EKCO-PB189-PB289-1938 电路原理图.pdf
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1、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
2、 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
3、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 PB2
4、89, 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
5、 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 distin
6、ctive 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
7、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 h
8、ad 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 m
9、otor 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 bro
10、adcast 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 Austr
11、alia, 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 driv
12、e 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 ba
13、ck 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
14、 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 a
15、ddition 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 am
16、plifier 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
17、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
18、 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 use
19、d 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 ca
20、pacitor 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
21、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
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