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American midgets are engaging little sets in two respects. One, they are masterpieces of the multum in parvo school of construction, and two, they often have very attractive cabinets, especially those made of the plastic material called Catalin, which enabled almost any shape in any colour to be produced with ease. Although midgets were inexpensive they were seldom cheap and nasty, as is evidenced by the large numbers of 50- and 60-year-old $5 bargains that are still in good working order. Servicing these sets is an art all of its own, requiring as it does a good knowledge and appreciation of US design techniques, which differed in many respects from those employed by UK manufacturers. For a start we have to bear in mind that midgets, which were invariably of the AC/DC type with series heaters, were designed to work with HT voltages of no more than 90 V, due to the US mains voltages which were between 110 V and 120V, either AC or DC. Most, up to about 1939, used heater chains rated at 0.3 A and employed resistive line cords to drop the mains voltage to suit. After this date tubes (valves) with higher voltage heaters rated at 0.15 A began to take over, and in many cases the chain voltage was close to that of the mains and no dropping resistors were necessary. The first thing to do when you have a midget on the bench for repair is to ascertain whether or not a resistive line cord should be fitted, and if so if it has the correct resistance. This is a necessary task because all too often previous owners or repairers will have either shortened the cord or even removed it altogether, and this must be put right before the set is plugged into the mains. Incident ally, it is worth repeating the advice given else where in this guide that it is far better to use a step-down auto-transformer to power midgets than to attempt to modify them for the UK 230V mains voltage. Look at the tube (valve) line-up to see if it consists of 0.3 A or 0.15 A tube (valve). If you are not familiar with the tube (valve) types find their heater ratings in the data section in Appendix 2. Add the voltages up to find the total chain voltage, and subtract the answer from 117.5, this being the accepted mean voltage for US mains. If, with a 0.3 A chain, the difference is more than about 30 V expect to find a resistive line cord. 0.15 A chains will usually be within 10V or 20 V of 117.5 V and seldom require one. You can work out the required resistance of a line cord using Ohm’s law, all that is necessary is to divide the difference between the chain voltage and 117.5 by the chain current. For instance, the average TRF midget with 0.3 A heaters has a total chain voltage, including that of a pilot lamp, of about 65V. Subtracted from 117.5V this gives 52.5 V to be dropped, and dividing this by 0.3 gives the answer 175 ohm. The usual standard was 60 ohm per foot for 0.3 A line cord so its length will be about a yard. A 0.3 A superhet with one more tubes (valves) needs about four inches less. Check the actual length of cord on the set, bearing in mind that there will be about four or five inches under the chassis, and if the lengths are more than a few inches less than those given don’t plug in the set but measure the exact resistance with a meter. Tackling faults on resistive line cords What can you do if the line cord is too short or, worse still, open circuit? As has been stated earlier, resistive cords worked perfectly well when left to themselves and gave no trouble. When problems occurred they were usually traceable to owners having wound them into neat coils which overheated them, or to snipping odd lengths off them. Remember, though, that most American midgets were converted to work on UK mains by the addition of an extra length of line cord, and although this might now be too short for 230V it would be more than adequate for 110/120V operation from an auto-transformer. For this reason it is a good idea to collect and store any odd lengths of line cord that you might come across, because they might well be of use in the future. Should you find that the cord is only a few inches short you may be able to get away with fitting an extra dropping resistor on or under the chassis. Six inches of 0.3 A cord equals 30 ohm, which could be replaced by a 30 ohm wire-wound resistor. The dissipation would be 2.7W, which should not be troublesome even in the smallest of midgets. Likewise, a foot of cord could be replaced by a 60ohm resistor; as the dissipation would be 5.7W a 10W type should be used. The heat produced is probably about as much as could be tolerated in a midget so don’t go beyond this. If the cord appears to be o/c, don’t give up hope until you have tried chopping off about an inch and a half from the mains plug end, where constant handling may have caused the resistance wire to break. Many a cord can be brought back to useful life in this way. If all else fails you may like to consider using an external mains dropper in a metal safety cage. Remember that in sets with 0.3 A chains the cord feeds only the tube (valve) heaters, with the anode of the rectifier going either directly to the live mains input or via a small stopper resistor of around 30 . Another possibility is to replace a 0.3 A chain with 0.15 A equivalents, this bringing up the heater voltage to the point where only a small dropping resistor, if any, is required. Extra precautions necessary with midget sets It was common practice to use only a single-pole mains switch to break the side of the mains lead that went to the HT— line and the bottom of the heater chain, which often was the chassis of the receiver. The interesting situation arises that if the mains lead is plugged in to make the chassis neutral while the set is working, it will become live when switched off and vice versa due to the path from the top of the heater chain back down to chassis. It is therefore essential to replace most carefully all the various devices such as covers for chassis bolts and grub screws employed to pre vent owners from accidentally coming into con tact with live metalwork. To their credit, many US manufacturers were aware of the danger of shock and eliminated it by isolating their chassis altogether. Instead of using it for the hearer and HT— returns a busbar was fitted within the set to do the job. As this was unfamiliar to British eyes it was all too easy for an ignorant engineer to frustrate the good design by shorting down the HT— line to chassis, typically when replacing smoothing capacitors. Always examine carefully the wiring of American midgets around the on/off switch. If one side does not go to chassis but to various insulated points these are almost certainly the HT— busbar in an isolated-chassis set. Check the resistance between the busbar and chassis — it should not be less than about 250 k-ohm. If a low resistance is registered the cause must be investigated and put right or the safety of the isolated chassis will be lost. Types of midgets Most of the midgets you are likely to meet will be either TRFs using an RF amplifier, a detector, an output tubes (valves) and a rectifier, or superhets using a frequency changer, an IF amplifier, a detector/AF amplifier, an output tubes (valves) and a rectifier. Thus most TRFs have four tubes (valves) stages and most superhets five, but in practice the use of combination types means that the actual tubes (valves) count may be less. For instance, a triode pentode such as the 12B8GT or 25B8GT may carry out the work of both the RF amplifier and the detector in a TRF, while rectifier-pentodes such as the 25A7GT or 70L7GT may provide both the output and the HT supply. By using, say, a 25B8GT and a 70L7GT together the equivalent of a four- tubes (valves) may be built with only two. However, the circuitry will be much the same as for all TRF midgets. Rectifiers and HT smoothing Although, as with all AC/DC receivers, half wave rectification was used in midgets, in 0.3 A Repairing sets you will quite often find that a full-wave type was used with the sections strapped. If the set also has an energized loudspeaker it is quite possible that one half of the rectifier may be used simply to supply the field winding, with the latter being connected between one of the cathodes and HT—. A capacitor of about 8 uF to 16 uF will be shunted across the field to smooth the HT passing through it. Incidentally, don’t be surprised to find that the speaker is of the moving-iron type. As regards the main HT smoothing, it was standard practice in all midgets to connect the top end of the output transformer primary directly to the rectifier cathode. From the same point a resistor of up to 10 k-ohm both smoothed and dropped the HT for the screen grid of the output tubes (valves) and the rest of the tube (valve) electrodes. It was usual to employ a twin electrolytic capacitor (typically 20 uF+ 20 uF) for smoothing. The working voltage rating normally was no more than 150V, so a UK type with a 250VW rating will be more than adequate — but watch the ripple current rating and how the negative is connected. When an isolated chassis is used a replacement capacitor should have its metal case insulated by a plastic sheath or by several turns of tape. No decoupling required Because the smoothed HT in a midget is no more than 90V it is unnecessary to employ dropper resistors and decoupling capacitors for the screen grids of RF or IF amplifiers, or frequency changers, these being connected directly to HT+. It is also possible in many cases to dispense with cathode bias resistors and capacitors for IF amplifiers, so the underside of a midget can looked surprisingly uncluttered. Output stages The same basic circuitry is used for both TRFs and superhets. The tube (valve) will be either a pentode or beam tetrode capable of giving more than ample output and it is common for the by-pass capacitor across the cathode resistor to be omitted, thus giving a measure of negative feedback to improve the tone. As with all types of radio set, always check the coupling capacitor that feeds the grid for leakage. Detectors in TRF midgets Either grid-leak or anode bend detectors were used. Note that when the former type is employed the grid capacitor and resistor may not necessarily be connected between the top end of the tuning coil and the grid of the valve, but may be inserted between the bottom of the coil and chassis/HT—. The necessary grid bias for an anode-bend detector is obtained by using a high value cathode resistor, typically between 10 k-ohm and 22 k-ohm This will have to be decoupled, but the capacitor seldom fails. Reaction was very seldom used in midgets; on the few occasions when it was a preset reaction capacitor was fitted. The most popular type of tubes (valves) for the detector position was a straight RF pentode although a few midgets used a high-mu triode. When a pentode is used the very low screen-grid voltage necessary may be derived from the cathode of the output valve. RF amplifiers in TRF midgets A variable-mu pentode is always employed, with volume control effected by a potentiometer (usual value 25k-Ohm—50k-Ohm) in the cathode circuit. Its wiper is connected to chassis/HT—, with one end going to the cathode of the tube (valve) and the other to the top of the aerial coil. As the volume is advanced the cathode voltage is reduced and the amount of resistance across the aerial coil is increased, but when the volume is turned down the cathode voltage is increased and the amount of resistance across the aerial coil is reduced. It is usual to employ some means of preventing the cathode from being taken directly to chassis/HT— at full volume, sometimes by a fixed external resistor of about 100—300 ohm, sometimes by an extra length of track on the control after the stop for the wiper at maximum. A point to watch: Americans tended to use M as a symbol for l000fl on resistors so a control marked 25 M will have a resistance of 25 k-ohm. Meg was used to indicate 1,000,000 ohm. The Americans were good at producing highly efficient tuning coils. A feature of them not usually found over here was the use of a coupling loop, a single turn of fairly thick wire, to improve the coupling between primary and secondary. This was employed in both the aerial and RF coupling coils, which were very similar in design. The standard coverage was from 550 kHz to 1700 kHz, with the last digit ignored on the dials, which were marked from 55 to 170. A typical feature of the dial was to have the word ‘Broadcast’ inscribed near the 55 end and ‘Police’ near the 170 end, a reminder of when many US police forces had radio communications working on the low end of the MW band. Before the Second World War a certain amount of American midgets were exported to the UK, and these were adapted to cover long waves by the fitting of extra coils and a wavechange switch. As mentioned elsewhere in the text, during the war some 100000 American midgets were imported into this country to make up for the shortfall in domestic radio production. At that time the LW band was not being used for domestic broadcasting so there was no need for the sets to be modified. Detector/AF amplifiers in superhet midgets Almost without exception a double-diode-triode was used with its two diodes strapped. Minimal IF filtering was used, possibly just one capacitor of about 100 pfd to chassis from the top of the diode load, which itself might well be the volume control. Grid current biasing was usually employed for the triode section of the valve, with the cathode taken directly to chassis/HT—. AVG bias was drawn from the detector load with seldom more than one decoupling resistor and capacitor to feed the IF amplifier and frequency changer. IF amplifiers These were simple in design and there was very little to go wrong, with no feed or bias resistors and decoupling capacitors to worry about. Note that the second IFT might be unscreened and fitted beneath the chassis, while in very small midgets it was sometimes omitted and the detector diode fed from the IF amplifier anode by inductance capacity, a simple tuned coil being fitted in the anode circuit. Very occasionally two IF amplifiers were employed, but not always exploited fully since one of the IFTs might be untuned. Frequency changers Heptodes generally were preferred to triode hexodes. In many cases the local oscillator coils were wired into the cathode circuit of the valve. When a feed resistor is used to supply G2 + G4 it has to carry a fairly heavy current and should be checked if oscillation ceases. The usual IF was 455 kHz, and the frequency often was printed on the cans of the IFTs. ‘Tracked’ tuning capacitors were popular in midgets as they enable conventional padding capacitors to be omitted. RF amplifiers in superhets These were sometimes employed, possibly more as a sales feature rather than for their actual performance. There was seldom room or the inclination to fit a three-gang tuning capacitor so the RE stage usually was aperiodic, with resistance capacity coupling being used between it and the frequency changer. Curiously enough, the RF amplifier tubes (valves) might well be a triode. Aerials TRF midgets nearly always had a ‘throw out’ aerial consisting of about eight to ten feet of thin flex which could be draped around a room more or less out of sight behind furniture, etc. Usually small clips were provided on the cabinet back or underside onto which the aerial could be wound when out of use or for transportation. Superhet midgets also often had throw out aerials but latterly frame aerials became popular usually glorified by names such as ‘wave master’. Experience shows that they sometimes leave much to be desired in the way of sensitivity, so if a midget with a frame aerial gives a disappointing performance try the effect of bringing your hand close to the frame to see if its pick up is improved. If so it might be worth considering adding your own throw out aerial coupled into the top of the frame via a 0.001 uF, 300VW isolating capacitor. |