Fullscreen HTML5Med Res (??MB)HTML4

Vintage Radio Philips Philips 1963 1963 “Musicmaker” “Musicmaker” MM1 MM1 mantel mantel radio radio By Associate Professor Graham Parslow Philips valve radios from the 1960s had excellent sound, came in a variety of interesting colours and were quite affordable at the time. This article covers three different models: the model 224, MM1 and MM1/01, as they look similar. Their performance is indistinguishable because they all use the same circuit and components. The design of valve-based superhets was fully mature by the 1960s. By then, they were pretty much all using tried and tested components and circuitry. The competing Kriesler range of valve mantels were comparable and sold in greater numbers, as judged by the number of remaining units held by collectors. The Philips radios had a more conservative style, and that may have been advantageous for a kitchen radio where function was the prime consideration. Philips also made other valve radios in the 1960s, notably the mantel model 172 using inductive (permeability) tuning, as well as valve radiograms. But the biggest competitors to these radios were the then-new transistor types. Philips made a good range of transistor sets, both for battery and mains operation, at prices comparable to the valve radios. Their transistor models included the Philadelphia model MM2, the Metropolitan model MT7 and the Leisuremate model RB290, all are pictured at the end of the article. The downside to these transistor radios was an audio output of only about 300mW before distortion became severe. Australian Philips valve and transistor radios were manufactured at Hendon in South Australia. Local production ceased in the early 1970s when tariff protection was lifted and imported radios took the market. Those imported radios usually cost less than just the local cost of the components! 94 Silicon Chip Three similar models The model 224 was introduced in 1961 at £27, and was marketed with the title “Futura Five”. It came in one of six colours: ember red, flamingo, charcoal, turquoise, grey and primrose. The escutcheon at the sides of the dial had a two-tone colour scheme, with one section usually being metallic chrome or gold. The other section was usually black, but sometimes colour-matched to the case. The knobs were cream with eight raised flutes. The volume and tone knobs at the left drove concentric pots, with a DPDT on/off switch linked to the tone control. The tuning knob used the same two segments to duplicate the appearance of the left-hand knob, but it was mounted onto a single shaft with stepped diameters to lock the sections together. The easiest way to identify the model is to look at the paper label pasted under the case. However, these labels are easily damaged and sometimes missing. An example of the label from an MM1 radio is shown at the end of the article. The model MM1, marketed as the The Philips model MM1 (1963) is nearly identical to the Futura Five 224, except it has provision for an external pick-up. Sadly, this particular example has a crack in the top of its case. Australia’s electronics magazine siliconchip.com.au easily when subjected to trauma. The mauve-coloured case shown here was particularly badly affected by heat. The last of the line was the model MM1/01, which was an entirely cosmetic change. The knobs were made more conical in shape, with more flutes, and the escutcheons were black in all sections. Circuit details Why add the pick-up facility to this mature product line? Possibly, it was to compete with the better-selling Kriesler radios that had such an input. Another reason may have been to promote the low-end phono turntables made by Philips that sold for around £5 at the time (see the accompanying advertisment from National Radio Supplies Sydney, originally in RTV&H, December 1963 on p114). These sets draw around 30-32W in use. Unfortunately, the thermo-mouldable plastic case is easily damaged by heat, including the internal heat generated by the valves. The cases also crack Referring now to the original circuit diagram, reproduced overleaf, L1 is an RF choke wound around a 6.8kW resistor, and is described as a loading coil. Loading coils are added to achieve more efficient coupling of RF to a tuned circuit from a short external aerial. The external aerial coupling coil, L2, is three turns around the 6.5inch (165mm) long ferrite rod, spaced 20mm away from L3. The circuitry around mixer/oscillator valve V1 is conventional, with L3C1 for tuning and C4-L4 to set the local oscillator frequency. Oscillation is sustained by feedback from L5. In all these Philips radios, the circuit data specifies a 6AN7 for V1, but the later radios had 6AN7As installed. The nine-pin 6AN7 mixer valve was released in 1948 and became widely used throughout the 1950s. The joint release of the 6AN7 and 6M5 by Philips is described in Radio and Hobbies magazine, January 1950, page 67 (a recommended read). The Philips MM1/01 (1965) is the same as the MM1 with changes only to its external appearance, such as the conical knobs, dial, escutcheon, grille and case colour. This extract of the RTV&H advert from National Radio Supplies Sydney, shows contemporary turntables for sale. The Philips Futura Five 224 (1961) is a 5-valve superhet mantel radio enclosed in a plastic case. “Musicmaker”, was introduced in 1965 and added a pick-up input to the circuit. The way this connected can be seen from the pseudo-3D chassis layout on the MM1 label. The almost identical model 224 label had a simpler 2D chassis diagram because it did not have a pick-up input. This offers another way of recognising the earlier model 224, because the 224 case has only one lower slot at the rear, positioned to view the chassis serial number. The MM1 radios have an additional narrow slot for pick-up access, as seen at the bottom of the mauve case on page 97. siliconchip.com.au Australia’s electronics magazine January 2021  95 The 6AN7 draws 0.23A of filament current at 6.3V. The slightly more efficient, but otherwise identical, 6AN7A valve was released in 1961. It had better cathode emission and required 0.3A for the filament. In a radio with parallel filaments supplied with 6.3V, there is no problem with interchanging the two valve types. However, farm radios powered by 32V DC often connected the filaments of five valves in series, with equalising resistors to regulate the 6AN7 filaments to 0.23A. If a 6AN7A is used as a replacement in these 32V radios, the equalising resistors should be altered to maintain correct filament voltage and current. The IF signal created by the mixer enters the first IF transformer from the plate of the 6AN7. IF amplification is carried out by a 6BH5 pentode. The 9-pin 6BH5 was released in 1952. This Philips-made valve was only built for the Australian market, and is uncommon in non-Philips radios. The amplified IF signal is detected by the diode connected to pin 6 of the 9-pin 6BD7. The 6BD7 triode-double diode is a commonly encountered valve dating from 1950. The IF transformers are the thin rectangular types that Philips introduced in the early 1950s. With age, some of these transformers have gone open-circuit. As the internals are set in resin, sadly they cannot be repaired. Fortunately, none of the IF transformers in the sets described in this article had failed. The bottom end of the second IF transformer secondary (L9) is grounded for the 455kHz IF signal by mica capacitor C15 (220pF). C15 has no effect on audio frequencies, so demodulated audio passes across R7 (47kW), superimposed on the negative DC output from the diode. The AGC circuit passes a negative bias to the preceding 6AN7 and 6BH5 grids via R6 (3.3MW). Delayed AGC is achieved by 47W resistor R14 between the centre tap 96 Silicon Chip Australia’s electronics magazine siliconchip.com.au This photo of an MM1 was taken before restoration, as can be seen by the dust and other debris on the underside of the chassis. The external links to a pick-up can be seen at the top of the photo with shielded cable to connect to the audio amplification section. of the power transformer and ground, with the low-signal grid bias for the 6AN7 and 6BH5 being derived from the transformer end of R14. This means that a higher AGC voltage is required before the gains of those valves are reduced. To listen to the radio, the A-B jumper link in the pickup connector needs to be in place. The demodulated audio signal is then fed to the 500kW volume control (R8). The link has a pull-string accessible through its cabinet slot. A ceramic or crystal phono cartridge can instead be connected between B and C on the linking socket. The audio signal from the cartridge is then amplified by the 6BD7 triode and conventionally passed to the 6M5 grid via 10nF coupling capacitor C18. Many radios have top-cut tone controls acting at a point of high voltage that compromises the reliability of the components. This circuit sensibly places R13 (a 250kW pot) and C19 in a position that is nominally at 0V DC. The 6M5 output pentode has 6.5V of grid bias, generated by R17 (220W). Unusually, there is no cathode bypass electrolytic across R17. A bypass capacitor here would provide a low-impedance path for audio, thereby maximising the amplified output from the pentode. This gave me a chance to see how critical, or otherwise, that conventional bypass electrolytic is. I found that adding a 22µF capacitor across R17 made no audible difference, so It’s a bit hard to see from this angle, but the connection for the pick-up is at the bottom rear of the chassis (circled in red). Here it has been fitted with a wire-link between points “A” and “B” for normal radio operation. siliconchip.com.au Australia’s electronics magazine January 2021  97 The Philips MM1 label originally had a red background, but it has been changed here to white for clarity. Philips did not make an unreasonable omission. There is no convenient place to mount the speaker transformer on the top of the chassis, so it is mounted below. The resulting ‘spare space’ above the chassis is occupied by the aluminium cans for filter electrolytics C16 and C17. There is no filter choke in the HT supply. The 4 x 5.5-inch (100 x 140mm) elliptical Rola speaker has the 1960s rounded edge magnet profile. This replaced the plain cylindrical profile that Rola used for magnets in the 1950s. The baked enamel frames of these speakers resist rusting, but the magnets often show rust. The Rola speakers mounted in these radios sound surprisingly good for their modest dimensions. Faults and troubleshooting Although these radios are ‘modern’, they are still over 50 years old. Most of the original Ducon paper capaci- tors remain serviceable. Nevertheless, in three of these radios, I found C18 was leaky and compromised the 6M5 grid bias. Editor’s note: some restorers prefer to replace paper capacitors regardless, as they will fail eventually. I selected a model 224 to listen to as a shed radio after replacing only C18. For some days, it behaved well, but then failed completely. Rocking the valves in their sockets revealed the problem. The fix at this time was cleaning the pins of the 6AN7 and 6BH5. After a few more days, it developed a crackle. Suspicion immediately fell on the two mica capacitors, C7 and C15. Against optimistic expectations, their replacement did nothing to help alleviate the crackle. The next step was to begin replacing the paper capacitors, starting with C20 across the output transformer. The crackle stopped after this single replacement, so the radio went back into service. The back of another MM1 radio which is badly deformed from the heat of the valves during operation (likely with high ambient temperature and poor ventilation). 98 Silicon Chip Australia’s electronics magazine However, the crackle soon started again, so it was back to the bench for a systematic replacement of all paper capacitors. After every capacitor was replaced, the radio still produced abundant crackle. Worse still, only strong stations now tuned in weakly and turning up the volume drove the set into audio oscillation at around 2kHz. All resistors checked as true to value, except R2 and R4, which had gone significantly high in value. Replacing them did not change anything. If the problem cannot be found below the chassis, then it might be above. As soon as I looked at the tuning capacitor, I saw that trimmer C2 had broken away minutely from the solder joint to Earth. Repairing that changed nothing, so it was back to looking below. It then became clear how I had compromised the set by a simple mistake. The original, large capacitor C6 was soldered into a cramped space and the more compact polyester replacement allowed me to connect it to a more convenient Earth point. The problem was that the free tag I used was above an Earthing solder point on the chassis, but the tag was not connected to it, so C6 was floating. A simple Earth link brought the radio back to full function, complete with crackle. Back on the top of the chassis, removing the mixer valve did not affect the crackle, so I determined that it was being introduced at a later stage. Removing the 6BH5 IF amplifier valve produced blissful silence. I had not previously run into a crackling valve, but a replacement 6BH5 was indeed the answer. Since replacing the 6BH5, the radio has been perfectly reliable. SC siliconchip.com.au These three Philips transistor radios were contemporaries of the MM1 radios in the 1960s. From top-tobottom: Metropolitan MT7 mantel; Philadelphia MM2 mantel; and Leisuremate RB290 portable. siliconchip.com.au Australia’s electronics magazine January 2021  99