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Originally Posted by peverett
Passive AGC just like the AM radio AGC. The signal is rectified and fed back to the tuner RF amp and a portion of the IF amplifier section. The strength of the signal directly determines the AGC strength with no amplification. Most, if not all, early TVs used this type of AGC.
Active AGC is amplified AGC. The rectified AGC voltage is amplified by prior to being sent back. Examples include keyed AGC, etc. This type of AGC was used in the 1960s and later.
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Great explanation..! However, wasn't keyed AGC was pretty common well before 1960..?
Let's see... flipping through the first two volumes of SAMS "Television Tube Location Guide" (published in 1950 and 1951 respectively), I see a few chassis with an AGC keyer, but mostly it's just a AGC rectifier. So, not many there.
Skipping on to volumes 5 and 6 (1955 and 1956 respectively), I see quite a few tube layouts showing keyed AGC. Certainly not all of them, mind you, but it's a pretty decent percentage. Also, don't basically all color TVs have keyed AGC..?
In the March 1960 issue of
Consumer Reports, they had a review of "Minor Brand" Television Consoles for 1960, which includes the following text:
"The fact is, for some years, CU has had a general rule that, if possible, no TV set is tested unless its circuitry includes three stages of Intermediate Frequency (IF) amplification, [...]; and keyed Automatic Gain Control (AGC)" [...]
The context of this statement was to point out that some of these "minor brands" had not generally been included in CU's usual reviews because they did not offer sets meeting those two criteria. The companies
not offering sets having keyed AGC in 1960 but reviewed anyway for this particular project were A-M-C, Muntz, Olympic, and Supre-Macy; all of which are definitely on the "low-end" side of the spectrum, and two of them are department store house-brands. [Even the Truetone (Western Auto house-brand) set reviewed in this project had keyed AGC and 3 IF stages. By the way, the other minor brands covered in this review were Andrea, Setchell-Carlson, and Trav-Ler, all of which also had keyed AGC and 3 IF stages, but those weren't low-end makes anyway so that's not surprising.] So, I'd take this to suggest that keyed AGC was pretty common well before 1960, except for low-end sets. (I'm sure even the major brands produced low-end sets without keyed AGC too)
Anyway, I didn't intend to get pendantic here (sorry about that!)-- it just seemed like 1960 seemed rather late, so I did a little spot research to check. I could still be wrong though; I'm just reporting what I saw. I'd agree that keyed AGC wouldn't have become ubiquitous until after 1960, but I think it was still pretty common before then.