Repair of a Philips Model 834A Superinductance

Before the advent of superhet receivers for the domestic market receivers used a design known as "TRF" or "Tuned Radio Frequency". These sets employed tuned circuits operating only at the signal frequency and if one needed a lot of gain, several tuned circuits associated with more than one valve were needed. In the UK triode valves were superseded by RF tetrodes or pentodes giving much more gain. In the earliest sets, which looked more like pieces of scientific apparatus you might have found in a school physics lab, there were often separate tuning dials for each set of coils and each knob had to be set precisely in order to get a decent speaker volume. In some areas close to a local transmitter the separate tuned circuits also provided a degree of interference rejection. Most TRF sets had dials calibrated from 0 to 180 degrees and a tuning aid chart was often fastened to the set so that users could readily set the dials for best reception of the desired station. Searching for new stations was quite laborious as the dials had to be set to roughly the right settings for anything to be heard. Tuning was therefore quite an art.

Front chassis view showing the volume/mains switch and the tuning/wavechange switch

The design of the Philips Superinductance receivers was a major step forward because only a single tuning dial was used. All the tuned circuits were coupled together, or ganged, so that a user could tune across the entire waveband without loss of gain and therefore even very weak broadcasts could be heard without difficulty. The Superinductance design required extreme accuracy in component manufacture and careful construction to make the components associated with tuning exactly the same. In particular, coils needed to be wound so that they were identical in characteristics otherwise the gain of the set would drop off across the tuning range. To aid alignment or to adjust individual sets for precise tuning alignment trimmers were used. In the Philips 834A these trimmers are tubular ceramic based components having a very low loss and excellent stability. The interior of the set looks different to most TRF sets as the coils are housed in copper screening cans. These cans are part of the overall design and help to make the set stable in use. Moving on to another feature of a British or European TRF receiver; the method of achieving sufficient gain for reception of distant stations. To explain this I should first mention the high price of receivers. A commercially manufactured set represented an enormous investment for a typical family and one of the reasons for the high price was connected with patents. Everyone will have heard the name Marconi, who was one of the foremost experimenters. During his experimentation he soon realised the future importance of wireless communication and made the decision to purchase the many patents associated with the science. At the time the basic principles of radio were looked on as rather interesting scientific curiosities and the experimenters had no idea about specific uses for the strange things they saw in their laboratories, and had no idea about the value of any patents that would later form the basis of wireless communication. When broadcasting began after World War 1 and organisations like the BBC began to operate radio networks there was a lot of discussion about money. Initially, in the case of the BBC it was compulsory to use only sets, and many components, that were registered for use with the BBC. Most early items have an oval BBC label printed on them. Once receivers began to be manufactured by commercial concerns, Marconi stepped in to demand royalties for using his patents and it was soon agreed that the amount of royalty should be calculated by counting the number of valves in the set.

The case, with bakelite front and a thin pliable matching material called Arborlite folded into a front panel slot

As sets were already fairly pricey for several reasons and, now with the extra levy demanded by Marconi, it became apparent that in order to cut costs designs should make use of the minimum number of valves. Making a valve do more than one job was possible. Some sets used the same triode as an RF amplifier and an LF amplifier, but the key design factor in a TRF set was regeneration. Regeneration or controlled feedback could dramatically improve a set's performance, so in the UK a new control soon appeared on receivers. If this control knob was rotated the volume of the tuned station got louder and louder as if by magic. Those users familiar with early crystal sets or single valve receivers must have been amazed at the improvement. Instead of stringing up longer and longer aerials they could just twiddle the reaction control, as it was commonly know, and listen in comfort to distant stations. The reaction control unfortunately had a serious drawback. As the knob was turned there came a point where the signal suddenly failed to get louder and a faint high pitched whistle could be heard. The whistle could also appear if a set tuned to one station was tuned to a different dial setting. For example tuning from the whistle free National Service to a regional station might cause the set to whistle, and it wasn't always obvious to the casual user. Next door however your neighbour, already listening to the local regional station, suddenly heard an extremely loud squeal or howl that completely drowned reception. I have only slight personal recollections of this problem and I suspect the vast majority of my recollections were in fact due to interference between medium wave stations at night. The problem of "howling" speeded up the design of the superhet receiver, whose local oscillator was on a different frequency to the tuned station and therefore didn't upset your neighbour. Most wireless listeners knew about the reason for interference from poor adjustment of reaction controls because, in those days, there were dozens of magazines published for the listener and radio constructor. These magazines were extremely popular because they informed the man on the street how to make a receiver at a fraction of the price of a commercial set. They also carried readers' letters complaining about interference from a neighbours set. The government even defined the maximum aerial length you could erect in your back garden. This at least would limit the range of interference. The Philips Superinductance design included measures to obviate howling. The set operated with sufficient feedback to provide a high gain but was arranged so that the set never broke into oscillation at the signal frequency. This was accomplished by careful design connected with valve biasing and by excellent screening between amplifying stages, hence the use of the copper screening cans. It's worth noting also that one of the coils is mounted at right angles to others. Although it looks odd and conflicts with matters of symmetry, this significantly reduces stray coupling which might have resulted in unwanted feedback. Now onto the overhaul of the receiver.... The first step in repairing an early mains receiver is to examine the mains input circuit. When this set was designed in 1932 for AC mains use, the domestic voltage could have been virtually anything so there should be a selector plug or some similar arrangement for setting the local voltage. This Philips set was satisfactory in this respect as it was already configured for the range that included 240 volts. Another Philips receiver in my collection was set to 90 volts and the primary of the mains transformer is open circuit so it's important to check before plugging it in! Next, examine the mains lead as this would normally have been cotton covered rubber and rubber of this age will usually be in very poor condition. In fact, if the mains lead is original, as was the situation here, replacing it with a modern lead is the only option. A purist would look for a lead of similar construction, but sensibly a plastic insulated lead terminated in a 13-amp plug is fine. This set uses a special 2-pin connector which must be unplugged before the back of the set can be removed. Should a new lead carry a safety earth or not? Safetywise the answer must be yes. For practical reasons however the answer is no. This model happens to be for use on AC mains and as such incorporates a transformer for developing LT and HT and the chassis, which has lots of exposed areas at the back, can be touched by the user. Philips recommends that the chassis is connected to a good earth point. Probably not for safety reasons, but for improved reception, and certainly not with a screwed connection, but via a wander plug.

Rear chassis view showing the mains selector arrangement and 2-pin mains plug

Any mains equipment if not earthed will have an AC voltage present on exposed metal parts because of capacitive leakage although the voltage source will have relatively high impedance and consequently have a very low current capability. Touching an exposed metal part of an unearthed receiver will give one a slight tingling or a rubbery sensation in the fingers. A mains safety check would result in failure. Back in 1932 the set would have been considered totally safe or more accurately safe when it left the factory. However there are specific faults which would make the set dangerous to use, for example a short circuit between mains live and the chassis. I guess to comply with today's standards the set should be powered via an isolation transformer? The next task is to check any other rubber covered cables. The Philips used a length of such cable between the loudspeaker and the output circuit. Many valve receivers employ a loudspeaker which has an integral output transformer. This means the connecting cable will be carrying HT and must therefore be in good condition. The cable in this example had lost all it flexibility and the rubber insulation was cracked and bare wire was visible. I replaced the cable with modern plastic covered wires as the owner was not worried about maintaining authenticity. There are now a number of checks that must be made in the interests of prudence. Two particular capacitors need to be removed and tested, or better, just removed and replaced. One is the capacitor connecting the anode of the output valve to chassis. If this were to go short-circuit the output transformer would be destroyed. Philips actually connect this capacitor across the output transformer primary so its failure would not damage the transformer and I found the capacitor (C14) to be reasonable in terms of DC leakage, but wildly out in terms of capacitance. It measured around ten times its marked value of 0.005uF and would result in lots of top cut, or loss of sharpness in tone. I fitted a modern capacitor having the correct value. The second capacitor (C11) which should be tested is the one connecting the anode of the LF amplifier to the grid of the output valve. The grid circuit has very high impedance and the grid voltage dictates the anode current. Even a tiny DC leak through the coupling capacitor will place a positive voltage on the grid resulting in a high anode current and an excessively hot valve and consequent short life.

Old repairs with the metal cased capacitor block just visible

Roughly speaking... as an example. If the anode of the LF amplifier carries 100 volts and the input impedance of the output valve is a megohm then a coupling capacitor leak of a megohm will result in 50 volts on the grid of the output valve. Strictly speaking this will not be the case because the output valve will start to draw grid current once the voltage rises above zero and the input impedance will then drop and reduce the voltage, but you can see the principle. I fitted a new modern capacitor. In fact all the capacitors in the set needed replacing. Philips use a metal cased block carrying six decoupling capacitors. Two had been replaced previously. I fitted four to replace those remaining. I also replaced one of the two earlier replacements. Next I looked at the HT circuit. The rectifier valve filament connects to a pair of 16uF wet electrolytics (C15 & C16). Both of these measured open circuit because the electrolyte had long since leaked away, as was clear from the corrosion.

Pair of leaky wet electrolytic capacitors

I fitted two modern 10uF 450V capacitors. A purist would no doubt remove enough of the contents of the original cases and fit the new parts out of sight. My solution was to disconnect the old and solder in place the new. A word of warning here.. These old valve rectifiers must not be connected to new capacitors having a value in excess of the original capacitors. Each valve type has a recommended maximum value of reservoir capacitor. An old 8uF can safely be replaced with 10uF but a 470uF would dramatically shorten the life of the valve. Sooner or later, if possible the valves can be tested. This is not a vital step as replacements can be tried by substitution, but certainly helps if there is a fault in the set. As I have an old Avo valve tester I tested the valves. These were originally MM4V 1st RF amplifier, 2nd S4VB RF amplifier, 994V 3rd RF amplifier/detector, PM24A output valve and 1821 rectifier. The 994V had been replaced by a 904V and the output valve by an Osram V6. All had good filaments, but all showed up as having very poor emission. Increasing the filament voltage provided the proof, as emissions shot up when I moved from 4v to 6v. This is not particularly serious as I believe emissions had probably dropped because of the set being laid up for 40 years and will improve with use. Next I measured all the resistors. To my surprise all were in excellent condition and very close to the values shown on the circuit diagram.

Although I have thousands of old radio valves it isn't always easy finding the right one, but after an hour or so I discovered several that were considered close equivalents to the S4VB with the microphony problem. Plugging an MS4 into the set produced a little more volume and an absence of crackling and microphonic effects. Next I checked the alignment across the medium waveband. By tweaking a couple of tubular trimmers I was able to increase the gain at the HF end of the two wavebands. Because of the very accurate coil manufacture gain was now pretty well flat across the whole of both bands. Ordinary sets might have both trimmers and padders. The latter for effectively balancing the coil inductance at the low frequency setting of the tuning capacitor (in fact it alters the tuning capacitor value as seen by the coil which usually has an adjustable core). The Philips coils are not adjustable because they are accurately matched in the factory. During testing I found the aerial worked fine in the socket marked A2 but not particularly well in socket A1. Checking the service sheet produced by The Wireless & Electrical Trader on March 13th 1943 and the circuit diagram explained why. Socket A1 isn't connected to anything! This socket was intended to be used by the listener that lived very close to a large local transmitter and relies on the tiny capacitance between the A1 and A2 sockets. Why was this? Well this set is an early TRF design and hasn't got AVC. Automatic Volume Control sometimes called AGC or Automatic Gain Control is the feature used by virtually all receivers made after this Philips set; that is in superhet receivers. The AVC system uses a simple receiver operating in parallel with the main set which produces a voltage proportional to the strength of the received signal. This voltage, generally negative, is used to apply bias to the RF amplifying stages and determines the overall gain of the set. A strong signal produces a large negative bias voltage which reduces the anode current of the RF amplifiers. This results in the received signal weakening in turn making the bias to be less negative and increasing the gain. The bias then goes more negative and decreases the gain and so on. The AVC circuit quickly establishes equilibrium, only changing if the incoming signal fades or strengthens. AVC was such an important design feature that special valves were introduced. Because changing the bias voltage basically only changed the anode current of a valve and didn't have a dramatic effect on its gain, a method of gain change needed to be introduced. The "variable mu" valve was designed to overcome the problem. This new valve had a different type of grid. Instead of being wound as a coil with the same spacing between turns, the turns were spaced differently along its length. As the anode current changed more or less of the grid became active. Using this principle an amplifying stage could be designed to operate at a given anode current at which the control grid voltage was able to change the amplification. The term "mu" is used to describe the amplification factor of a valve, hence "variable mu" means variable gain.

No expense spared screening

Because this Philips receiver has no AVC a strong signal would overload the circuitry and produce distortion, hence A1 was available if one was listening to one's local station only a few miles away. Plug the aerial into A2 and tune away from the local station and the set could hear really weak stations. Because the tuned circuits are of high quality the set will not then be troubled by the local station. Note that the word "quality" is an early technical term referring to the physical construction of the coils. For any tuned circuit using a coil there will be a resistance, or impedance, at the frequency to which it is tuned (resonant frequency), and in fact, a range of impedances at other frequencies lower or higher than the resonant frequency. One could wind a coil from thin wire or very thick wire, steel or silver plated copper and the difference between these is described by their quality or "Q". The lower the DC resistance of the wire the better the Q and the lower the impedance (or resistance at AC) the better the Q. What this means in practice is the better the Q the louder the received signal and the better the rejection of signals that are not tuned in. The Philips Superinductance range of sets, which includes this model, use high Q coils enabling them to operate well even in the vicinity of strong local stations because the narrow passband rejects all but the station to which one is listening. The term "passband" is a technical term which defines the width of a tuned circuit in terms of the voltages at a point off-resonance when the voltage drops to half that measured at resonance. In theory the Q could be so high that one couldn't hear any modulation on the received station, but in practice especially when several circuits are operated in series as in a radio, the manufacturing tolerances will stagger the centre tuning points. "Stagger" is another technical term which indicates tuned circuits in series are adjusted to slightly different frequencies. When the Philips set is tuned towards a station one suddenly hears it emerge from the background noise with good clarity. As the station is tuned in it gets louder but also begins to lose it clarity or sharpness and becomes mellow sounding. I think this explains the quality of sound one associates with old receivers.

Modern-looking loudspeaker

Modern receivers are usually designed completely differently. RF tuning as such may not exist because a broadband front end is used which gives a flat gain over the whole range of operation. In these sets the passband is fixed by a special device inserted in the amplifying circuits. These amplifying circuits have a flat-topped frequency response which is wide enough to provide high fidelity reception of a signal so gone is the mellowness of the old designs. Back to the overhaul of the 834A... Putting the chassis back into its case revealed a practical problem. At some time the knobs had been replaced. Not normally a problem, but the switching from medium to long waves requires one to pull the tuning knob and vice versa. The replacement knob on the tuning control had not been pushed in for years because the clearance to the case wasn't big enough. When it was first fitted it pushed the spindle in and selected medium waves, but after it had been pulled out for long waves there wasn't enough clearance to push it back in again. The repairer, perhaps the one that fitted the replacement decoupling capacitors in 1953 or whenever, switched on the set and checked medium waves, then pulled the knob and checked long waves. All being well he delivered the set to its owners who must have listened only to Droitwich on 1500 metres, not realising medium waves were missing.   I have a theory that most early radio sets that still exist today were put away in the loft when they didn't perform properly because they were still worth lots of money. This set had certainly not been worn out and looks to be in remarkably good condition. Maybe the owner decided to put it away till they could afford to find out why it wouldn't receive the Home Service? After all they'd just paid a small fortune to have a couple of valves and capacitors changed so they certainly weren't going to throw it away; just put it in the loft for a few weeks. That was probably 60 years ago. As a postscript... The set used a Cossor V6 output valve, but little is listed about this type (I think there is also a "V5"). I wonder if the code resulted in so much confusion that the numbers were changed.

Maybe someone knows about the V6?

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