KMPC 710 Transmitter
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)
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
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
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
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
|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
|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.
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