This model of receiver was made by Philips from 1928 and was one of the first types made for use on AC mains. It uses early types of valve, predating the universal use of the British 5-pin base. Slightly later, this fifth pin was used to carry the cathode connection to a mains valve but in these early models the cathode connection is brought out to a terminal on the side of the 4-pin base.
The set uses an early tetrode valve for mains use, the Mullard S4V, as RF amplifier and a Mullard 154V mains triode as detector. The loudspeaker output valve is a directly heated type, a Mullard PM24, and relies on hum reduction by an artificial centre tap at the 4-volt heater supply winding. This also provides a degree of auto-bias. Further bias for this valve and the RF amplifier is obtained by feeding the negative HT current through a choke and a series of fixed resistors. The rectifier valve is a Mullard DW2.
As was the case before about 1932, when broadcast stations had not been allocated firm wavelengths, the receiver's dial is merely calibrated 0-180 degrees requiring that a log was normally kept by users so that they could readily tune to their favourite stations.
The configuration of the circuit is known a "straight" or "TRF" design rather than the superheterodyne type used in later years. The reason for the use of a TRF design was quite involved, being partly due to the calculation of tax or levy imposed on users and partly due to patents. The main shortcoming of a TRF design, of the type used by Philips in this model, is the need to simultaneously adjust two tuning controls together with the "reaction" control for best reception. The station log would have recorded the settings for the two controls.
Another shortcoming is the inherent lack of being able to distinguish between two adjacent stations especially when the receiver is off-tune. The method used by Philips for overcoming this sort of interference is a choice of three aerial sockets. These allow for a trade-off between volume and interference-free reception.
In company with many models made in the early to late-twenties the loudspeaker is external to the set. It must also be noted that the output transformer driving the loudspeaker is designed for what is termed a "high impedance" loudspeaker. A recommended type of speaker was used with this model but most horn types will have the correct impedance.
The set is extremely compact. Every cubic inch of space is used and the wiring, which is in a uniform brown colour, is run in cableforms and consequently very difficult to trace. Decoupling capacitors are mostly in multiple blocks and were generally in excellent condition.
Faults revealed after an examination of the set
(1) The RF amplifier valve was faulty.
Repairs carried out
(1) As a direct replacement for the
S4V was not available, a 5-pin equivalent (type AC/SG) has been
used. This necessitated a modification to the receiver to add
a fifth socket to accommodate the extra valve pin. To reduce
manufacturing costs in this mass-produced model, the valve holders
are integral to sections of the Bakelite chassis.
Note that the voltage for which the receiver was designed is marked as 196. This probably reflects the mains supply voltage of the first household in which the set was used, for example one of the London boroughs
I applied 196 volts AC to the sets input and an aerial about 50 feet long to the top socket. I connected a small horn loudspeaker to the output and was able to tune in many stations at full loudspeaker strength.
It takes a little practice to manipulate the controls but the following sequence will work. Tune the two dials roughly in step until a station is heard. By trial and error adjust the two dials separately until the best reception is heard then gradually alter the setting of the reaction control until the desired quality and volume is obtained.
The set is most sensitive at the point
immediately before oscillation takes place.
The final work carried out on the set was to add a ballast resistor to allow the radio to be used on 230v mains. As the overall current taken by the set from the mains supply is quite low, and fairly constant, a simple series resistor has been added. The value of this has been calculated to drop about 40 volts, thus providing close to the original 196V at the primary winding of the mains transformer. The new component is a metal-cased device incorporating a heat sink and is bolted to the chassis, close to the chassis safety switch, to which it is wired in series.
Parts fitted: New AC/SG valve, mains cable, resistors, capacitors and materials etc.
Note: It is unusual to find so many faults present in a set but this appears to have been a result of the owner plugging it into a modern 230volt mains supply. In many London boroughs in the 20s and 30s it was common to find the rated mains voltage to be less than 200 so when the set was plugged in, after over 60 years of being laid up, a combination of the very high HT supply and excessive valve dissipation caused many components to burn out. The original resistors proved to be very unreliable despite being open-wound on half-inch glass formers, overall, probably 20 times larger than modern versions having the same rating.