The BC-604 is an FM transmitter employing the principle of phase modulation. The carrier wave is modulated with audio frequency which changes the points in the signal that cross from positive to negative and vice versa. At these "zero" points, voltage kicks are added which become the FM signal that gets transmitted. The entire SCR-508 system is built around the main concept of the BC-604.

Frequencies are pre-set using crystals that are contained in a special drawer on the top left side of the transmitter. 10 crystals can be used at one time that correspond to the 10 buttons on the control panel of the BC-604. The range of frequencies possible are 20 MHz to 27.9 MHz in 100 KHz increments.

Power is supplied by an internal dynamotor with either 12 Volt or 24 Volt operation specification. The dynamotor produces 625 Volts at 0.225 mA when powered by 25.5 Volts from either a generator or the vehicle battery. When the transmitter is powered on by activating the "Transmitter on/off" switch, 25.5 Volts are suppled to the filaments and also heat the crystal oven. When the PTT button is pressed on the microphone, 25.5 Volts are connected to the "dynamotor relay" and the dynamotor starts to generate the 625 Volt anode voltage, placing the transmitter on the air.

The BC-604 was developed by Bell Labs in the 1930s, and was basically adapted from a civilian form-factor and militarized. During the 20s, 30s and 40s civilian radios employed a rectangular chassis where the vacuum tubes were on top and their related components (wiring, capacitors, resistors, inductors) were below the chassis. This allowed for a nice and neat display of glowing vacuum tubes inside the radio, but hid the spaghetti-like mess of wires and components below the chassis away from view. Both the BC-604 and BC-603 devices used this civilian radio form-factor. This not only did not make changing tubes incovenient, working on the radio was extremely difficult. The pictures below show how incredibly cramped the components are.

The BC-604 transmitter mounted in the turret of the Jumbo Sherman.

The DM-37 D 24 Volt dynamotor.

 

 

The DM-37 D 24 Volt dynamotor.

 

 

The DM-37 D 24 Volt dynamotor. Note the voltages 25.5 Volts for the filaments and 625 Volts for anode.

 

 

The connectors on the bottom of the dynamotor. These connectors act as both power inputs and outputs, along with the (-) connections to ground.

 

 

The male connectors in the dynamotor bay. Note that everything is covered with the weather-proofing varnish.

One side of the dynamotor with the cover off. The brushes are inside the plastic caps on either side of the motor.

 

 

The low voltage portion of the dynamotor showing the brushes and the windings.

 

 

Another interior shot of the dynamotor.

 

 

Here the ball-bearings have been cleaned and are ready to be freshly greased.

 

 

Here the dynamotor is installed in its bay. Four screws secure it to the base and make sure there is a good electrical contact.

 

 

The installed dynamotor is shown here from the rear of the BC-604.

Here I'm using an I-177 WWII tube tester to test one of the VT-164 pentodes. The BC-604 uses 7 x VT-164 vacuum tubes and one JAN-1624 power amplifier. The VT-164 (JAN-1619) tubes are used in the following stages: oscillator, 1st RF amplifier, 1st audio stage, 2nd audio stage, rectifier, doubler, and tripler.

 

 

Most of the VT-164 tubes read at around 2750 Micromhos (transconductance unit). Since most of them had this reading (even the new ones) I concluded this was normal for the tubes.

 

 

Tubes installed in BC-604 viewed from the rear of the unit.

 

 

The drawer containing crystals for the transmitter. Only 10 crystals could be used at one time, therefore these 10 frequencies would have to be pre-selected before a mission or battle and the participating vehicles would have to configure their SCR-508 sets to these frequencies.

 

 

Crystal number and frequencies, and final output frequencies.

 

 

Crystal number and frequencies, and final output frequencies.

The crystals are installed in the oven container. The oven was necessary to maintain a specific temperature around the crystals to maintain their resonant stability.

 

 

One of the crystals being tested for the resonant frequency. The test involves "reverse" excitement of the crystal by applying RF voltage at one end through a 10 k-Ohm resistor and connecting an oscilloscope through another 10 k-Ohm resistor. As I slowly changed the RF frequency around the crystal frequency under test on the signal generator, the voltage signal amplitude on the oscilloscope visibly diminished as I crossed the crystal's actual resonant frequency - this happened when the crystal and RF source were connected in parallel with the oscilloscope. The effect was opposite ( voltage signal amplitude on the oscilloscope visibly increased) if I connected the crystal in series with the RF source and oscilloscope.

 

 

Another photo of the parallel crystal connection.

 

 

Signal when the RF source was not in resonance with the crystal.

 

 

Signal when the RF source WAS in resonance with the crystal. This tells you that the crystal resonates at the correct frequency, but it does not tell you the condition of the crystal and how strong it is. In other words, what excitation voltage is necessary to make it oscillate.

 

 

Here is the actual resonant frequency of the crystal. Based on the SCR-508 manual, Channel 75 has a base frequency of 509 KHz which is very close to the reading here. Not bad for a 60 year old piece of technology. The 509 KHz gets multiplied by 54 times before it's amplified and transmitted.

 

 

Crystal sockets

 

 

Corresponding frequency pre-set buttons. These buttons have several purposes: crystal frequency selection, antenna tuning memory set and recall, crystal resonant frequency tuning memory and recall. Above the buttons are the antenna and ground connectors, these are not often used because of the existing base connections. Below is the RECEIVER TUNE / OPERATE switch. To the right is the antenna current meter. Below the meter is the TUNE / ANTENNA CURRENT switch which allows the tuning of each crystal frequency through a dummy load. Below is the ON/OFF switch, neon power light and the RADIO / INTERPHONE switch which toggles external and internal vehicular communications.

 

 

A very lucky and timely acquisition - a TS-710 crystal test set for frequencies under 1 MHz.

 

 

Here I've inserted a Channel 0 crystal with ~ 370 KHz resonant frequency.

 

 

I set the frequency with the tuning dial to ~ 370 KHz.

 

 

A coax cable from the rear of the crystal tester conveniently connects to a frequency counter.

 

 

Here I've tuned the tester to the Channel 55 crystal (472 KHz)

 

Perfect reading on the meter - half-way, just like the instruction book stated.

 

 

And the drive voltage is nice and low at 0.25 Volts. This crystal is in pristine condition.

 

 

The exact resonant frequency of the crystal.

 

 

Crystals installed in oven.

 

 

Reverse of the crystal oven. The antenna tuning coil is at the left bottom of the frame.

 

 

Metal cover over the filter tuning controls.

 

 

With the cover off, the tuning controls are exposed, along with the complex INTERPHONE / RADIO relay switch. This relay has 21 connection terminals!!!!

 

 

Another photo of the complex INTERPHONE / RADIO relay switch.

 

 

Another photo of the filter tuning controls.

 

 

Antenna relay switch with 6 connection terminals. This relay switches between tuning and operating the transmitter, and between transmitting and receiving through the antenna. It controls transmitter and receiver access to the antenna through the base unit.

 

 

Another photo of the antenna relay switch.

 

 

A photo of the entire underside of the BC-604. It is a jumbled mess. Not only is it exceptionally difficult to service it from this side, but the weather-proofing varnish makes soldering a smelly affair. One positive aspect of the weather-proofing varnish is the good condition of capacitors and resistors after many years. After testing a good cross-section of the components, all were found to be stable and working within specifications.

The large variable tuning capacitor in the middle of this section is used to tune each pre-set frequency along with the buttons are used to mechanically remember the tuning settings.

Relay S103, this controls disabling the receivers when the transmitter is on the air.

 

 

The reverse of the dynamotor male connectors.

 

 

Microphone connection structure. The little "L"-shaped hex key is used in the transmitter frequency tuning process.

 

 

I've attached probes to the microphone terminals to test the signal voltages.

 

 

Another photo of the frequency tuning capacitor.

 

 

This device is used to simulate the pushing of the microphone switch by a)supplying power to the dynamotor and b)connecting to an audio generator and supplying a constant audio source.

When the transmitter is turned on, about 25 volts are supplied to the filaments of the tubes. When the microphone switch is pushed, the dynamotor relay is triggered (S102) supplying the dynamotor with 25 volts. The dynamotor then provides the ~600 Volts to the anode circuits and puts the transmitter on the air.

 

 

The FT-384-B adapter is connected to supply the basic 25 volts from an HP power supply. The red lead creates a loop that aligns with the dynamotor schematic. The other leads connect to my 1000 Volt power supply to separately provide the anode voltage.

 

 

The transmitter is operating: 600 Volts / 170 mA draw by the anode circuits.

 

 

Filament power: 25 Volts / 4 Amps - this is without a dynamotor. With the dynamotor, the total draw of the BC-604 would be about 13-16 Amps.

 

 

Close up of the power connections.

 

 

200 Watt, 30 DBm attenuator, so as to not damage my spectrum analyzer.

 

 

Antenna and ground connections.

 

 

Left-bottom: crystal frequency tuning lever. Middle-top: antenna circuit tuning controls for each crystal frequency. Middle-bottom: meter switch and side-tone volume. Right-bottom: antenna coupling control.

Crystals are installed.

Loosening and tightening the retaining gear for the crystal frequency tuning capacitor.

A fully-tuned frequency response for Channel 15 (21.5 MHz). Note: the middle vertical line on the screen is set to exactly 25 MHz, with each horizontal division of 1 MHz. So the primary frequency response is exactly at 21.5 MHz after tuning at that frequency. After button #3 is released, the tuning settings are mechanically remembered, so if button #3 is pressed again, the frequency will be fully tuned.