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Classic circuit uses The “Au an old-fa “Wi Cen 58  Silicon ilicon C Chip hip siliconchip.com.au “state of the ark” technology ussie Three” : ashioned valve st ireless” – 21 ntury Version! So you thought valve technology was dead! Well it is – but we have exhumed enough of it to produce a 3-valve radio which has quite a respectable performance. It is a superheterodyne circuit, based entirely on readily available components. It is suitable for moderately-experienced constructors – even those who’ve never touched a valve in their lives! B y K e i t h Wa l t e r s W HY WOULD ANYONE want to build a valve radio, one that doesn’t even pick up FM stations? If nothing else, to get a feel and understanding for old-fashioned technology. There are lots of people who are attracted to valve amplifiers (particularly musicians) and lots of people busily restoring vintage radios, television sets and all manner of thermionic technology. So why not build a valve radio from scratch? Despite the relatively few parts the radio uses, this is certainly not a toy and it illustrates how much performance you can get out of just a few valves. As far as its lack of FM reception is concerned, there were no FM radio stations in Australia during the valve siliconchip.com.au era! (While experimental broadcasts started back in 1948, the first FM radio stations, 2MBS and 3MBS, did not start transmitting until 1975). The “cabinet” The prototype radio is housed in a whimsical gothic cabinet which pays homage to some of the “cathedral style” radio cabinets of yesteryear. Some people will hate it and others will like it. If you’re in the first category, then build a more conventional cabinet. Why “Aussie Three”? Well that’s a dig at the “All-American Five” concept that January 2008  59 Here’s the front view of the Aussie Three removed from its Gothic-style cathedral case. We’re willing to bet that the vast majority of Aussie Threes built will remain in this state! emerged in the USA in the 1930s. As an alternative to the grandiose (and expensive) timber cabinet radios that are the delight of collectors now, some manufacturers started marketing the virtues of a basic, no-nonsense but perfectly serviceable superheterodyne that the “regular guy” could afford; the “Model T” of radios if you like. There was no RF stage (which wasn’t really necessary in urban locations anyway) but any lack of sensitivity could be overcome by connecting a decent aerial and earth. The valve line-up was the now-classic rectifier, mixer/ oscillator, IF amplifier, detector/audio preamplifier and a pentode audio power output stage. Our Aussie Three uses three triode-pentode valves, deletes the valve rectifier in favour of semiconductor diodes and adds a ferrite rod antenna to come up with quite a respectable performance. 60  Silicon Chip To any non-technical user, it’s just a radio: you turn it on and it works! Despite its tiny PVC tuning capacitor, there’s surprisingly little frequency drift, even right up at the top of the AM band. From my home in the outer suburbs of northwest Sydney, it picks up all the Sydney stations with just its ferrite rod antenna, all at about the same volume. Bake a cake – then build the radio The hardware comes from a variety of sources. There are no PC boards, as all the wiring is “point-to-point” using old-fashioned tag strips and hook-up wire. The chassis is actually a cake tin, purchased for less than $3 at Big W! Some of the other parts and materials came from Bunnings Hardware and no doubt you may want to improvise with some items you have in your junk box. siliconchip.com.au Parts List – Aussie Three Valve Radio 1 tinplate baking tin, approx. 245 x 222 x 50mm (eg, “Willow” brand) 3 9-pin valve sockets 2 sets AM IF/oscillator coils 1 ferrite rod and coil assembly 1 24 VAC 24VA (or higher rated) plugpack (see text) 1 240V to 7.5V mains transformer (for speaker transformer – see text) 4 8-way tagstrips (E-6-E) 1 4W 125mm or larger loudspeaker 1 2.5mm “DC” chassis-mounting power socket (for AC connection) 1 chassis-mounting RCA socket (for speaker connector) 1 RCA plug (to connect to speaker) 1 chassis-mounting screw terminal (for antenna) 1 100mm length stiff tinned copper wire (for mounting LED) 1 10mm length of wooden dowel 2 wooden drawer knobs 4 assorted hose clamps 1 dial drum assembly with dial cord (see text) 1 station dial (see text) 3 metal pergola hangers (L-shaped steel, 37mm wide, 130 x 50mm) 2 steel brackets, 45 x 45 x 110mm (to hold tuning assembly) Small block of timber to mount ferrite rod Various lengths of single and figure-8 hookup wire, various colours (some need 100V+ rating) Wire for antenna (if required) Cable ties Screws and nuts as required 2 flat steel washers, 10mm internal Valves 2 6BL8 (V1, V2) 1 6BM8 (V3) Semiconductors 4 1N4004 1A power diodes (D1-D4) 1 5mm white LED (LED1) Inductors 2 10mH miniature chokes (RFC1, RFC2) Capacitors 5 47mF 160V electrolytic (C12, C19, C20, C21, C22) 1 22mF 16V electrolytic (C17) 1 10mF 160V electrolytic (C13) 1 10mF 16V electrolytic (C15) 2 220nF 200V polyester (C1, C4) 1 56nF 200V ceramic or polyester (C8) 1 47nF ceramic or polyester (C11) 1 10nF 200V ceramic or polyester (C14) 1 6.8nF 630V polyester (C18) 1 4.7nF ceramic or MKT polyester (C7) 3 3.3nF ceramic or MKT polyester (C2, C5, C16 ) 1 680pF ceramic (C9) 1 100pF ceramic (C10) 2 12pF ceramic (C3, C6) 1 60/160pF miniature tuning gang (variable capacitor) (VC1) Resistors (0.5W, 5% unless otherwise specified) 2 470kW (R7, R11) 2 220kW (R3, R9) 1 100kW (R2) 3 47kW (R1, R4, R10) 1 15kW (R16) 1 10kW (R6) 2 3.9kW (R5, R8) 1 470W (R13) 1 330W (R12) 1 39W 10W (R15) 1 8.2W 10W (R14) 1 10kW horizontal trimpot (VR1) 1 500kW switched log pot (VR2) siliconchip.com.au Silicon Chip Binders REAL VALUE AT $13.95 PLUS P & P These binders will protect your copies of SILICON CHIP. They feature heavy-board covers & are made from a dis­tinctive 2-tone green vinyl. They hold 12 issues & will look great on your bookshelf. H 80mm internal width H SILICON CHIP logo printed in gold-coloured lettering on spine & cover H Buy five and get them postage free! Price: $A13.95 plus $A7 p&p per order. Available only in Aust. Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or call (02) 9939 3295; or fax (02) 9939 2648 & quote your credit card number. Use this handy form Enclosed is my cheque/money order for $________ or please debit my  Bankcard   Visa    Mastercard Card No: _________________________________ Card Expiry Date ____/____ Signature ________________________ Name ____________________________ Address__________________________ __________________ P/code_______ January 2008  61 The “dial” is made from an old CD, the dial drive is a length of dowel held in by hose clamps, the dial cord is brickies’ string, the tuning assembly brackets are intended for pergolas . . . hang on, what’s a LED doing in a valve radio? Other components came from Dick Smith Electronics, Jaycar Electronics and Wagner Electronics. This design uses three triode-pentode valves, two 6BL8s and one 6BM8. These are old 1950s-era “workhorses” that are still easy to get from Wagner Electronicss and other suppliers (check the internet). Shop around and don’t get suckered into buying so-called “audiophile” valves at inflated prices. They won’t work any better than the regular types. Low high voltage! There is a common misconception that valve equipment needs dangerously high voltages to work properly. In fact, 100V is more than adequate for a radio like this and is much safer for the casual tinkerer. Although 100V DC can theoretically give you a dangerous or even fatal shock in the wrong circumstances, with dry hands in a normal workshop situation, the worst you’re likely to get from this circuit is a bit of a “nip”. Circuit description As already mentioned, the circuit of the Aussie Three is a conventional superheterodyne radio. This means that the incoming broadcast signal is mixed (ie, heterodyned) with the local oscillator signal and the difference frequency between these two signals becomes the “intermediate” frequency. This is amplified in the IF amplifier (funny, that) and then fed to the detector where the original audio modulation is recovered and fed to the audio amplifier stages and thence to the speaker. And where does the “super” prefix come from in the word “superheterodyne”? This merely refers to the local 62  Silicon Chip oscillator signal being “above” or higher than the incoming broadcast signal. In our circuit, the incoming signal is picked up in the ferrite rod antenna which is tuned by the 160pF section of the plastic dielectric tuning gang (using the terminal marked “A”) and then fed to the grid of the pentode section of V1 (valve1, 6BL8). The local oscillator uses a red “transistor” oscillator coil, L2. (Actually, this is not a coil but a conventional RF transformer with two windings). The secondary winding is tuned with the 60pF section of the plastic dielectric tuning gang (using the terminal marked “O”) and then connected to the grid of the triode section of V1 via resistor R4 and capacitor C10. Oscillation is maintained by the feedback winding which is fed from the plate of the triode via capacitor C9. The grid-cathode circuit acts as a diode that conducts slightly on the positive excursions of the grid signal, resulting in a standing DC bias (ie, voltage) across C10. This tends to reduce the gain of the triode, damping down the oscillation and so stabilising the output amplitude. Note that all the coils and transformers in this circuit were originally designed to be used in low-voltage transistor radios. This is why all the windings are capacitively coupled, to keep high DC voltages away from the flimsy insulation of the coil wires. Experienced vintage radio enthusiasts may have noticed that there appears to be no mechanism for coupling the oscillator signal into the pentode mixer. In fact, the oscillator signal is fed to the mixer using just stray capacitive coupling! This works well, possibly due to the high gain of the valve. siliconchip.com.au “O” C A K LED VC1b 6-60pF R4 47k C10 100pF 9 IFT1a (BLK) 5 8 1 V1b ½ 6BL8 C11 47nF 160V 4 3 5 2 6 1 7 8 9 VR1 10k DAMPING C12 47 F 200V 2 B 3 AGC RFC2 10mH R15 39  10W 8.2  10W R14 POWER SWITCH ON VOLUME CONTROL VR2 500k VOLUME C D R7 470k C14 10nF 4 5 4 5 5 A 4 B C6 12pF 8 4 V1 (6BL8) V2 (6BL8) A D1 C15 10 F 16V A C16 3.3nF AUDIO AMP R8 3.9k 1 9 V3a ½ 6BM8 9 9 1 7 R2 100k 6 5 2 R16 15k C19 47 F 200V A D2 K  A K +48V A D1-D4 1N4004 AUDIO OUTPUT K C20 47 F 200V D3 C8 56nF C8 56nF R3 220k D4 C22 47 F 200V K 4  SPEAKER C21 47 F 200V A C18 6.8nF 630V T2 HT1: +100V R3 220k C7 4.7nF C17 22 F 16V R13 470 R12 330 B 3 AGC AUDIO AUDIO V3b ½ 6BM8 4 8 LED1 (WHITE) K R11 470k R10 47k (SHIELDED AUDIO LEAD) C7 C 4.7nF 8 4 AUDIO DETECTOR & AGC DETECTOR AUDIO & AGC DETECTOR V2b V2b ½ 6BL8 IFT2b (YEL/CRM) B 1 ½ 6BL8 V3 (6BM8) R9 220k IFT2a (YEL/CRM) C5 3.3nF HT3: +85V S1 C13 10 F 160V R6 10k 7 5 6 IF AMPLIFIER V2a ½ 6BL8 24V AC INPUT R2 100k HT2: +90V C4 220nF IFT1b (BLK) 6BL8, 6BM8 LOCAL OSCILLATOR R5 3.9k R1 47k RFC1 10mH C3 12pF “AUSSIE THREE” VALVE radio K D 3 TUNING 7 4 6 C2 3.3nF Fig.1: it’s a fairly traditional superhet circuit with three valves – a mixer/oscillator, an IF amplifier/detector and an audio preamplifier/amplifier. However, the audio and AGC detector is somewhat unusual and its operation is explained in the text. 2007 SC  A D1–D4: 1N4004 OSCILLATOR COIL (RED) L2 C9 680pF C1 220nF “A” VC1a 6-160pF 2 FERRITE ROD ANTENNA L1 MIXER V1a ½ 6BL8 240V EXTERNAL ANTENNA 7.5V siliconchip.com.au January 2008  63 C CAPACITORS D DIODES RFC RF CHOKES R RESISTORS V VALVES IFT IF TRANSFORMERS C13 R7 R16 C15 R8 R1 C4 C1 R11 C10 R9 R6 C16 C17 R10 C14 R5 C2 IFT2b R12 RFC1 C3 VR1 IFT2a C6 C22 D4 C5 C12 R3 C7 D3 D2 D1 V1 IFT1a V3 C18 R4 C9 IFT1b C8 V2 RFC2 R2 C11 C20 C21 C19 R13 R15 R14 No wiring diagram is supplied for this project – use this photograph and the one adjacent to identify and locate the components. It’s not particularly critical but this layout should be roughly followed because it works (moving the valves around, for example, could introduce instability or unwanted interaction). The plate load of V1a (ie, the 6BL8 mixer pentode) is a 10mH choke (RFC1) and the mixer’s output (ie, the intermediate frequency, or IF) is capacitively coupled via capacitor C2 to the tuned winding of the first IF transformer IFT1. IFT1a is lightly coupled to IFT1b via a 12pF capacitor (C3). This small value is necessary, otherwise the two coils would be over-coupled, producing a broad, double-humped IF response. VR1, the 10kW trimpot wired across the secondary of IFT1, allows the tuned winding to be damped to prevent unwanted oscillation. The “hot” end of IFT1b is connected directly to the control grid of the second 6BL8 (V2). This pentode section 64  Silicon Chip is configured as a straightforward IF amplifier. The “cold” end of IFT1b is bypassed to ground by capacitor C4 and AGC (automatic gain control) is fed to this point. We will talk about AGC in a moment. The IF amplifier has a 10mH choke (RFC2) as its plate load and is coupled to the second IF transformer, IFT2, in the same manner as for the mixer, via another 3.3nF capacitor. Unusual detectors The triode section of the second 6BL8 (V2) is used as the detector for the audio signal and for AGC signal. This siliconchip.com.au LED1 VC1 L2 VR2 L1 V1 SPK V3 V2 T2 ANT ACV is unconventional, as most old valve radios used the same diode for both signal detection and AGC, which results in audio distortion, particularly with the heavily compressed near-100% modulation routinely used these days. (Back in earlier days, radio stations modulated their carrier at less than (and often significantly less than) 100%. Apart from being less taxing on transmitter equipment, one reason for this is that the lower the modulation, the less power was consumed (and therefore lower transmitter electricity bills for the station!). In this circuit, the IF signal is applied to the cathode of the triode and the grid and plate act as the anodes of siliconchip.com.au separate diodes. The diodes conduct on the negative swing of the modulated IF signal and the result is a negative DC voltage. The audio signal is taken from the grid and the AGC from the plate of the triode. AGC Gain is controlled in the traditional manner by applying the negative voltage generated by the AGC diode to the grids of the mixer (V1a, via the antenna coil) and IF amplifier (V2a, via IFT1b) valves. As the signal strength increases, so does the negative control voltage, which reduces the gain of the valves. The January 2008  65 To make the ferrite rod antenna movable, so it can be aligned to the wanted stations, it was mounted in this block of wood, itself hinged on a screw through the bracket. The grommet is used to protect and attach to the very fine wires of the coil. result is that the difference in volume between weak and strong stations is greatly reduced. (In the old days it was called “AVC” – Automatic Volume Control but this isn’t really an appropriate term for the same principle applied to other types of receivers, so the term AGC came to be preferred). As mentioned earlier, the detected audio signal is taken from the grid of V2b, serving as the plate of a diode. The diode load is the 500kW volume control potentiometer (VR2), which is bypassed by C7. Purists may argue about the validity of having DC across the volume control potentiometer as it can cause it to become noisy. A separate detector load resistor could have been used, with a coupling capacitor to the volume control, but this would introduce some detector distortion. The audio amplifier uses the triode and pentode sections of a 6BM8 (V3). The triode is a standard connection with grid cathode bias generated by the voltage drop across the 3.9kW cathode resistor. To briefly explain, the gain and operation of any valve is controlled by the negative DC voltage applied to the grid – ie, the grid must be negative with respect to the cathode. In this case, the cathode is at about +0.75V, so the grid will be -0.75V with respect to the cathode. In valve parlance this is known as “cathode bias” or “self bias”. The same bias scheme is applied to the pentode which drives the speaker transformer in class-A mode. No negative feedback is used around the transformer as it was found to cause operating problems. Power supply The power supply is based on a 24V AC supply (in fact, a Christmas lights transformer). The valve heaters are wired in series across the 24V AC supply, together with series and shunt resistors to make sure that each heater filament operates at the correct voltage. Pin 4 of the oscillator/mixer 6BL8 (V1) is connected to earth, as it is the one most likely to be subject to induced 66  Silicon Chip hum from the heater supply. Pin 5 of V1 is connected to pin 4 of the IF 6BL8 (V2) and pin 5 of V2 goes to pin 5 of the 6BM8 (V3). R15 is connected from pin 5 of V2 to ground to compensate for the lower heater current requirements of V1 and V2. Pin 4 of V3 connects to the 24V AC input via R14, which drops the 24V down to the necessary 18V. The high voltage supply uses two voltage-doubling rectifiers (diodes D1-D4). The cold end of the second doubler (D3, D4) is returned to the output of the first, rather than ground. Each doubler gives about 2 x 1.4 x 24 = 67V or so, which means the open-circuit voltage is about 135V. This will drop to around 100V, depending on the particular 6BM8 used. The 24VAC supply is switched by the volume control, although of course, the external power transformer will remain on all the time. Aerial coil and tuning capacitor The aerial coil is a standard AM radio ferrite slab/coil unit, used by the hundreds of millions in transistor radios, and available cheaply from DSE and Jaycar. I used a DSE unit and although it works quite well “as is” I replaced the supplied ferrite slab with one of their 100mm ferrite rods, which fits nicely inside the aerial coil. Adjustment of the inductance is made simply by sliding the coil along the rod. I then held it in place with a cable tie. To enable the rod to be oriented to the appropriate radio station, I drilled a 10mm hole in the end of a piece of wood and glued one end of the ferrite rod into it. I then mounted the piece of wood with a nut and bolt as shown. To make solder tags so I could lengthen the flimsy wires of the antenna coil, I fitted a rubber grommet over the ferrite rod and pushed some short loops of copper wire through the rubber. The tiny tuning capacitor (again intended for a small transistor radio) is mounted on a right-angle metal bracket from Bunnings. Because they sell these things by the thousand, they’re very cheap. There is a 20mm diameter hole punched on each face and by filing semicircular notches on opposite sides of the hole, the tuning capacitor can siliconchip.com.au be mounted nice and firmly with the supplied 2.5mm screws! Order of construction A sensible order of construction is to drill and modify the chassis as required, solder in the tagstrips and valve sockets (as these handle the point-to-point wiring) and then start with the electronics. As mentioned earlier, the chassis for the radio is actually a “Willow” brand tinplate cake tin, presently available from Big W for $2.60. Start by cutting the holes for the three valves in the positions shown in the photographs. You’ll need a 20mm hole saw for this but you don’t need to spend a lot of money as you’re only cutting into tinplate. Even cheapie hole saws from a bargain shop should be fine. The actual positions of the valves are not critical; just remember to leave room for the capacitor mounting bracket (you’ll get a good idea from the photographs). The layout is designed to keep the audio output valve as far away from the mixer as possible in the interests of RF stability. You could drill six extra holes and use 3mm nuts and bolts to attach the valve sockets but you’ll find it quite easy to simply solder them in. The same applies to the tag strips. A good place to start actual electronic construction is the power supply section, since without that, nothing else will work. Using the labelled photograph as a guide, solder the four capacitors and four diodes onto the tagstrip. You should find the cake tin is easy to solder to. Whether you solder this tagstrip in first or solder the components to the tagstrip then solder it in is up to you – both have their advantages. Just remember that the outer two positions of the tagstrip go to earth so don’t solder components to these! Be careful with polarities – all of these components are polarised. The diodes are easy because they are all cathode to anode, with the anode of D1 soldering to earth (the cake tin). When soldering the electrolytic capacitors in, make sure their leads don’t short to anything. Testing the supply Before soldering in the 10W heater resistors, (carefully!) check your power supply by temporarily connecting the 24V AC. Make sure that you have about 130V or so across C22 and 50V or so across C20. (As we mentioned, these voltages will drop to those shown on the circuit when current is being drawn). If OK, install the tagstrip holding R15 and solder it and R14 in place – but remember that Capacitor Codes o o o o o o o o o o o No. 2 1 1 1 1 1 3 1 1 2 Value 220nF 56nF 47nF 10nF 6.8nF 4.7nF 3.3nF 680pF 100pF 12pF mF Code IEC Code EIA Code 0.22mF 220n 224 .056mF 56n 563 .047mF 47n 473 .01mF 10n 103 .0068mF 6n8 682 .0047mF 4n7 472 .0033mF 3n3 332 n/a 680p 681 n/a 100p 101 n/a 12p 12 the unloaded electros will take some time to discharge – a 1kW 1W resistor on a pair of alligator clips makes a useful discharger. Check the heater line It’s a good idea to check the resistance of the valve heaters before going any further – naturally, you’ll need to have completed the valve heater wiring to all three valves before this check. Make sure that you don’t have either the power connected or any valves plugged in. As you would expect, the heater line (ie, between points A and D on the circuit) should measure open-circuit. With just the 6BM8 plugged in you should measure about 50W and about 14W with all three valves plugged in. (Just as with a lamp filament, this resistance will increase as the valves heat up). Audio preamp and amplifier Once you have the power supply finished, the next logical step is to get the audio amplifier stage built and working. The amplifier stage includes all capacitors from C11-C18, resistors from R6-R13, the wiring to the speaker transformer (and obviously speaker) and the connections to the power supply. As these components are spread across the other three tagstrips it makes sense to solder them in now. Construction of the amplifier stage is fairly straightforward but be careful where components cross over each other that they don’t short. Modifying the speaker transformer It’s becoming more and more difficult (and expensive!) Resistor Colour Codes o o o o o o o o o o siliconchip.com.au No.   2   2   1   3   1   1   2   1   1 Value 470kW 220kW 100kW 47kW 15kW 10kW 3.9kW 470W 330W 4-Band Code (1%) yellow purple yellow brown red red yellow brown brown black yellow brown yellow purple orange brown brown green orange brown brown black orange brown orange white red brown yellow purple brown brown orange orange brown brown 5-Band Code (1%) yellow purple black orange brown red red black orange brown brown black black orange brown yellow purple black red brown brown green black red brown brown black black red brown orange white black brown brown yellow purple black black brown orange orange black black brown January 2008  67 No, the hose clamp is not used in case of a grid leak. And it doesn’t hold the valve together! The earthed hose clamp is effectively a magnetic shield to reduce instability at the low end of the band. Don’t knock it: it works! to heat up but then you should hear music coming from the speaker – and its level should be adjustable with the volume pot. The headphone output from a personal stereo should be able to drive the speaker at a reasonable volume, depending on the particular player. Don’t expect it sound like your hifi system; 20% harmonic distortion in a domestic mantel radio at full output was considered average, 5% was high fidelity! An alternative is the “blurt” test – a damp finger on the pot wiper (say at mid-range) should get you a healthy raspberry from the speaker! If you can’t get any sound from the audio stage refer to the troubleshooting section later in this article. Topside hardware to buy speaker transformers. So the “speaker transformer” is actually a DSE 240V to 30V mains transformer with tappings at 7.5V, 15V, 22.5V and 30V. I used the 7.5V section with a 4W speaker. The transformer will work as it comes from the manufacturer but it can be made better by removing and re-stacking the laminated iron core. The core consists of equal numbers of “E” and “I” shaped pieces, interleaved so that half the “I” sections are on one side and half are on the other side. This is fine for a power transformer because it minimises magnetic flux leakage, giving the best efficiency. However, the transformer has DC flowing through the primary and this will magnetise the core, which can lead to distortion if the core saturates on peak plate current excursions. It also tends to limit the high-frequency response of the transformer. If you pull the core stack apart and rearrange the pieces so that all the “E” sections are on one side and the “I” sections are on the other, this will tend to prevent saturation. It will make the transformer less efficient at low frequencies but this radio won’t be reproducing much below 150Hz. The transformer is easy to pull apart and reassemble. All you have to do is pull the aluminium frame off with a pair of pliers, put the stack in a vice and pull out one of the “E” sections, also with pliers. Once you get the first one out, the others will pull out much more easily, and after that it will more or less fall apart. When reassembled, mount the transformer on the top of the chassis, soldering its feet to the chassis. You may need to scrape away some of the passivation on the transformer feet to get a clean surface to solder to. Connect its primary leads to the top of C18 and to HT1. Testing the amplifier You can easily test the amplifier section using the audio from the headphone socket of a portable CD or MP3 player. Temporarily, wire the player output directly across the volume control (outer terminals). Connect your speaker to the transformer secondary (0V and 7.5V taps), plug in the 6BM8 valve and apply power. Naturally, you’ll have to wait a little while for the valves 68  Silicon Chip We’ll leave the underside of the chassis briefly and look at the hardware on the top side. You can see what we have added to the cake tin in our photographs. The metal L-shaped “legs” fitted to three corners of the chassis are pieces of cheap pergola ironmongery and their main purpose is simply to allow you to turn the chassis upside down without breaking the valves! At the same time, they make handy mounts for the volume control and ferrite rod antenna. They come pre-drilled and in this case I’d recommend the use of small nuts and screws for mounting, as they are quite thick and would be hard to solder without a really large iron. The front two (horizontal) L-shaped brackets screw together to form a “U” shape. These hold the oscillator coil, the tuning capacitor with its dial drum and the tuning drive shaft. The tuning drive shaft is actually a piece of 9.5mm Tasmanian Oak wooden dowel! 10mm holes are drilled in the front and rear brackets, the holes are smoothed down with sandpaper, a bit of grease is applied, and the shaft turns as smooth as silk! If wood sounds like an unlikely material, remember that in days gone by, wooden wagons used to go for hundreds of miles with wheels that turned on “bearings” like this! A pair of small diameter rubber hose clamps (from a $2 bag of 10 from a cheap shop!) keeps the shaft from moving out of position. The tuning capacitor mounting plate obviously mounts between the two front chassis “legs”. For a drive cord, I used some “brickies’ twine” which is a slightly stretchy polyester string but I have also used dental floss quite successfully. (You can of course get some real dial cord from Wagners!) Since the dial cord doesn’t directly drive a station display (with a pointer and so on), the stringing is not particularly critical. More adventurous constructors could try their hand at a traditional slide rule display, possibly running the string across a couple of pulleys mounted on the front legs. A suitable source of such pulleys might be a discarded venetian blind assembly. Hose clamps We’ve mentioned the hose clamps on the dial drive assembly – but what’s that hose clamp doing around V1 (the IF amplifier valve)? Ideally, this valve should have a socket that incorporates a shielding can to reduce the possibility of instability at siliconchip.com.au the low end of the broadcast band. However, I found a makeshift shield made from a piece of aluminium foil and held on with an earthed hose lamp worked fine! And then I found just the earthed hose clamp was enough! When you tighten this clamp, don’t overdo it. You don’t want to let the air into V1 (or let the smoke out when you turn it on!). The coils The IF, aerial and oscillator coils will require some care with their mounting, as they are quite small and fragile. If you are experienced with metalwork, you could drill a set of small holes in the tinplate chassis and mount the coils more or less in the traditional manner but this will require accurate drilling and great care with the soldering. Another approach is to make 10mm holes with a wood drill, carefully file them out so that the coil pins don’t touch the chassis, and then solder the metal cans to the chassis via their mounting lugs. The problem with this approach is that as you unscrew the ferrite cores, there is a tendency for them to push the coil assembly out through the bottom of the can. You can prevent this by directly soldering the coils’ earth connection pins to the chassis but this will make the coils difficult to remove if that becomes necessary. When soldering wires to the pins, only use flexible hookup wire (from rainbow cable or the like). The coils are wound with very thin enamelled wire, with no slack where it attaches to the pins, and any tension on solidcore wire will tend to twist the coil pins and break the connection. Just in case you hadn’t worked it out from the photos, the aerial coil and IF transformers mount under the chassis, while the oscillator coil mounts on a bracket close to the tuning capacitor on top of the chassis. The rest . . . Once you have the audio working, you can tackle the tuner, IF and detector stages. Capacitors C1-C10, resistors R1-R5, two valves (V1&V2) and all the IF transformers and coils make up this section. There are no tricks to this – you just wire it as per the photographs and it should work straight off, at least after a fashion. I’ve made four versions of this circuit now and provided everything is correctly wired, the chances are that, with a reasonable antenna, you’ll pick up stations straight off. If the radio sounds completely “dead” even with the volume turned right up, you most likely have a wiring fault. Once again, refer to the troubleshooting section. However assuming that you have everything wired up correctly and are receiving stations of some sort, the next step is alignment of all the tuned circuits. Alignment – it’s not too daunting The best way to align any AM radio receiver is with a 455kHz oscillator, modulated at about 400Hz. If you don’t have one, check out the Minispot Modulated Oscillator in this month’s SILICON CHIP – see page 72. Connect the oscillator’s output to pin 2 (pentode grid) of V1. With any luck (and if you haven’t fiddled with the cores of the IF transformers), you should hear some sort of 400Hz tone from the speaker. Using a proper core-adjusting siliconchip.com.au Audio Troubleshooting If you can’t get any sound form the audio stage, the time-honoured checklist is as follows: • Is the valve heater glowing? • Does the glass envelope feel hot? (Not just warm). • Pull the valve out while it is still running. Do you hear a loud click or thump from the speaker? If not, check the speaker and the speaker wiring. If all the above check out, you’ll have to start comparing the voltages on the valve pins with those marked on the circuit. The most likely cause of problems is simply incorrect wiring or poor soldering. You should measure around 6V on pin 2 (cathode) of the 6BM8, indicating a plate current of about 18mA. If the plate and screen voltages are significantly higher than 100V and there is no cathode voltage, it means that the valve is not drawing current and may be faulty. If all that checks out, try touching the grid (pin 3) with your finger (or with the shaft of a metal screwdriver held in your fingers). If the pentode is working properly, you should hear a 50Hz buzz from the speaker. If you do, the pentode amplifier is working but there’s something wrong with the triode preamp. Check and double check your wiring and components. Tuner/IF Troubleshooting Assuming you have the audio section working, if you can’t find anything obvious, you’ll need to check some voltages. First check the screen and anode pins (3 & 6) of the 6BL8s. You should measure about 90-100V. If that seems in order, check pin 1 of V1b which is the oscillator triode anode. It should measure around 60V. If the voltage is too low, check pin 9, the oscillator grid. If the oscillator is working, you should measure a negative voltage somewhere between 12-18V. If not, the most likely cause is either incorrect wiring, poor soldering . . . or an open-circuit oscillator coil, possibly damaged during the construction process. tool (or a sharpened knitting needle, NOT a metal-bladed screwdriver), adjust the cores for maximum volume from the speaker. (As the volume increases, turn down the output level of the test box, not the volume of the radio). More critical adjustment requires measuring the AGC voltage across C4, preferably using a digital multimeter or any other meter with a sufficiently high input impedance. You can just do it by ear if you keep the input signal level right down. You’ll find there is some interaction between the adjustments, so you may need to go over them a couple of times to get it exactly right. If you don’t have an accurate source of 455kHz but you have access to a basic digital frequency counter (even one built to a multimeter), you can still accurately align the IF by a more roundabout route. First, you need to find out the frequency of one of your local radio stations. The announcer or station jingle usually tells you what their frequency is quite often, or if you have access to any sort January 2008  69 Here’s a close-up view of that “unique” dial drive assembly we talked about earlier – a length of dowel held in place by a couple of hose clamps. You can also see the two “L” brackets that combine to form the U-shaped mounting bracket. of radio with a digital tuner you can identify it that way. In this example, we’ll use Sydney station 2SM, on a frequency of 1269kHz. If you’re in another location, choose a reasonably strong station towards the top of the band. What you have to do is monitor the frequency of your radio’s local oscillator at the junction of the oscillator coil and C3 and adjust the radio’s tuning until you get a reading of the chosen station’s carrier frequency plus 455kHz. In the case of 2SM it will be 1269 + 455 = 1724kHz. It’s then simply a matter of adjusting the IF cores for maximum output of 2SM’s signal. (Caution: it is entirely possible to mistune all the IF coils to some frequency other than 455kHz and so pick up some other station, so just be sure you are listening to 2SM or whatever!) If you don’t have access to any sort of test equipment, you can simply tune the radio to any station you can find and simply peak up the IF coils for maximum volume. While this will still work, you may not get coverage over both ends of the broadcast band (more about this later). Adjusting the oscillator circuit Once the IF is aligned, you then need to adjust the oscillator circuit so that the radio covers the entire AM band and adjust the aerial tuning to match. Here is where some compromise may be needed. If you simply want a standard radio that covers from 530kHz to 1602kHz, the aerial and oscillator alignment will be quite straightforward and will present no surprises to anyone experienced with vintage radios. However, in Australia, the AM band has been extended up to 1.7MHz, mostly for special interest stations (mostly ethnic broadcasts entirely in foreign languages). It is just possible to get this radio to tune up to 1.7MHz but only at the expense of the “bottom” end of the band. However, not everybody needs to tune right down to the bottom of the AM band and for those who do, chances are they may not need the extra coverage at the high end. 70  Silicon Chip In commercial receivers, the alignment procedure was generally based around getting the receiver tuning to line up with the frequency or station markers printed on the dial scale. However, since we are going to make our own scale, in this case it’s simply a matter of getting it to tune over the desired frequency range. If you have access to a modulated signal generator, the procedure is quite easy. (We’ll just describe the standard tuning range here to start with). You start by setting the signal generator to 1602kHz and with the tuning capacitor turned fully clockwise, you adjust the oscillator trimmer capacitor until you clearly hear the 1602kHz signal. You then adjust the aerial trimmer capacitor to give maximum sensitivity at that frequency, measuring the AGC as you did for the IF alignment. Next, turn the tuning capacitor fully anti-clockwise, reset the signal generator to 530kHz and adjust the oscillator coil’s ferrite core until you clearly hear the 530kHz signal. That done, adjust the position of the antenna coil on the ferrite rod for maximum sensitivity. If you now turn the tuning capacitor fully anti-clockwise again, you’ll find that your previous adjustments will now be slightly off and so some readjustment will be needed. Old repair manuals used to state that you need to repeat these adjustments several times but twice should be good enough. If you don’t have access to a signal generator but have a frequency counter, you can measure the local oscillator frequency instead. To receiving 1602kHz, you need a local oscillator frequency of 2.057MHz and to receive 530kHz, an oscillator frequency of 985kHz. This will get the tuning range right but to adjust the aerial trimmer and aerial coil, you’ll need to find two stations as close as possible to 1602kHz and 530kHz. If you do this at night with a reasonable aerial connected, you should have no trouble finding suitable stations, and setting the aerial tuning is easier with weaker stations anyway. If you want your radio to tune up to 1.7MHz, you will siliconchip.com.au probably need to adjust the oscillator trimmer to its minimum position and also screw the core of the oscillator coil out slightly to get coverage of those frequencies. The only problem with that is that you will probably find the radio will no longer tune right down to 530kHz, although this will depend on the actual tuning capacitor used. You may also find that the aerial trimmer still has too much capacitance even at its minimum position and you may have to compromise with the position of the aerial coil. The dial drive, pointer and scale The reduction drive assembly is unconventional in construction but works extremely well. I used a plastic trolley wheel from Bunnings, with the rubber tyre pulled off. This was and attached it to the tuning knob that comes with the Jaycar tuning capacitor. The actual tuning dial was made from a discarded recordable CD (or DVD)! The dial pointer was made from a large diameter plastic cable gland, simply pushed through a hole cut in the front of the cabinet. The dial cursor is simply a sewing needle pushed through the slots in the gland. The assembly of the dial scale is as follows: first, carefully ream out the centre hole of the CD so that it just fits snugly on the axle part of the trolley wheel. It has to be tight enough that it won’t move by itself but not so tight that you can’t easily adjust its position with respect to the dial pointer. Wrap sufficient insulation tape around the centre boss of the tuning knob that comes with the tuning capacitor, so that it fits snugly in the axle hole of the trolley wheel, and push it in onto the same side that the CD will be mounted on. Drill or otherwise cut a hole in a scrap piece of wood so that the trolley wheel can lie down flat on it. Draw a line through the centre of the tuning knob and drill two 3mm holes on the line, on opposite sides of the hole. The drill holes need to go through the knob and the trolley wheel, and you need to drill them as straight as you can. If at all possible, use a drill press. (The $79.95 ones you often see in hardware stores are more than adequate for the job and are well worth considering – you’ll wonder how you got by for so long without one!) Once the holes are drilled, pull the tuning knob out, remove the tape and using two 25mm M3 screws, attach it to the opposite side of the trolley wheel. After that, you simply mount the original knob onto the tuning capacitor as the manufacturer intended, with the supplied screw. After the radio has been constructed and aligned, the dial-scale markings are made by first fitting the CD to the trolley wheel boss and then mounting the chassis in its correct position in the cabinet. You then have to identify all the stations and write their positions on the CD with a fine-tipped CD marker or similar, using the sewing needle cursor to rule guide lines. If you have a frequency counter, and know the station frequencies, the easiest way to do this is to set the local oscillator frequency to the station carrier frequency plus 455kHz and mark the dial accordingly. You could just make a “generic” frequency dial similar to those found on commercial radios today but most vintage radios have the actual station markings (which may not be terribly accurate anymore!). Of course, there’s nothing to stop you from providing both! Once you’ve got all the stations marked, remove the CD siliconchip.com.au from the assembly and scan it into a photo editing program such as Photoshop. The actual procedure from here will vary with the particular software you have but basically you create a new layer on top of the scanned image (usually referred to as the “background”) and overwrite your handwritten station markings with whatever font you think looks authentic! More advanced packages allow you to distort the shape of text so you can produce even fancier results. Some packages such as Corel Photo-Paint allow you to copy and paste WordArt objects from Microsoft Word into a drawing and it’s a very quick and easy way to import fancy lettering. Once you have the station markings done, you will then need to erase your handwritten ones from the background layer (leave the image of the CD itself as a guide to lining up the label), add whatever background colour you want, and print it out. (Leaving aside jokes about blonde typists and whiteout on the screen, if you have an LCD computer monitor, a very useful technique is to cover the screen with a piece of Glad Wrap and rule your guide lines on to that with a fine-tipped felt pen!) If you don’t have a fancy drawing package that can do the necessary text rotation, you can always print the station call signs out on paper, and physically cut and paste the dial in the time-honoured fashion! More recent versions of Microsoft Word give you various “WordArt” options that allow you to print vertically. Then you can simply paste the printed call signs over your handwritten ones on your CD, scan that into the Paint program that comes with all copies of Windows, and do any touching-up necessary with that. I originally tried printing the dial scale onto a stick-on CD label (these are A4–sized sheets of adhesive label paper with two CD stickers per sheet) but I found it difficult to get the positioning right and you only get one go at it! Since we’re not worried about contaminating the CD surface, it’s much easier to print out the label on ordinary photo-quality paper, cut it out with scissors and stick it on with sprayon adhesive, which will allow you to slide it into position before the glue sets. (You don’t normally see the inner or outer edges of the label, so it doesn’t have to be all that neat a cutting job). If you have one of the new printers that can print directly onto CDs or DVDs, well of course that will be even better! If you want to have a back-lit display, you’ll need to glue the paper onto a transparent CD-sized disc. Many “spindle” blank CD and DVD packages have a transparent plastic packing piece that is perfect for the job. Another possibility is cutting one out of one of the cheap round polythene “clamshell” CD cases. I tried soaking a white label CD in paint thinner but the whole thing started to dissolve! SC Where do you get it? It’s unlikely that there will be a kit made up for this project. However, most electronic components are quite common and should be easy to obtain from normal parts retailers (eg, Dick Smith Electronics, Jaycar Electronics and Altronics). References are given in the text for some of the more obscure bits, especially the “hardware” items. The valves (and many other parts) are available from Wagner Electronics in Sydney; (02) 9798 9233 or www.wagner.net.au January 2008  71