Fullscreen HTML5Med Res (??MB)HTML4

Vintage Radio By Rodney Champness, VK3UG More Philips Twins – the Dutch 209U and the Australian 112A Continuing on with our series of Philips twins, this month we take a look at the Dutch Philips 209U multi-band receiver and its Australian “twin”, the model 112A. Under the skin though, these are two very different receivers. L IKE OTHER Dutch/Australian Philips twins, the 209U and 112A receivers look the same at first glance but on closer inspection, are as different as chalk and cheese. In this case though, there is a slight difference in cabinet size, so the Dutch parent company obviously produced more than one variant of this particular cabinet style with only minor differences between them. As before, the cabinet moulds for the Australian-built receiver were obtained from the parent company, probably after the parent company had finished with them. The Dutch 209U receiver was manufactured from 1946-1947 while its Australian look82  Silicon Chip alike was produced somewhat later, from 1948-1949. Model 112A circuit details Fig.1 shows the circuit details of the Australian Model 112A. It’s a fairly conventional 4-valve superhet receiver with 455kHz IF stages. As shown, the antenna signal is fed to a tuned circuit consisting of L1 and C1 and resonates at a frequency just below the broadcast band. This boosts the sensitivity of the receiver at the low-frequency end of the band, while trimmer capacitor C2 boosts the performance at the high-frequency end by feeding signal into coil L2. The signal in L1 also inductively couples into L2. Basically, the tuned antenna circuits in sets of this era and later were designed to extract the maximum amount of signal from relatively short antennas. In effect, this was done by partially tuning the antenna using fixed value components. This technique significantly improved receiver performance compared to sets using the antenna-tuned circuits of the 1920s. As an aside, to get the best performance from crystal sets, additional tuned circuits for the antenna are used. These must be capable of tuning the antenna right across the band, as in the crystal set described in the April 2007 issue. In addition, highfrequency 2-way radios must also siliconchip.com.au Fig.1: the Australian model 112A receiver is a conventional 4-valve superhet with 455kHz IF stages. Note the tuned antenna circuit. L1 & C1 boost the sensitivity at the low-frequency end of the band, while trimmer capacitor C2 boosts the sensitivity at the high-frequency end. have fully tunable antenna circuits (ie, the antenna must be tuned to the operating frequency) if they are to work efficiently. Getting back to Fig.1, the RF signal from the antenna circuit is tuned using C3 (one section of the dual tuning gang). The resulting signal, in the range from 530-1620kHz, is then applied to the signal grid of V1, an ECH35 converter. Note that no provision has been made to adjust the inductance of L2, so it cannot be peaked at the lowfrequency end of the tuning range. However, by adjusting the position of the dial pointer and adjusting the oscillator padder capacitors to suit, some peaking the set’s low-frequency performance is possible. It’s a fiddly process though and doesn’t always achieve perfect results (the article on alignment in the February 2003 issue described the techniques necessary for good results). At the other end of the band, C2 is adjusted to peak the performance at around 1500kHz. Local oscillator The local oscillator is based around V1, coils L3 and L4, the other section of the tuning gang (C4) and their associated components. The values siliconchip.com.au The view inside the model 112A receiver. The chassis is well laid out and all parts are readily accessible. of padder capacitors C8 and C9 are adjusted so that stations appear at the correct position of the dial at the low-frequency end of the tuning range, while C10 is used to do the same thing at the high-frequency end. V1’s output appears at its plate and this is coupled to a double-tuned 455kHz IF (intermediate frequency) transformer. The resulting 455kHz IF signal is fed to V2, an EBF35, where it is further amplified and then applied to another double-tuned 455kHz IF transformer. Its output is in turn fed to a detector diode in V2. The resulting audio signal from the detector appears across R8 and volume control potentiometer R9. From February 2013  83 Fig.2: the Dutch 209U receiver is also a 4-valve superhet design but with performance equivalent to a 5-valve circuit due to the dual role performed by valve B2 (see text). It’s a multi-band design and is more complicated than the Australian model 112. there, the signal is fed via C19 to the grid of V3, a 6V6GT audio amplifier stage. This stage in turn drives the loudspeaker via an output transformer (L12/L13). No negative feedback is used in the audio amplifier. Simple AGC The IF signal level at V2’s plate is greater than it is at the detector diode and this signal is also applied to the AGC diode via C14. This set has simple AGC and as soon as there is any signal (including any interference or other noise), a small AGC bias appears on the AGC diode. This is a good design feature as V2 has no standing bias and the set will normally be tuned to a station. However, it would not be considered good design practice in a communications receiver. By contrast, the converter stage (V2) does have standing bias due to the voltage across R2. Note that the converter receives just 20% of the AGC voltage applied to the IF valve due to the voltage divider formed by resistors R1 and R5. The power supply is designed to operate off voltages from 220-260VAC 84  Silicon Chip at 40-60Hz. Australia now has 50Hz mains but 40Hz was used in Western Australia for some time and 60Hz is used in the USA (although it’s doubtful that Philips exported this set to the USA). However, it was a good selling point and it’s possible that some of the home-lighting plants of the era ran at 60Hz. Power transformer The power transformer has two secondaries: (1) 6.3V for the valve heaters and dial lamps and (2) a 376V centretapped secondary for the HT (high tension). Note resistor R13 (250Ω) between the centre tap and the chassis. The voltage developed across this is fed to V3’s grid via R10 and R11 and provides a bias of -9V for this stage. Finally, resistor R12 decouples the HT supply at V3’s plate from the HT line fed to other sections of the receiver. This is good design practice as it minimises IF and audio feedback between the stages. The Dutch 209U circuit Now let’s take a look at the circuit for the Dutch 209U receiver – see Fig.2. This is a very different circuit to the one used in the 112A. It’s also a 4-valve superhet receiver but in this case, the first stage uses a UCH21 triode heptode (B2). This valve can be used as a separate pentode and triode and/or as a converter. In this receiver, it is used as both and so it has the performance of a 5-valve set. Unlike the 112A, which tunes the broadcast band only, the 209U is a multi-band receiver. The tuning ranges cover three bands: 157-400kHz long wave, 530-1600kHz medium wave (broadcast band) and 5.88-18.2MHz shortwave. In addition, this receiver is designed to work on both AC and DC mains and as such, can have a live or “hot” chassis. Hot-chassis sets are considered by most vintage radio collectors to be dangerous to work on, as any carelessness can lead to a severe electric shock or even death. In fact, they are often shunned because of this and only those who know what they are doing and are aware of the dangers should work on them That said, a receiver that uses a power transformer to isolate the siliconchip.com.au mains from the chassis can be quite dangerous too, as these may have an HT rail voltage in excess of 500V DC (ie, across the main electrolytic filter capacitors) soon after switch-on. This HT rail is also capable of delivering a fatal electric shock and so all sets need to be treated with respect, not just the hot-chassis AC/DC types. Valve heater voltages The valves used in the Dutch 209U receiver have 20V, 50V or 55V heaters, each rated at 100mA. As a result, when these are connected in series, a heater supply rail of 20 + 20 + 55 + 50 = 145V is required. Connecting a resistor or resistors in series with these heaters allows the set to be operated from 220V. If the set is to operate from 125V, the heaters are instead switched into two strings with three heaters in series on one string and the rectifier heater on the other. Series resistors are then used to reduce the voltage drop across each string to the required 125V. The necessary switching to do this is achieved via a plug-and-socket arrangement on the rear apron of the chassis. By changing the wiring to a couple of resistors in these strings, it’s also possible to run the set on either 110V or 200V. The wiring is such that when used on AC, the HT voltage is of the order of 150V. When operated from 220V, the current drain is around 100mA for the heaters plus a further 70mA for the plate and screen loads of the valves. Because this is an AC/DC receiver with a “hot” chassis, the antenna lead has a capacitor in series with it, so that no voltage appears on the antenna. There is no earth on this set; instead, it relies on the mains to effectively act as the earth. Foil plate antenna In addition to the external antenna, there is also a foil-plate antenna on the inside of the back panel of the receiver. This is also isolated by a capacitor to make sure no mains voltage appears on it. This antenna is shown at the top-left of the circuit diagram and is connected to the signal grid of the UCH21 triode-heptode converter valve (B2) via C120 and C101. When aligning the receiver, this plate antenna must be attached and the back panel fitted in place as it affects the antenna input tuning adjustments siliconchip.com.au This view of the model 112A shows just how few parts there are underneath the chassis. Note that this photo was taken before the power cord was rewired and properly clamped into position (the Earth connection was also later improved). The Dutch 209U’s chassis is more crowded than the model 112A’s and is further complicated by the band-switching arrangement at lower left. As a result, it’s the more difficult of the two sets to service. at the high-frequency end of each tuning range. As shown in Fig.2, all the antenna input coils are wired in series and various sections shorted to earth as required. At the same time, the secondaries are switched to valve B2’s signal grid, depending on the selected frequency band. The oscillator circuits are also switched as required to the triode section of the B2 converter valve. The mixed signals appear at the plate of B2 and are fed to a doubletuned 452kHz IF transformer (S51S54). However, one version of the receiver has an IF of 468kHz. Next, the resulting 452kHz IF signal is fed to the signal grid of the heptode section of valve B3 (UCH21). B3 amplifies this IF signal and it is then fed through a second double-tuned IF transformer. It then goes to the detector and AGC diodes in B5, a UBL21 duodiode-power pentode valve. The detected audio appears across resistor R12 and volume control potentiometer R11, with the signal at R11’s wiper then fed to the grid of the triode section in valve B3. The amplified signal from B3 is then fed via C83 to the pentode section of audio output February 2013  85 The Dutch 209U receiver is a hot-chassis design (ie, no power transformer), so care is required when working on this receiver – see text. stage B5. B5 in turn drives the speaker via a transformer. As with the model 112A, there is no negative feedback or tone control circuit in this receiver. Power supply The power supply is a typical AC/ DC configuration. As stated above, a number of valve heaters are wired in series and a resistor or resistors are wired in series with this heater string to bring the total voltage drop up to the applied mains voltage. Also as stated, around 145V is dropped across the heaters, so a further 75V is dropped across the series resistor for 220V operation. The rectifier valve (B6) is a UY1N, which is a half-wave type. This has C110 (22nF) wired across it to filter out mains interference and artefacts generated by the rectifier itself. The HT rail appears on B6’s cathode and is filtered by C1 (the first filter capacitor). This HT rail is fed to valve B5’s plate via the primary winding of the speaker transformer. The remainder of the receiver derives its HT supply via a HT filter made up of R1 and filter capacitor C2. Back-bias for the amplifying valves is derived from the voltage across resistor R75 (below B5). B5 receives around -8.5V but B1 and B3 receive only a portion of this due to R34, R35, R11 & R12. The bias voltage applied to valves 86  Silicon Chip B1 and B3 before AGC is applied is around -1.5V. Delayed AGC As with many other European Philips sets, the 209U receiver features delayed AGC. Both diodes in valve B5 are strapped together for the detector and AGC functions and no AGC bias is applied to the front-end of the set until the signal exceeds 0.5V at the AGC diode. In addition, the detector doesn’t function until the -0.5V bias on the diodes is overcome. This means that when tuning between stations, the set will normally be silent unless there is quite a bit of external interference. Once a received signal generates more than 0.5V at the detector/AGC diodes, the receiver bursts into life. However, if the amplified signal strength is only just greater than 0.5V at these diodes, considerable distortion will be present in the audio. In practice, the signal will either be well under 0.5V at these diodes or well above it in normal operation, so that’s not a problem. Comparing the two receivers From the circuit descriptions above, it’s obvious that these are two very different sets under the skin. Both sets have four valve envelopes, with three active stages in the 112A and four in the 209U. As a result, the Dutch 209U is the better performing receiver of the two. As previously stated, the 112A is a broadcast-band only set whilst the 209U has three bands. Europe uses both the long-wave band and the medium-wave band, whereas Australia ceased using the long-wave band in the late 1920s. And although Australia used the shortwave band from time to time, it has been used much more often in Europe. Because the 209U is an AC/DC live chassis set, the rear panel must be kept in place because contact with the chassis could prove fatal. It should be safe if the Neutral lead is the one that’s connected to the chassis but don’t depend on this. In fact, when I serviced such sets in times gone by, I always made sure that the mains plug was wired so that the chassis was “cold”, ie, with the Neutral connected to chassis. By contrast, the 112A has a conventional power transformer but you still have to be careful of the HT rail. Both sets can be easily accessed for routine serving by removing a panel on the underside of the set. However, the 112A has a lot fewer parts. The 209U has 35 capacitors and 18 resistors while the 112A has just 21 capacitors and 13 resistors. Restoration John de Haas, the owner of these two sets, restored them to working order mainly by replacing any faulty capacitors and out-of-tolerance resistors. He also found that the wiring in the 209U was in better condition than in the 112A. In fact, quite a bit of the wiring in the 112A had perished and this will have to be replaced in the near future. The valves used in these sets are as different as they can be. The valve bases in the 209U mainly use the American 8-pin lock-in base, with an octal base used for the rectifier. By contrast, the 112A receiver uses all octal valves. What I do like about the Dutch 209U receiver is the use of a UCH21 triode heptode for the converter valve (B2). As stated previously, the triode and heptode sections can be operated independently and this makes this valve quite versatile. When used as a converter, the triode grid and grid three of the heptode are strapped together at the valve socket. The service manual for the 209U is siliconchip.com.au The two sets, side by side, with their distinctive “flip-up” dial scales. Because it’s a hot-chassis design, the Dutch 209U receiver (right) must never be operated without its rear cover in place. comprehensive and all service work on the set is covered in detail. That’s probably just as well, particularly when it comes to the dial-scale stringing as this is not otherwise easy to figure out. On the other hand, the manual for the 112A is quite sparse but its circuit is easy to follow and service without much detail being provided. The cabinets of both of these sets cleaned up quite well and they are prime examples of how good a Bakelite cabinet can be made to look. Summary Both sets is capable of quite good performance although the 209U has the edge on the 112A. If you want multi- band operation, then the 209U is the only choice but there’s little to choose between them on the broadcast band. Finally, the 112A receiver is the easier of the two to service. It has far fewer parts than the 209U, doesn’t have complicated band switching and has a conventional power supply with SC a mains transformer. Radio, Television & Hobbies: the COMPLETE archive on DVD YES! NA R MO E THA URY ENT QUARTER C NICS O OF ELECTR ! Y R O T IS H This remarkable collection of PDFs covers every issue of R & H, as it was known from the beginning (April 1939 – price sixpence!) right through to the final edition of R, TV & H in March 1965, before it disappeared forever with the change of name to EA. For the first time ever, complete and in one handy DVD, every article and every issue is covered. If you’re an old timer (or even young timer!) into vintage radio, it doesn’t get much more vintage than this. If you’re a student of history, this archive gives an extraordinary insight into the amazing breakthroughs made in radio and electronics technology following the war years. And speaking of the war years, R & H had some of the best propaganda imaginable! Even if you’re just an electronics dabbler, there’s something here to interest you. • Every issue individually archived, by month & year • Complete with index for each year • A must-have for everyone interested in electronics Please note: this archive is in PDF format on DVD for PC. Your computer will need a DVD-ROM or DVD-recorder (not a CD!) and Acrobat Reader 6 or above (free download) to enable you to view this archive. This DVD is NOT playable through a standard A/V-type DVD player. Exclusive to SILICON CHIP ONLY 62 $ 00 +$10.00 P&P HERE’S HOW TO ORDER YOUR COPY: BY PHONE:* (02) 9939 3295 9-4 Mon-Fri BY FAX:# (02) 9939 2648 24 Hours 7 Days <at> BY EMAIL:# silchip<at>siliconchip.com.au 24 Hours 7 Days BY MAIL:# PO Box 139, Collaroy NSW 2097 * Please have your credit card handy! # Don’t forget to include your name, address, phone no and credit card details. siliconchip.com.au BY INTERNET:^ siliconchip.com.au 24 Hours 7 Days ^ You will be prompted for required information February 2013  87