1947 RCA BTF-250A FM Transmitter...

KPFA Folio Oct 1991, ad for moving saleOn October 12, 1991, my father, my brother and I went to KPFA's Shattuck Avenue location for an advertised garage sale. The station had just completed a move to their new facilities on Martin Luther King, Jr. Way, and they were liquidating old equipment. We bought this RCA transmitter from them.

In 1954, KPFB became the first license in the FM educational sub-band to a foundation that wasn't a school, making it the precedent to all subsequent community radio on the left end of the dial. At that time, Pacifica acquired this RCA BTF-250A FM transmitter. It is possible that this unit, or parts of it, was used as KPFA's first transmitter on top of the Koerber Building.

It is a 250-watt unit from the first generation of RCA FM transmitters for the new band, 88-108Mc. The total height, with the original mounting base, is specified at 84-inches, but the base was not present when we bought it. An 80-inch cabinet with a 31 X 28 footprint, in a residential garage is more than enough. The side panels, doors, and meter panel are removed to reveal a 25 X 25 X 80 welded frame of 1/16"-thick angle stock. This cabinet series is similar in appearance and construction to the BR-84 equipment cabinet, and would have been a perfect pairing in a spartan transmitter room.

It weighs just over 1000-pounds, so wherever you put it, it had better be where you want it to be.

You can click on some of these images to see larger versions.

RCA BTF-250A KPFB & WPIK-FM Transmitter


MI-7016 Exciter

MI-7016 FM Exciter and MI-7017 Power Supply.
RCA MI-7016 exciter

Yep, that's the "iron fireman" exciter, RCA's Master Item (MI) 7016, mentioned in Lorenzo Milam's Sex and Broadcasting. It is accompanied by a MI-7017 power supply, providing DC supplies of +600, +400, +250, +150, +105, +6.3, and -50. The mount is not a standard 19-inch. This is 22-inches wide with a varied mounting screw spacing.

MI-7016 ID tag and label.

The AFC is by motor control. The modulated oscillator is divided by 240 through four synchronized divider stages. A crystal controlled oscillator in the 120kc range, is divided five times. The two signals are mixed in the grid circuits of four 1614 (6L6) tubes, the plate circuits of which are supplied through four separate windings of this 2-phase control motor. When the modulated oscillator is on frequency, no current is pulled through the 1614s, but when there is a difference, the current pulls the motor to one side or the other, turning a split-stator capacitor a fraction of a degree to bring the M.O. back on frequency.

Master Oscillator tank tuning capacitor.

Lock is indicated on a built-in CRT by an oval pattern in switch position two. Because the phase is synced throughout the division process on both sides of the chain, this is close to a true PLL. The motor damping is smoother than several electronic AFCs I've worked with.

RCA MI-7016 exciter

The motor cover is filled with a viscous silicone oil damping fluid. The resulting friction on the brass disc smoothes the AFC action, like a capacitive filter in an electronic AFC. I fabricated this motor cover by machining an acrylic block to dimensions shown on some blueprints included with the paper documentation, and finally happened upon the same RCA Thunderleg monogram as the original. Photo evidence from the first-edition of FM Simplified, by Milton S. Kiver, shows this reproduction to be fairly accurate.

T115, master oscillator tank coil. The tuning screw is for fine adjustment of the frequency.

The Modulated Oscillator consists of a tank coil and a 6V6 tube running at 4.5 to 6 megacycles and is reactance modulated by a pair of 6V6s the grids of which receive a bit of feedback from the M.O. tank. The feedback exerts capacitive reactance through the tank from the current of one modulator, and inductive reactance from the other. The AB1 push-pull of the modulators allows a wide swing of direct FM on the M.O. and also some common mode cancellation of microphonics and distortion.

I replaced the shattered ceramic mounting base of this complex coil with a 1/8" sheet of poly carbonate plastic. Inside that can are a multi-tapped tank coil, a sample coil for the modulator grids and two concentric ferrite slugs for a coarse tuning (back) and a fine tuning, plus six solder posts.

The M.O. is multiplied by 3 with another 6V6. All of the four 6V6s are supplied by a rectified and filtered 6.3VDC supply from the MI-7017 to minimize AC hum. All other tubes and rectifiers operate from one of two heater transformers. The critical areas in this exciter are the reactance tubes, M.O., and first multiplier.

A second multiplier, 2E26, brings the carrier to half the frequency of the FM band, 44 - 54Mc. Thus, MI-7016 was to be RCA's motor controlled entry into the original FM band, but the change to the higher frequencies was too late in the design to build the necessary enclosed chassis necessary to multiply the frequency any higher. This exciter design was also configured in a variety of ways for the aural carrier exciters in the first generation of RCA television transmitters.

A second 2E26 serves as a 50Mc amplifier. After complete component replacement, I am able to milk 9-watts out of this stage, well enough to drive the doubler in the next stage of the transmitter.

RCA MI-7016 Exciter
The rear of the MI-7016 is open with no shielding. The manual touts, "Vertical chassis type of construction has been followed with surface mounting of components and exposed wiring for easy and speedy circuit tracing and servicing." The exciter B+ interlock switch was bypassed by KPFA when it broke, though making it easier to service the coarse tuning on the MO tank with B+ applied. You just keep your fingers away when it's running.

The audio input to transformer T301 can be configured for 150- or 600-ohms. Input level here is -14dBm at 400cps. for 100% modulation, because the supplied MI-4926A passive pre-emphasis network has an insertion loss of 26dB. The input to the network is the customary +10dBm. Before the widespread use of limiting amplifiers, manual gain riding by a watch engineer left the modulation at around 30%. Click Here for a PDF of a pre-emphasis weighted sweep of this network.

Three-position switch, S107, applies a small amount of DC to either side of the secondary of T301 to test the AFC and aid in tuning the tank feedback into the reactance modulator grid circuit.

KPFB's audio input was driven by a CBS Labs Volumax-FM which was not available at the sale. They were using it correctly, ahead of the pre-emphasis network, without modification to the internal Volumax pre-emphasis curve. Most of the time the audio was a sum of the L & R program busses from the C-2 console, but could be switched to a sum of the audition busses, or to any of several other sources within the building.

This exciter is capable of mono operation only. The audio frequency response of this exciter conforms well at 30Hz to 15kHz per FCC requirements.

This article in Broadcast News shows us what an incredible product the MI-7015 exciter was going to be.

Or, just go to this guy's website and see all the wonders there.

Audio input terminals.


MI-4926A pre-emphasis network.

Doubler and Power Amplifier

The "RF" section consists of a doubler circuit and the power amplifier. Both use 4-125A/4D21 radial-beam, power tetrodes. Putting the exciter output on channel with doubler in the exciter, would have required serious redesign of a product RCA was already committed to producing, so the doubler had to be here in the high level.

Both circuits are fed from the same 2200VDC plate supply. A 400-volt "low voltage" supply is located underneath the RF compartments for screen supply. The compartments are cooled by suction through two centrifugal blowers, but restoring that is a work in progress now.

Front door open.

RCA BTF-250A doubler and PA compartments

With the door open: On the right, the doubler compartment with a 5U4G to rectify the screen supply. The left compartment is the PA with two 4-125A tetrodes. These circuits are tuned with variable inductors which keeps the overall Q down and maintains wide enough bandwidth for a 180kc FM mono signal. The four tunable inductors are rotated by controls on the front panel, between the circuit breakers, and connected by flexible shafts.

The doubler compartment is where RCA got their 45Mc-band exciter onto the 88-108Mc FM band we are familiar with today. It includes a B+ shorting switch (left) and a plate supply interlock switch (right), which engage when the hinged door is opened. The 5U4G rectifier for the screen supply is included in this compartment since there is no better place to put it.
Doubler compartment: 4-125A and 5U4G
RCA BTF-250A doubler
Power amplifier compartment.
RCA BTF-250A power amplifier

The power amplifier uses two 4-125 tetrodes in parallel. The doorknob capacitor on the B+ feed is a replacement for an Aerovox molded-mica transmitting cap. The choke is the now hard-to-find Ohmite Z0 (zee zero), and is a replacement because the original exploded possibly from a failure in the aging fillament bypass capacitors. C317 in the upper-left corner is set to about 90% per the instructions, and there are no further words on using it to tune this circuit. The 104-108Mc, 1-1/2-turn coil is installed here. The 2-1/2-turn, 88-104Mc coil was in use when this was tuned to 89.3 for KPFB. Luckily, both were included. Some things survive the decades, others are lost.

A 250-watt output is achieved with a PA voltage of 2100V and 215mA input current. It has proven itself a capable transmitter still to this day.

Below the rear PA tube socket, 10,000pF Faradon model "T" mica capacitors bypass the filament leads to chassis through #6 spacers and these also support a plate to which the ground end of the screen neutralizing capacitors are connected. This serves to tie the neutralization into the cathode circuit. This type "T" capacitor was used at other points around the doubler circuit, but all were replaced at KPFA.
Underneath the rear PA tube.

High Voltage Power Supply

This supply delivers 2100-volts to the doubler and PA circuits at full load. It matches the vertical position of the doubler/PA assembly on the front side, so, unlike many AM transmitters, the heavy iron of transformers and chokes is not located on the floor of the cabinet. Copper tube formed into conductors directs the secondary of the power transformer to the rectifier anodes. The rectifiers are supposed to be 866A mercury rectifiers, but were replaced by KPFA engineering with solid state units sometime after February 1960. The filament transformer primaries are fed by a rheostat for adjustment of the proper voltage. This input voltage is recorded from the meter, and operating practice is to check and adjust it periodically. The back door interlocks disable the HV supply and allow a shorting switch to short B+ to ground for safety. Another interlock is supposed to kill the exciter B+ since there is no protective cover on its rear panel. All of these interlocks conspire against the operator who must take the station off the air to make any adjustments on rear of the unit. RCA's higher-power transmitters all have the filament control and it's meter on the front panel. KPFA seems to have added the "FIL HOURS" and mains voltage meters. The main filter caps were replaced by KPFA in an effort to rid the station of PCBs. The Exciter B+ interlock is useful to stay completely off the air durring warm-up as the second harmonic of its output is readily heard on nearby receivers.

Where did it come from?

KPFB's went on the air February 14, 1954, to fill-in parts of the Berkeley Hills and El Cerrito areas that are shadowed from the Grizzly Peak transmitter location that KPFA acquired in a reorganization in 1950. Today, this is a common application for translators and on-channel boosters, up to 250-watts.

KPFA Transmitter Supervisor, Steve Hawes wrote on the ba.broadcast group in 1994:

Regarding "the original KPFA was KPFB", as I understand it the original KPFA _transmitter_ was used for KPFB, but I'm not positive about that. The first KPFA transmitter was 250 watts, and I know that the 250 watt RCA transmitter that became KPFB in 1954 was in possession of the station at the time of the STA to use the KSFH facilities, and that the KPFB transmitter contained the KSFH modulator panel [MI-7016 exciter unit], which had some missing parts because of a theft when KSFH was dark. I was never really clear about this, but I surmise (conjecture mode on) that the modulator panel was taken out of the original KPFA xmtr to get the old KSFH unit on the air, and probably went with it when the 3kW RCA was sold. That left the KSFH panel with the missing parts in the 250w that later became KPFB. Those missing parts had to [be] replaced locally and some had to be built in a machine shop of the one of the listeners. The KPFB transmitter never looked (or acted) quite like a stock RCA.

The manual we got from KPFA shows some evidence of a different previous owner. The table of crystal frequencies for output channels has an underline and a mark at 98.3 m.c. and the call letters "WPIK-FM" written there. WPIK was a 1000-watt, daytime-only AM station on 730 in Alexandria, Virginia, which went on the air in 1945. Like many existing AM stations of the day, they tried parallel operation on FM and got a grant on 98.3, a class-A channel, and put it on the air in October, 1947. This was probably a temporary frequency until it moved to the 105.9 allocation, which still remains today in the Washington, D.C. metro area. The new frequency was prabably part of a power increase as well, and the 250-watt unit was not enough. The WPIK call letters now belong to an FM in Summerland Key, Florida.

A hand-drawn block diagram of this transmitter is marked with frequency calculations for 98.3 which are crossed out and replaced with those for 89.3, KPFB's frequency. Neither of the TMV-129G crystal ovens is marked for 98.3. One has factory-original markings for 89.3. Another was originally marked for 100.1, but this was covered-over and relabelled for 89.3. Both ovens had 89.3 crystals in them in 1991.

Perhaps this copy of the manual is not the original shipped with this transmitter. How did WPIK get rid of their transmitter? Did they unload it through a buy-back offer from RCA? Through an equipment broker? Was this a direct sale or donation to Pacifica Foundation?

Steve Haws also writes on ba.broadcast:

The original [KPFA] operation began on April 15, 1949 on 100.1 MHz with an ERP of 550w. It referred to to itself as "KPFA Interim" (it really was an experiment in listener-sponsored radio).

Hmmm, this is interesting knowledge. The crystal oven on the left is factory marked for 100.1, and probably does have a place in KPFA history. This channel, like the 98.3 initially used by WPIK-FM, was a class-A only channel. Obviously, if the first transmitter on the 6th floor of the Koerber Building really was a 250-Watt RCA unit, and if KSFH had really abandoned another unit up on Grizzly Peak, then certainly a lot of parts could have been swapped between the units. If our 250A is a third unit acquired in anticipation of the KPFB allocation, then we really have quite a complicated shell game to contend with.

Further speculation could be that the Exciter unit was stolen entirely from the KSFH site, and that only the exciter shown here and the manual were acquired from WPIK.

The Condition Of The Equipment

KPFA was essentially running their studios at a minimum in the time leading up to their move from Shattuck Avenue to today's Martin Luther King, Jr. Way location. KPFB seems to have been a somewhat neglected burden on the engineering staff. A transmitter like this needs to be operated in the presence of an operator. In the early days, even KPFA had full-time watch engineers, but the transition to "combo' operators who are on the log in charge of both the discjockying and the transmitter(s) means that this transmitter's meters and off-switch needed to be close-by and within view. In the 1970's, KPFA built a main, on-air studio called Control 2. The BTF-250A was built-into a closet positioned along the Alston Way, north wall of the building. This closet was supposed to vent heated air outside with a blower. Various blower failures and other problems seem to have left the transmitter to overheat on many occasions. A hand written note among the maintenance log pages, on April 12, 1989, shows the air in at the bottom of the rack duct to be 65F and the air out at the top of the transmitter was 93F. After taping leaks in the rack room adjacent to the closet, the temperature dropped to 84F. The author of the log complains that a more sensitive manometer was needed. The closet helped to isolate the sound of the blowers from the operator and guest microphones at the console. Remote metering would have been impractical.

The mains voltage to the transmitter was chronically low, at one point recorded at 201 volts. Today, the available voltage is a little too high, often in excess of 250 volts.

There is strong evidence that the exciter fell-over on both it's front and back while out of the cabinet. This may be part of the 'theft' to which Steve Hawes refers. The motor damping cover was replaced with a home-made, all brass unit. L115, the master oscillator tank coil, was damaged extensively; it's aluminum shell, protruding from the front of the exciter was squished inward and its ceramic base at the rear was shattered. Various inductor cans along the bottom edge have damage. The cathode ray tube (2BP1) mu-metal shield is not original based on close examination of manual photos and blueprints. Could this have happened in transit to Berkeley, or by some clumsy handling by the engineering department?
In 2009, I began a lengthy restoration of the exciter, replacing all of the mica and ceramic capacitors in the M.O. and multiplier stages. Two of the bypass caps in the second doubler and amp had slowly shorted to ground, burning-out two hard to replace National Radio Corp. R100 chokes. Several carbon resistor values were way off, and those were replaced too. The ceramic base of L115 was rebuilt with 1/8" Polycarbonate plastic. The biggest project was mostly cosmetic in rebuilding the motor damping disc cover and gluing an original RCA emblem to its center. This new cover doesn't leak. The old capacitors were 500-volt, axial, molded-mica components. A compromise had to be made as modern ones are all dipped-mica and radial form. The scope still needs a rebuild pending acquisition of adequate wire.

Here are the X-Y oscillogram patterns viewed on an external scope after major exciter component overhaul. The CRT circuit displays only comparisons between stages for tuning purposes, and does not have any sweep mode to show waveforms. While the lissajous paterns are not perfect, correct tuning at each switch position is indicated fairly obviously. Later manuals show example shapes that are very close to these.

Restoration of the circuits beyond the exciter has progressed to Tune-up of the doubler. Some components to be replaced are difficult to find in form factors close enough to the originals. Faradon "T" capacitors used in the filament bypass on each of the 4-125A tube sockets have no equivalent and must be replaced with ceramic disc, radial caps. Much of the wiring in this compartment is compromised and needs to be replaced. All subchassis wiring is specified for at least 1000V with 3000V for B+ runs. The 10KV jumper wires between the big power supply, the meter panel and the PA are disintigrating.

The most staggering fact in all of this is that Pacifica Foundation was able to operate an FM station, in a major radio market, for 37 years with this model of transmitter, and that it possibly also served as the first KPFA transmitter.


Here is the RCA BTF-250A manual, IB-30131, dated 10-11-1946, PDFs in five parts:

A page from the KPFB maintenence log describing the steps taken to make KPFB PCB free!!

 To be continued...

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