KMPC 710 Transmitter 
Historical Photos
Taken at North Hollywood, CA by Marvin Collins (Then Chief of KFI) in 1996
(Text and edits by Steve Blodgett, Chief Engineer of KMPC at that time)
The pictures on this page were photo prints until scanned and digitized by Marv in 1999.   The main attraction is the old RCA 50F transmitter that KMPC ran for years during its history as a pioneering broadcaster in Los Angeles.  The photos speak for themselves.  Since this page was created, we have added additional and helpful historical comments (In blue) from Jack Sellmeyer. - Steve Blodgett

Thanks to Steve Blodgett for writing the photo captions and letting me photograph the beautiful old RCA 50 KW transmitter in 1996. I have fond memories of often bicycling to the site as a kid about 1948.   -  Marvin Collins, Chief Engineer, KFI

Jack Sellmeyer adds, on KMPC's transmitter and history...

The KMPC transmitter was acquired as part of a group purchase by G. A. Richards of Detroit, Michigan who, at the time owned WJR, Detroit, WGAR, Cleveland and KMPC. The WGAR transmitter was the first 50F manufactured by RCA and the KMPC transmitter was probably in the first ten although it was delivered after the Cleveland radio. 

Mal Mobley was a Staff Engineer for the Richards group headquartered in Cleveland. He relocated to southern California to oversee the power increase at KMPC and remained there for the rest of his career. He later opened an office as a Consulting Engineer and practiced for several years into the 1960's.

The G.A. Richards group was dissolved in 1953 following a revocation hearing which began on "news slanting" charges by some disgruntled "newsies" at KMPC. The company was ultimately absolved of all charges, the licenses of all stations were renewed, but Mr. Richard's health was severely affected by the hearing and advanced age and the company was liquidated. WJR was sold to another company; WGAR to Peoples Insurance Company, later "Nationwide" and KMPC to the Autry interests.
 

J. S. Sellmeyer, P. E.
Sellmeyer Engineering
McKinney, Texas
January, 2002

Click on photos for a larger image.
The KMPC transmitter building on Burbank Boulevard in North Hollywood. This facility was expanded in 1946 specifically to accommodate KMPC's power increase from  10kw to 50kw and house the then-new RCA 50F Transmitter. It's location is in a busy neighborhood now, but at the time of its construction, the surrounding area was completely agricultural. The design and construction project was under the supervision of KMPC's long-time transmitter supervisor and engineering pioneer, Mal Mobley, who spent the major part of his life as a broadcast engineer with KMPC.
At the far end of the room, on the left in the photo, is the RCA 50-F transmitter, with its familiar grey RCA look, with cabinets so big you could walk into them. This facility was run by a large staff of operating engineers, who kept the plant and its equipment in perfect operating condition, a testament to the pride of station ownership and management of the day. Each day, at sunrise, the flag was raised in front of the building as the morning operating staff began its shift. The facility was kept in spotless condition and was regularly inspected to insure that maintenance was kept on schedule. The RCA 50F transmitter  was operated during daytime hours, and its companion RCA 10kw transmitter operated during night hours to meet the reduced power requirement of KMPC's license. In front of the RCA 50F we see a standard RCA operator console, where the staff of engineers would keep track of audio levels and were able to do switching functions on the RCA. At this desk, meter readings were reliably taken and logged every 30 minutes, on schedule! In the photo, closer to us, we can see the blue Phasor cabinets which house the tuning equipment for KMPC's 3 towers. Closer still, there's a door that leads to the room areas behind the transmitter equipment, and this side of the door are the two equipment racks that housed a rather complex switching system used to automatically control the transmitters at this facility. Closer to us, but not seen, are two newer transmitters (1980), a Harris MW50, and MW10. On the right, you can see a side view of the 4 equipment racks that housed remote audio lines and associated audio amplification and control equipment, plus remote control systems which were added in more recent years.
Click on the Photos for a Larger Image.
Here are what the RF Power Amplifier and Modulator tubes of the 50F looked like. They're heavy. We used a special mechanical lift device to move these tubes in and out of the cabinet. The tubes used in this transmitter  reportedly ran 80,000 hours or more before replacement. That's amazing when you think of the relatively short span of tubes used in today's transmitters. The engineering staff kept meticulous records of tube installation dates and operating history of each tube put into service. 
Here's the oscillator cabinet which housed the crystal oscillators for 710 KHz and the low level amplifiers which brought the RF signal up to a level which could be used by the Power Amplifier tubes. Typical of the design of RCA transmitters of the time, the components were layed out almost in schematic drawing fashion. You can look at the cabinet and see the layout of the drawing, oscillators at the bottom, amplifiers above, in order, each increasing in power level. 
In this cabinet, the High Power Audio Modulators are shown. 2 are used at any one time, with a spare left in place just in case  one tube should fail. These tubes are so heavy that it took a long time to properly replace a tube, so having a spare in the cabinet "just in case" was a big help to the operating staff, which could get the transmitter back on the air in a hurry if  necessary by using this spare tube. Whether or not the spare was ever necessary, we'll never know. But it should be said that the RCA 50F was so reliable that when it was replaced in 1980 with a Harris MW50, the engineers at the time noted that the RCA had less failures in the 34 previous years than the new transmitter had after only ONE year of service at the same site.
These magnificent looking tubes are the high voltage rectifiers of the RCA 50F transmitter.  We fondly referred to them as the  "Frankenstein Tubes". They produced 10,000 volts with enough current to power the RF amplifier and audio power amplifier stages. The power supply of the 50F was a big, no-nonsense, industrial power system, running at 480 volts, 3 phase, Engineered to run continuously with ample "headroom" so that transformers and other components ran relatively cool. The one thing that would strike the person when looking at the 50F was the word "BIG". Everything was large.  Transformers, tubes, conductors, cabinet housings, and the transformer power supply vault were all Big.  The building itself was virtually built around the transmitter, or more correctly, built for it.
This is the front panel of the cabinet that housed the many relays, contactors, and industrial sized mechanical switches used to operate the 50F. Although this transmitter was used primarily during the day at full power, it could also be operated at 10kw by manually moving switch contacts which would cause the power level to be satisfactory for night time use. The 50F seemed happier though, when it was at its full power level. It was fully capable of 150% positive modulation and was run that way before the mandated 125% limit was put into effect by the FCC. 
This is the inside of the control cabinet. Above we can see the terminal blocks where interconnections are made to and from other cabinets of the 50F. Typical of the RCA's 50F cabinets, one could open the door and walk  right in.  The overhead bulb supplied sufficient lighting for routine cleaning and maintenance tasks. 

Here are some notes from Jack Sellmeyer on this cabinet...
The item (bottom of photo) ...in the control cabinet is the 480 volt line reactor "6L-1", not the voltage regulator. The 480 (then 460) volt buss regulator fed all the low level 480 volt equipment, i.e.: the filament supplies, bias supplies and the 1500 volt DC supply for the exciter and low level audio equipment on the back door of the modulator cabinet. It is a GE "Inductrol" regulator which is mounted in the left rear of the cabinet.  There are three of them, one for each phase, They are mounted behind the metal enclosure on the floor at the left rear of the door identified on the schematic as "6T-1".

Below the 50F transmitter is the blower room. These are redundant, high volume blowers that created negative pressure in the room. Blowers were operated, one at a time, alternating on schedule. Air was drawn through two walls of filters that comprised a plenum room (off the photo to the left). The engineering maintenance crews made filter replacements on schedule in order to keep the transmitter interior clean. The outlet ducts from these blowers went off to the right and entered the bottom of the RCA 50F through the floor beneath. 

High volume and low speed cooling from this isolated room made the transmitter almost silent, with only the sound of a hushed air movement as it ascended from the blower room below, through and around the Power Amplifier tubes. Because these blowers were housed in a separate room beneath the transmitter room level, these motors were not heard in the main operating room above. 

Additional notes: 

At the time these photos were made the station was still owned by Gene Autry and Golden West Broadcasters and was in the long process of transferring control to Cap Cities/ABC, now Disney, who changed the call sign to KDIS, Radio Disney.  The KMPC call sign has since been removed from the building.  The site has become a faded memory of the once great station of earlier days.  - Steve Blodgett
 

Back to Main Site
email: steve@earthsignals.com

EarthSignals.com Website

Users Since December 3, 1998