EKCO-PB189-PB289-1938 电路原理图.pdf
RadioFans.CN BVWS Trader Sheet CD-RomRadioFans.CN BVWS Trader Sheet CD-RomRadioFans.CN RadioFans.CN Most of their receivers had distinctive plastic cabinets, but Ekco chose wood for the 1938 model PB289. At the top centre of the dial is the magic eye tuning indicator. Note the row of tuning selector buttons down the right. Left: Dominating the rear of the chassis is the motor tuning assembly. Two semicircular rails carrythe fingers which contact the commutating segments on the large Paxoline disc. iimtage II?iidIiit by PETER LANKSHEAR Something different from the UK By the late 1930s receiver design was generally standardised and predictable, with a host of locally-made radios dominating the Australasian scene. But during the short period from 1937 to 1940, New Zealand was fortunate in having the Ekco brand receivers imported from England, and one of their 1938 pushbutton models, the PB289, is worth studying as an example of up market British design. Founded in 1922, the E.K. Cole Com- pany of Southend-on-Sea soon became a major British manufacturer, with exten- sive facilities including a plastic mould- ing plant. At one stage they even made their own valves, which, although given their own type numbers, were equivalent to the standard Mallard range. Having in 1931 pioneered the use of plastic, Ekcos Bakelite cabinets became a major specialty and in 1933 they em- ployed leading industrial designers to create innovative and imaginative styles. Although concentrating on distinctive moulded cabinets, they did use wood for some of their top line receivers, includ- ing the model were going to look at here. The PB289 has a nicely proportioned cabinct with a very large square dial cov- ering three bands the European long wave band from 150 to 300kHz, the standard medium wave or broadcast band, and short waves from 6 to 18MHz. To the right of the dial is a row of 12 pushbuttons. Pushbutton tuning, originally used in car radios, was the fashion feature for 1938 domestic receivers. According to one authority, of the 665 new British models for that year, no fewer than 231 had pushbutton tuning. Three major systems were used. Most common were switched preset semi fixed-tuning capacitors or inductors, and telephone-type dials with finger stops linked to the tuning capacitor. More complex and expensive was the motor- driven tuning capacitor used in the PB289. The PB289 motor can be used in the pushbutton mode to select broadcast band stations, and also to assist manual tuning. As it also controls bandswitch- ing, there is no handswitching knob! In- stead, the three lower white pushbuttons are used to select the manually tuned long and shortwave bands as well as broadcast band manual operation. Enclosed back The PB289 incorporates two good fea- tures frequently found in European re- ceivers. One was to protect the rear of cabinets with fibre panels, which al- though of questionable acoustic value, served to prevent contact with live fermi- 88 ELECTRONICS Australia, February 1992 RadioFans.CN 89 ELECTRONICS Australia, February 1992 RadioFans.CN The underside of the chassis can be accessed by removing a panel on the cabinet bottom . Although many components are mounted on tag panels, the wiring has the familiar rats nest appearance. Note the motor and drive shaft in the centre. VINTAGE RADIO nals and which now provide a bonus for the collector by their having discour- aged meddlers, dirt and mice! The other feature was a removable panel on the underside of the cabinet, providing access to the wiring without the need to remove the chassis. With the back removed, the British metal-sprayed valves are immediately apparent. This 4-volt heater series was rarely seen in locally made receivers which, at the time, generally used Amer- ican pattern valves, with a sprinkling of the Philips side-contact P based series. Dominating the rear of the chassis is the motor tuning control system disc, with its silver-coated contact plates and a frame fitted with two rows of adjustable contact fingers. A circuit of the PB289 is not readily available, and the accompanying dia- gram is of the slightly simpler PB189. Differences are minor, the PB289 having the addition of a magic eye tuning indi- cator and motor drive for the wavechange switch. Although the circuit appears to be complex, the PB receivers were basically conventional band-switched superhet- erodyne receivers comprising a triode- hexode mixer, an IF amplifier, a diode triode detector-audio amplifier and a power amplifier plus of course, a rec- tifier. Each stage is significantly different in detail from contemporary local prac- tice, and the component count is greater than for equivalent locally made receiv- ers. Two additional valves, V3 and V4, are the heart of an automatic frequency control system, necessary to compensate for any lack of precision in the pushbutton tuning mode. An eighth valve is a P based type TV 1 magic eye tuning indicator. The design is conserva- tive, with plenty of bypassing contribut- ing to stability. Image problem The very low intermediate frequency of 126.5kHz simplifies tracking and pro- vides considerable gain and selectivity, but also creates serious image problems, especially on short wave. Extra tuned circuits, following the aerial, are used to minimise images on the long and me- dium wave bands. A different method of aerial coupling is used for each band. LA is a conven- tional primary winding for shortwave, and longwave signals are connected through a loading coil Ll. Broadcast band coupling is to a tap on L2, an effi- cient method commonly used for car ra- dios, but ideally must be tuned for indi- vidual aerials. C2 is a phasing capacitor for further reduction of broadcast band images. The oscillator circuit of the triode- hcxode mixer Vl is complicated by the automatic frequency control valve V3, a general purpose type 354V triode, con- nected to HT via extra oscillator coil windings. V3 pulls the oscillator fre- quency, to an extent governed by the po- larity and amount of its grid voltage derived from the discriminator valve V4. A type VP4B, having a screen grid rat- ing of 250 volts rather than the more familiar 100, is used as the IF amplifier valve V2. The second IF transformer has a centre-tapped winding (L21) to feed V4, a 2D4B double diode discriminator. Similar to those used in FM receivers, the discriminator in this application gen- erates the AFC control voltages. When the receiver is accurately tuned, there is zero voltage at the junction of R13 and R15, but off tune a voltage is generated, with a polarity and magnitude depending on whether the signal is above or below resonance, and the degree of mistuning. By controlling the anode current of V3, this voltage corrects any tuning errors. Effective AGC The diode detector configuration is slightly unconventional. Instead of the usual IF secondary winding, a small coil (L20) closely coupled to the primary of the second IF transformer is connected to L22 and C44, the combination being res- onant at the intermediate frequency. As AFC requires an effective auto- matic gain control system, the PB289 has an effective system with a delay of 2.5 volts, the voltage of the cathode of V5 above earth. C25 (which is rated in centimetres, an obsolete unit equal to 0.9pF) couples the anode of the IF ampli- fier anode to the second diode of VS, a type TDD4. The negative voltage from the rectified signal is the AGC voltage, and is applied through R11 to the grid of the TH4A mixer. Only half the available control voltage is fed to the IF amplifier control grid. This is good practice, as the anode cur- rent of V2 is not reduced sufficiently with large AGC voltages to limit its sig- nal handling ability. The usual terminals were provided for a gramophone pickup. However, in the case of the PB289, they are labelled Pickup or Television Sound and could be used in the UK with a low priced add-on TV unit made by Ekco for recep- tion of the recently inaugurated Alexan- dra Palace television transmissions. The medium-mu triode section of V5 operates as an audio amplifier resistance coupled to the PenA4 output pentode. The PenA4 was one of a family of Euro- pean high transconductance pentodes, which had no American designed equiv- alent. Similar valves, but with 6.3-volt heaters, were the EL3 and EL33 bet- ter known locally. These valves were twice as sensitive as the 6V6G, and in many receivers were successfully driven directly from a diode detector. Negative feedback One feature put the Ekco output stage considerably ahead of its time. Negative feedback had been developed by the Bell Telephone Laboratories to reduce cross- talk in multiplexed telephone amplifi- ers. By 1938, primitive negative feedback was being used around the out- 90 ELECTRONICS Australia, February 1992 RadioFans.CN put valve in some receivers, but usually this was simply a sample from the anode coupled back to the control grid. Al- though design becomes critical, feed- back is more effective if it includes the output transformer, and also is around more than one stage. Some contemporary Australian HMV receivers did use feedback from the voice coil winding over two stages. Around 1936, the BBC had patented the use of a separate feedback or tertiary out- put transformer winding for improved stability. Ekco used this method in the PB289, the feedback signal being ap- plied through R24 to the bottom end of the volume control. It is surprising that the system of con- necting the feedback to the volume con- trol was not used more, as it has some good features the chief being that, due to the shunting of the detector diode, the amount of feedback decreases as the vol- ume control is advanced and conse- quently, gain is not limited by feedback A further uncommon feature is the combination of L25 and C34, connected across the output transformer primary and used as a series-tuned 9kHz whistle filter. Permag speaker The power supply is conventional. using choke L8 instead of a speaker field for filtering. Unlike contemporary local and American loudspeakers, which still used electromagnetic field magnets, Ekco loudspeakers had permanent mag- net fields. British manufacturers had adopted Alnico alloy in 1936, and were well ahead in permanent magnet devel- opment. Rather than the usual 8 speaker gen- erally found in larger mantel receivers of the period, Ekco managed to fit in a 10 unit, with an improvement in bass re- sponse. Rugged, reliable The motor-driven tuning mechanism of the PB289 is rugged, simple and well built reasons for the unit in the re- ceiver illstrated still working flawlessly after more than 50 years. At the heart of the system are a twin field motor and a fibre disk about 15cm in diameter. Attached to the rear face of the disc ;are silver-plated commutating segments, in the form of two half circles with a lmm gap between them. Surrounding the disk is a frame carry- ing adjustable clips carrying fingers in contact with the commutating segments, each one being connected to the return of a motor field winding. Each finger is in turn connected to its own pushbutton, which when depressed, completes the circuit between a segment and earth, energising the motor which rotates the tuning capacitor and disc to- wards the gap between the segments. As the finger concerned encounters the gap, the motor is open circuited, and the rota- tion of the tuning capacitor stops at the position of the desired station. As a clutch ensures that the stopping is instan- taneous, location accuracy is quite good, with any minor tuning errors corrected by the AFC. Instead of a wavechange knob, the PB289 has a pushbutton for each of the three bands. Connected to the wavechange switch is a small disc, also with motor control segments. When a wavechange button is depressed, an elec- tromagnetically activated dog clutch couples the motor drive to the wavechange switch, which is rotated to .the required position. If the medium wave change pushbutton is left de- pressed, tuning becomes manual but with motor assistance if required, con- trolled by buttons either side of the main tuning knob. How does the PB289 perform? The pushbutton tuning works well, and there is good sensitivity. Tonal quality is above average. Used as intended, primarily for listening to local stations, it is an excel- lent receiver. The only real criticism is the image reception, which is apparent to a degree on the broadcast band and is very bad on the 6 to 18MHz band. Motor tuning was a short lived fash- ion, but for the historian, is a significant development. The wartime austerity of the 1940s discouraged such non-essen- tial frills, and after the War, switched capacitors or inductors and cam-driven mechanical pushbutton tuning methods proved to be adequate. Motor tuning is unlikely ever to be resurrected, for today non-mechanical remote controls provide pushbutton features that were once only possible in the dreams of science fiction writers. : ELECTRONICS Australia, February 1992 91 RadioFans.CN RadioFans.CN EKCO PB289 SN A9095. Photo: James Davidson RadioFans.CN
