Late 1940's RCA TV design- Adjacent Channel Rejection

[Penthode]

It is interesting to study the evolution of the design of various manufacturers televisions set from after WWII. RCA was a leader and as such introduced popular circuits as their synchroguide horizontal AFC in 1947/48. RCA was also one of the last manufacturers to stick with their split sound design until 1952 with audio take-off from the tuner and later after the second Video IF amplifier.

The earliest post WWII RCA televisions, the 630TS, 621TS and 721TS featured minimal adjacent channel rejection. The 630TS featured full 4MHz video bandwidth and a single adjacent channel video trap (19.75MHz) and adjacent channel sound trap (27.25MHz). The 621TS/721TS design had no adjacent channel rejection other than the roll-off response of the Video IF amplifier.

You may recall 1948 was a crucial year for the television broadcast industry as set sales were booming. But 1948 also yielded the peak of the 11 year sunspot cycle. Sunspot activity in 1948 interfered greatly with VHF television propagation causing co channel interference from distant broadcast television stations. The result was that the FCC issued a halt on the construction of new stations for the next four years which resulted in the opening of the UHF band (470MHz to 890MHz) band for television.

The 1948 series of RCA television sets beginning with the KCS28 chassis included not one but two pairs of adjacent channel video and adjacent audio traps. I always thought it excessive but during the days of analog television broadcast, it did make a brilliant DX'ing receiver!

It occurred to me that it was too much of a coincidence that this design was introduced coinciding with the sunspot cycle peak. You will find the dual traps pairs in almost all RCA designs until 1952, when the traps disappeared. By the mid '50s, only the most expensive RCA sets featured both traps but most often included only an adjacent channel sound trap or no adjacent channel traps at all.

Below I have included the shematic diagrams for the Video IF amplifiers of the 1946 630TS, 1946/7 621TS/721TS and 1948 8T241/3/4 using the KCS28 chassis. The adjacent channel traps have been boxed in red.