in this game at all, just intrigued by the possibility someone having, and eating, one's cake.

AGC was meant to correct for fading signals or station differences. Now we are feeding our sets with a converter signal or some other artificial source, so ACG is not really needed. That's my opinion.
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Ron

I agree. Unless a collector just wants the challenge of tweaking vintage TV's, they work fine as-is. I speak for myself when I say that although I enjoy using my vintage TV's, it's for fun. I enjoy each one's quirks and making frequent slight adjustments.
By the way, the basic 630 chassis was used in a slew of different sets into the '50's. The list would be extensive if someone jotted them all down!

+1 not really needed today.

There were electronics magazines back in the day that would discuss how to do such mods.

The 630 was one of the longer lived and more heavily licensed designs. 1946-60 so about 14 years. At the time Dumont was their biggest rival both in quality and in Licensing, but the licensing dropped off around the time Emerson bought Dumont and dumed down their chassis engineering from the finest to average.

Zenith around 1958 to ~1973 (~15 years) Zenith had the 14N22 monochrome console chassis (early on it went by other numbers and have some minor design revisions) which was an unkillable tank of a chassis.

GEs tube portacolor lasted from 1965 to ~ 1979 (~14 years) with one electronic revision, atleast one CRT revision and countless cabinet variations.

RCA from the beginning was licensing their color sets and a number of the off brand CTC4s and 5s made no attempt to disguise that. The CTC12/15/16 (they were nearly the same chassis) was probably the most popular among other brands...there were clones going in to the 70s, and after RCA stopped selling assembled chassis to other makes in 1964 the other makes started making interesting twists on chassis design and construction.

My understanding of keyed AGC is that the fundamental problem with a standard AGC is that the system (say typically used in a radio scenario) produces an AGC control voltage that is proportional to the average carrier level out of the detector, which, when averaged ignores the audio modulation, but the time constant of the filtering needs to be quite long to filter out the lower frequency range audio components. This applies too, to filtering off 50 or 60 Hz components in the detected video.

However, in the case of a video signal modulating the vision carrier, the average carrier level out of the detector varies widely depending on the scene brightness and over extended time frames, say a low light vs a high light scene, the level out of the video detector could change by up to 70% depending on the picture content. So a standard AGC design is not ideal to gain control the video IF & tuner's gain.

(The sound IF carrier is even less useful as it gets amplitude limited in an FM system)

So the purpose of a gated or keyed AGC in a TV, was to only have the AGC active during the H sync periods, so that part of the signal, unaffected by the scene contrast or brightness is used to represent the amplitude of the received vision carrier and it controls the AGC proportionally to that level, ignoring the picture content.

In addition, there is better noise immunity in fringe areas just sampling in the H sync or H flyback period, the AGC voltage is easier to filter and have a shorter AGC time constant and the AGC can respond more quickly to rapid changes in carrier level, such as RF carrier amplitude flutter from Aeroplanes. Also it was a requirement for portable & mobile sets used in cars where the RF carrier level could by dynamically fluctuating. This is why Sony for example used keyed AGC's in their early 12V operated portable sets.

So, keyed AGC is worth having of a vintage TV that's being fed from a fixed level signal from a modulator, especially if your TV is designed to reproduce black level correctly.

Because of the difficulties of maintaining stable black levels in the days of tubed cameras and signal chains, it often wasn't done very well. So a receiver with simple AGC and AC coupled video could actually disguise bad practice at the studio / transmitter.

I for one prefer not to watch a TV with AC coupled video, I find it annoying since most of the TVs in my collection do a reasonable job of holding a stable black level. However I have a 1957 vintage TV waiting for restoration at the moment (an RCA-influenced design by AWA) that has simple AGC and AC coupled video. I will be modifying it to remove these deficiencies using circuits derived from those in later models from the same manufacturer. Originality purists be damned!

Interestingly, the net change in number and value of components to make the necessary alterations will be close to zero. A little more intelligence at the time of design could have made this cost-reduced model perform so much better. I can add keyed AGC without adding or changing a tube and DC coupled video by actually removing a component....

Seems like we are confusing AGC (automatic gain control of the rf or if statges) with DC restoration.
Anyway, If we don't need AGC, we don't need keyed-AGC.

I am guided by the philosophy that "A job not worth doing is not worth doing well."

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Ron

No confusion here, they are separate issues in the design of a TV that both work to maintain the image as intended.

As has already been explained, with simple AGC the IF and RF gain will be controlled by the APL (Average Picture Level) of the incoming signal rather than by the sync tip level. This results in incorrect contrast levels.

So to be able to obtain a stable black AND white level, keyed AGC is required. Not just to correct for incoming signal variations.

Now of course if the incoming signal level is fixed, there is no need for any kind of AGC, the gain could just be manually set. Then the APL change issue goes away as well.

I guess what I am saying is that even with a modulator, simple AGC is a bad idea.

Remember the set being discussed here is the 630 chassis which atleast in it's original RCA incarnation didn't have any AGC just a manual RF gain adjustment in the form of the "picture" control. That set on a fixed modulator would only need correct adjustment of the picture control and not an AGC circuit of any kind added to it to handle DC video well.

One of the main reasons for keyed AGC is to reduce the effect of impulse noise
especially on the lowest two channels.

Absent that problem, one notes that a plain average AGC would work just fine if
it ran off the audio carrier. That's independant of whether its AM or FM. Even
intercarrier sets could do that with a separate, at IF frequency, AM detector. I assume
that keyed AGC was easier, in addition to the low band noise problem. The British with
positive modulation had fits getting an inexpensive, working, AGC.

Intercarrier sets were mainly of FM design in the sound.

Most UK sets that persisted with AM sound had separate sound amplifier stages at the sound carrier frequency, though some had a tuner and then had sound and vision IF's that were separate.

The AM sound carrier is perfectly good so as to use its detected output to make an AGC, much better than the video stages output. American sets all (that I have seen) went to FM and intercarrier sound post war though.

In FM intercarrier sets there are some issues trying to use the sound IF signal, to run the AGC. For most FM IF stages, by the time the 4.5MHz signal is amplified up to any reasonable level, that would work to drive an AGC rectifier, it would feed a limiter stage prior to the discriminator, so the carrier amplitude information is lost. So it could work but probably an active stage of amplification would be needed.

However, I did once modify a UK made set which had originally 41.5MHz AM sound and separate sound and 45MHz vision amplifiers for more modern use by adding a tuner (turning the original sound and vision TRF stages into "IF's") and converting to FM sound by installing a Ratio detector and I found it would work if the ratio detector's stabilization voltage was used for an AGC (which is not an uncommon technique), to control the Tuner which I had added to the set's front end, I still have the circuit somewhere. But this was still not an intercarrier system as the sound and vision IF's remained separate.

Philco used the ratio detector voltage to control tuner AFC in the 48-1000.

RCAs first AGC used in their KCS28 chassis (8t241 1948) was based on peak sync level. That is the sync peak level formed the AGC. This required amplification and an AGC amplifier was included. Note that this was one step away from keying out the sync peaks to improve noise immunity.

The peak sync AGC worked very well in airplane flutter. It however had limited noise immunity.

Most 1946-7 American sets were split sound past the tuner output. A LARGE portion of American sets 46-50 were split sound. RCA and Dumont which were the biggest licensers to smaller TV brands then hung onto split sound till the new decade. RCA was particularly stubborn....They hated using any tech they didn't invent and had split sound sets into the early 50s (those sets didn't handle chroma well and we're cited as part of the reason NTSC shifted sweep rates for color).

IIRC the Motorola VT-71 was among the first American postwar sets to use intercarrier sound.

Intercarrier sound required a change to the broadcasting standards. It required that the video carrier never drop below a prescribed minimum so that the 4.5mhz mixer output would not disappear.
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Ron

Thanks ACORNVALVE for the detailed explanation of Keyed AGC. Is Gated AGC the same ?

Same thing, different ways of describing it. Today we would say that the AGC is sampled from a known, fixed point in the video waveform.

Split sound sets would not have handled chroma well because most of them were full 4.0 MHz luma bandwidth. The split sounds sets had a better black and white picture than intercarrier sets which were forced to limit the luma response to minimize audio interference into the video. Also there was no intercarrier buzz in the audio.

The overriding disadvantage of split sound was it cost more (an extra audio IF stage was required) and local oscillator drift meant on the UHF and even higher VHF channels you may have to frwquently readjust the fine tuninggfor best audio. This would later have been corrected by including afc for the RF tuning. Bottom line: intercarrier sets were cheaper.

The shifting of the sweep rate for color was to accommodate intercarrier sets. Split sound sets would have been unaffected if instead of changing the sweep rate, the NTSC would have increased the audio video carrier spacing beyond 4.5MHz to reduce the aural carrier- chroma subcarrier 920kHz beat.

Yes that is right. RCA's first offering post war, the 621TS, had the sound split off just after the tuner. But only 2 or three years later the Admiral 19A11S had intercarrier sound and I think most post 1950 American sets had intercarrier sound.

I'm not 100% sure on the terminology here, but I think it is the same thing, AGC that is gated or keyed to be active in the horizontal blanking or H sync time.