SP - 1970 - Cartridge (Cart) machine was the workhorse for thousands of radio and television stations. The RP/SP series were among the most popular of all cart machines. The venerable "cart" began its ascension to the standard for broadcast stations in the early 60's.
Magnetic recording on tape or wire was not available for commercial use until after World War II, which means that anything that aired before 1947 or so was either a live broadcast or presented via the miracle of "electrical transcription". Electrical transcriptions, or "ETs", were records but not in a form that could be played on a typical home record player. Usually 16" in diameter (as opposed to the 10" records of the day) and recorded at 33 RPM (instead of the 78 RPM then used for commercial recordings), ETs were used for the distribution of syndicated programming and other broadcasts that were not time-sensitive. ETs were relatively expensive to produce, as they were often made on a heavy aluminum-backed disk, and their sound quality tended to diminish rapidly after the first or second playing.
International Tapetronics Criterion 90-3 cartridge machines.
  


Schafer 903 Radio Automation System (circa 1974)
Schafer model 800 automation system (tube type)  in 1969 at KDTH Dubuque, Iowa.
RCA 70-D Transcription Turntable-  This is the RCA Type 70-D, Model MI-11801,  Transcription Turntable with RCA Type 72-D, Model MI-11852  Recording Attachment. This piece of equipment was used in broadcast stations for playing records and for recording onto transcription discs for future playback. (Courtesy of  Western Historic Radio Museum  Vintage Radio Equipment and Memorabilia  From 1910 through the 1950s P.O. Box 73 - Virginia City, Nevada 89440  Owner/Operators: Henry Rogers - W7HTR   Sharon Rogers - KK7EI  www.radioblvd.com )  R- Most ET's were 16 inch in diameter and required a large turntable and special needle to play.

Ampex model 351 reel-to-reel tape machine. The workhorse of the time.
Scotch brand reel to reel magnetic recording tape.
Broadcasting Equipment 1930's to 1990's

Here are few photos of some of the equipment used by broadcasters beginning in the 30's through the 80's.  The digital era started in the early 80's leading the way to the digital revolution which has completely taken over the broadcasting and telecasting industry today.    In the 30's and 40's radio station programming was mostly live because the means to record commercials or programs had not been developed.   Wire recorders followed by electrical transcriptions became the early means of recording prior to 1947 but was rarely used .   Magnetic tape became available in the early 40's and with improvements over the next several years became the most popular means of recording content until 1959 when Collins Radio Co. invented  the cartridge  magnetic tape format designed for use by radio broadcasters to play commercials and announcements.  Cart and magnetic tape gave way to computers in  the 80's and music in mp3 format which launched  the digital broadcasting industry we have today. 
Electrical Transcription  (ET)
Technician cutting an ET (Electrical Transcription)
In the early 50's the Mackenzie Repeater tape cartridge machine brought to the broadcasting industry the first machine designed for continuous endless tape loop operation.  The magazines (cartridges) were made of metal with an exposed loop of tape.  Tape lengths varied from seconds to a maximum of 14 minutes.  The exposed loop of tape from the magazine required the operator to position the magazine in the repeater carefully placing the exposed loop between the repeater's pinch roller and capstan which pulled the tape from the inside of the loop in the magazine and wound itself back around the outside of the loop.  The tape was specially treated with graphite embedded to reduce the friction in the loop and permit it to turn freely in the loop. 

By the mid 60's the repeater had been replace with fully enclosed cart's housed in high impact plastic holders. The MacKenzie repeater was offered in numerous configurations - with as many as 10 decks in a unit. The popular Model 500 (shown) featured 5 decks and independent audio outputs for each. The development of the continuous loop tape cartridge replaced the repeater and quickly became the standard in the industry in the early 60's.    



Schafer Automation Systems - The first automation systems came to broadcasting in 1956  when the first Schafer Automation System was installed at KGEE in Bakersfield, California. The owner wanted to program his station all night long without a person being there.  The automation system included  a couple of Seeburg record player changers to play 45s and several Ampex reel decks for commercial. 
KTRB's first transmitter.  A homemade 250 watts transmitter built by Bill Bates in 1932.
Engineers working on the construction of KBOX in 1949.  R- KBOX's first transmitter manufactured by Collins transmitters.
It used standard 1/4 inch magnetic tape, in continuous loops, with the "cueing" of cuts on each loop accomplished with adhesive foil strips that were carefully applied to the tape  Later models used a separate cue track with analog tones which replaced the foil strips.

By the late 60's  the MacKenzie repeater  had taken its place with 16-inch transcription turntables, wire recorders, soft-cut acetates and other early machines and technologies that were used to record and reproduce sound in the 50's. What made the MacKenzie machine so special is that it was used in Top-40 radio stations, just prior to the emergence of the continuous loop tape cartridge. The venerable "cart", of course, began its ascension to the standard for broadcast stations in the early 60's.   It used standard 1/4 inch magnetic tape, in continuous loops, with the "cueing" of cuts on each loop accomplished with adhesive foil strips that were carefully applied to the tape  Later models used a separate cue track with analog tones, and still other Repeaters stopped in the absence of a "sub-sonic" tone on the audio track.)
J.D. West (Lee Mackenzie) KEJC at the computer.    
Early Automation Equipment
Circa 70's & 80's
KDJK production room control board in 1985
1948 control board built by Collins Radio.
Automation system including two cartridge turntables load with carts. 
RCA 44 BX
An old Rekokut turntable modified to play ET's by Gary Avey, Chico, CA.  Note the penny on the pickup arm for added weight.  A time tested method to help prevent skipping during playback.

Cartridge and reel tape bulk erasers. 
Computer automation,  the standard today.
Electrical Transcriptions, or "ETs"-
Mackenzie Repeater Tape Cartridge Machine
RCA 77DX