E-624, S - 27 „Ultra High Frequency Receiver“; developed and produced by Hallicrafters Co., Chicago IL.
The Ultra High Frequency Receiver Hallicrafters S-27 is a very early all wave receiver covering the VHF range up to frequencies in the 2 m band, this frequency range was in those days adressed as „ultra high frequency“, nowadays we are speaking of „very high frequency“ (VHF).
The set was usually used for band monitoring purposes, even when it was developed in the forties, this was still many years ahead of the use of the VHF range for broadcasting purposes. VHF broadcasting started in the USA in the 6 m band on frequencies around 50 MHz and was later reallocated to the 3 m band up to 100 MHz. Also the aircraft communications were moved to the VHF range from 118 - 137 MHz not before the end of World War II.
The receiver, which was developed around 1940, covers frequencies of 27 - 145 MHz in three ranges, the B-version S-27B the frequency range of 36 - 165 MHz .
The standard S-27 UHF receiver is powered by 115 Volts AC, the common mains voltage found in the USA. A part of the receivers shipped to Europe came with an optional „universal transformer“ (model designation S - 27U), on which, different voltages from 110 - 250 Volts can be selected. So do not simply fix a plug on your newly acquired S-27 and connect it to the European 230 V mains, but make sure the voltage, your receiver is set to, before powering up. If there are any doubts, use a variable isolation transformer and increase the voltage slowly up to 110 V and check, whether your radio is already running properly, so it will be wired for 110 V operation.
The receiver has a metal cabinet with a hinged lid, which - as found in many Hallicrafter's sets, gives access to the chassis to change tubes.
The S - 27 is a hollow state receiver covering the VHF range. The set developed around 1940 covers the 27 - 145 MHz frequency range, the later S - 27B version covers the 36 - 165 MHz frequency range.
The receiver comes in a rugged metal case with a hinged lid giving you access to the chassis to exchange the valves, it's dimensions are 48,3 x 23 x 25,6 cm and it's weight around 20 kg.
On the left side of the front panel, you find the rotating backlit main frequency dial, in contrast to the metal disc dials on older Hallicrafters models, only a part of the dial is visible behind the dial window. In the left lower corner of the frequency dial, you find the three position bandswitch. This lets you select one of three frequency ranges (27,8 - 47 / 46 - 82 / 82 - 143 MHz).
Just below, you find the R.F. gain control, the antenna trimmer, a bandwidth control (SHARP - BROAD) which also acts as mains switch and the A.F. gain / volume control at the right.
At the right of the main tuning dial, you find the fine tuning dial, which is mechanically coupled to the main tuning and is operated with the the typical Hallicrafters „steering wheel“ tuning knob, below you find the switches for ANL (which activates automatic noise limiter), the AM / FM mode control, below the BFO switch and at the right the BFO pitch control used for single sideband and CW reception.
At the right, below the signal strength meter, you find the tone control and at the right the Send / Receive switch (to mute the receiver when used in connection to a transmitter) and the headphones jack. A high impedance speaker (5000 Ohms) can be connected at connectors at the rear of the receiver, to drive an 8 Ohms speaker, you need a matching transformer.
When You have managed to apply the correct power to the receiver, it's operation scheme is different in AM and in FM mode.
Use the selectivity control to turn the radio on (yes - that's typical Hallicrafters, not to combine the mains switch with the volume control), turn the R.F. gain to the maximum setting clockwise and the A.F. gain somewhere in a middle position and select the frequency band. The AVC should be activated (switch to the left) and the BFO should be turned off, make sure, the SEND/RECEIVE switch is in the RECeive position.
In AM mode, You can try to catch some morse code transmissions or communications in the 10 m ham band above 27 MHz with the BFO turned on, sometimes, You get better results in copying CW or single sideband signals with manual R.F. gain control. There is also some AM communications activity in the Air band in the 118 - 137 MHZ, e.g. airport weather reports around 127 MHz.
In the FM mode, look for some FM broadcast stations in the standard FM broadcast band from 88 - 108 MHz, BFO, AVC and ANL is inactive in this mode, the selectivity has to be set to BROAD for broadcasting stations, and the TONE control can be set to HIGH FIDelity or even BASS BOOST. The reception uses a deemphasis constant of 100 usec., the meter is switched to act as a tuning indicator with optimal tuning when the indicator is in the middle position. In some cases, the antenna has to be tuned to maximum signal strength. Besides FM broadcast transmissions, you can hear some amateur radio communications in the 2 m band around 144 MHz and maybe some test transmissions of vintage military gear in the military FM band between 30 and 76 MHz; there is some amateur radio activity around 50 MHz in the 6 m band. Television does not use VHF band I anymore in Europe.
The signal coming from the antenna is amplified in a RF amplifier stage using a acorn miniature tube (V1, acorn tube 956) and is fed to the mixer stage (V2, acorn tube 954), where the oscillator signal (V3, acorn tube 955) is used to generate a 5,15 MHz intermediate frequency. After a first IF amplifier stage (V4, 6AC7/1852), in AM mode, the signal is fed to a second / third IF amplifier stage (V6, 6AB7/1853 and V7, 6SK7) and a diode for AM demodulation (V8, 6H6). For FM operation, the signal has to pass a limiter stage (V9, 6AC7/1852) and is fed to the discriminator stage (V10, 6H6) for demodulation.
An 6C8 is used in as AF preamplifier and two 6V6 in a push pull arrangement in the AF output stage. For the BFO, a 6J5 valve is used.
V1 (956, RF preamplifier); V2 (954, mixer); V3 (955, oscillator); V4 (6AC7 (1852), 1st IF amplifier); V6 (6AB7 (1853), 2nd IF amplifier); V7 (6SK7, 3rd IF amplifier); V8 (6H6, AM demodulator); V9 (6AC7 (1852), limiter); V10 (6H6, discriminator); V11 (6C8, AF preamplifier); V12 / V13 (two 6V6 , AF output stage in push-pull arrangement); V5 (6J5, BFO).
Gl (5Z3, mains rectifier), stabilizer VR-150 / 0D3.
The S-27 is a very early example of a receiver with VHF coverage, it was used primarily for military radio monitoring. On a historical photograph of the VHF monitoring station in the Sphinx Observatory on the Jungfraujoch an E-624 / Hallicrafters S-27 can be recognized as a VHF surveillance receiver.
Even after the set was developed in 1940, it took several years, until the VHF band was used for FM broadcasting in the United States, first the 6m band around 50 MHz was assigned to broadcasting, and shortly after, the 3m band up to 100 MHz was assigned to broadcasting. Air traffic radio services, which use the 118 - 137 MHz frequency range, were moved only in the post-war years from shortwave to VHF frequencies.