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VINTAGE RADIO By JOHN HILL How to solve unusual problems When one's hobby is restoring old radio receivers, it is inevitable that every now and then a set comes along with an unusual problem. Solving some of these odd problems is often easier said than done and the new chum to the vintage radio scene may find some repairs quite difficult. Some time ago, I reworked a few of the sets in my collection. These receivers were ones that l had restored several years ago but, in some instances, the restoration work was not as good as I would do today. The reasons are many. First, I did not know then what I know now. Second, some recently acquired test equipment (such as a valve tester, a radio frequency generator and an output meter] now makes it possible to perform much more precise tune-ups than in the past. These reworked sets included a number of 4-valve receivers , several of which responded extremely well to the fine tuning they received. A 4-valve radio, in particular, requires spot on alignment if it -is to perform well. There was still another set that needed attention and I had been well aware of its shortcomings for quite some time. While it looked a nice enough receiver, it was a terrible performer due to poor alignment. This mid-1950s 5-valve Mullard was so far out of tune it was "double spotting". As the dial pointer was moved across a station, Small JOpF trimmer capacitors can be used to tune an IF transformer when the normal tuning slugs are immovable. The alternative is to replace the IF transformer but suitable replacements are not always available. 4 SILICON CHIP the station would tune in, tune out, then tune in again, thus occupying two side-by-side positions on the dial. It was a problem I could not handle at the time because the iron cores in the IF transformers had been butchered. The screwdriver slots had been gouged out by someone trying to get the rather stiff slugs to move. When I last worked on the Mullard, I did manage to move the slugs a little but it appears that they were moved in the wrong direction because the alignment got worse. In desperation, the chassis was put back into the cabinet and the set placed in the "too hard basket". Alignment techniques Anyone familiar with old radios will know that IF transformers can be tuned by two methods. The method used in the Mullard and most other more recent receivers is by means of adjustable iron cores. However, back in the early days of superhets, the IF transformers were tuned with small trimmer capacitors. It therefore seemed reasonable to assume that the Mullard could be pulled back into alignment by adding some 30pF trimmer capacitors to the base connections of the transformers. Four trimmers were soldered into the circuit to see if the theory would work - and work it did! As it happened, four trimmers were unnecessary and two were removed from the circuit for the better. Such a modification is a bit of a trial and error affair. To cut a long story short, the set tuned up really well and the double Besurreetion Radio The original vintage wireless specialists Our skilled technicians offer QUALITY repairs & restoration. We have a large stock of bakelite and timber radios fully restored and for sale. Valves, high voltage capacitors and other specialised parts available. Intermediate frequency (IF) transformers can be tuned by either adjustable iron cores (left) or adjustable capacitors (right). Correct alignment of the IF stages is vital if the receiver is to perform correctly. ADDITIONAL SERVICES COPIES OF: CIRCUIT DIAGRAMS $5.00 SERVICE MANUALS $10.00 AVAILABLE FOR MOST MAJOR AUSl . MAKES FROM 1934 ONWARD. WANTED - valves , radios, etc., purchased for cash. Call in to our showroom at: 51 Chapel Street, Windsor, Vic 3181 PO Box 1116 Telephone: (03) 529 5639 ACTIVE SHORT WAVE ANTENNA TECHNIKIT AT4SW 't (SEE SC JAN '90) COMPLETE KIT $59 BUILT and TESTED $119 (BATTERIES INC) CASE $10 This is the rear view of the old Seyon receiver mentioned in the text . Note the 2.5V SX245 output valve (centre) in the 4V socket (see text). You should always be prepared to improvise to restore an old set to full working order. spotting disappeared. While there were other ways of solving the problem (eg, replace the IF transformers), the simple capacitor trick got me out of trouble and that is what this story is all about. Harsh sound Another problem often encountered when restoring valve radios is harsh sound in a set that lacks a tone control. Quite often a set will restore OK but the tone is either harsh or, at the other extreme, dull and lacking treble response. This change of tone is often noticeable after a loudspeaker has been replaced and so some tone adjustment is necessary in order to give the set a more pleasant sound. This is easily done by installing a high voltage capacitor between the anode of the output valve and the chassis. A capacitor in this position has the ability to bypass the harsher high frequencies and they are shunted off to earth instead of going through to the loudspeaker. The larger the capacitor, the greater the effect. By simple trial and error, the LOOP ANTENNA Q TECHNIKIT PX1 COMPLETE KIT $44 BUILT and TESTED $69 (SEE SC JUNE 89) Improved signal strength & signal quality in a portable tunable antenna. PACKING & POSTAGE IN AUSTRALIA INCLUDED IN PRICES QUOTED. WRITE OR RING FOR BROCHURES ORDERS ACCEPTED ANYTIME PAYMENTS BY BANKCARD, VISA, MASTERCARD, CHEQUE or MONEYORDER TRADE ENQUIRIES WELCOME JILOA PTY LTD (TECHNIKIT DIVISION) P.O. BOX 73, GLENHUNTL Y, VIC 3163 Phone (03) 571 6303 ]ULY 1990 5 This is the Mullard receiver with the butchered IF transformers. It lived in the "too hard" basket for quite some time until a remedy was found. Always use a high voltage capacitor (400V minimum) in the plate circuit of the output valve. By changing the capacitor value, you can alter the tone of the radio to give the best result. These heavy wattage wirewound resistors are the types to use as filament resistors when using a 2.5V valve in a 4V or 6.3V socket. You can calculate the resistor value using the technique described in the text. tone can be varied by trying capacitors of different sizes. A 0.022µF capacitor would be a good starting point. On the other hand, if a set sounds dull and lifeless, then it is most likely due to too large a capacitor in this anode to chassis position. When this is the case, the capacitor must be located (it could also be on the first audio valve) and replaced with one of less capacitance. Most receivers without a tone control have a bypass capacitor which serves as a fixed tone control. In some instances, it may be switched in or out of circuit by a tone switch. However, remember that a set 6 SILICON CHIP sounds a good deal different on the bench compared to when it is in its cabinet. The baffling effect of the cabinet greatly improves the sound, so any adjustments to the tone should be done with this point in mind. The old Seyon One of my very early sets is an old Seyon, an electric model with 4V valves (not counting the 5V rectifier). Replacement valves for such radios can be a problem for they are indeed a rarity today. When I first bought the set, the output valve was a B406. Being fairly green about such things at the time, I didn't realise that the B406 was in fact a battery valve and as such, was not suited to an AC filament supply. The original output valve could have been any of a number of valves and I have since discovered that an E406 works quite well. However, before the E406 came along I was faced with the problem of using an appropriate output valve. Although the B406 worked, it didn't look the part and it produced a certain amount of hum due to its lightweight battery filament. It so happened that I had several SX245 and CX345 valves in my collection. These were appropriate output triodes for such a radio except that they had 2.5V filaments instead of the required 4V filament. To solve this problem, a resistor was placed in series with the filament, after which the 2.5V valve worked quite happily in the 4V socket. Mind you, there was a surprising amount of heat generated in the resistor but it did get me out of trouble and that's what this particular article is about. Now if you are clever and know the filament or heater rating of the valve, the value of the resistor required can be worked out by Ohm's Law. On the other hand, if you're not clever or have no specifications on ancieut valves, there is another way. All one needs is an old wirewound rheostat (a 200 one or less) and a multimeter. The rheostat and the valve filament are connected in series and placed across the filament supply voltage, making sure that the rheostat is set to maximum resist- If you want to use a 2.5V valve in a 4V socket, the required resistance can be determined using a rheostat and a multimeter. You simply connect the rheostat (set to maximum) in series with the valve heater, and connect the multimeter (set to AC volts) across the valve heater. The rheostat is then wound back until the meter indicates 2.5V and its value measured to obtain the resistance. odes in the form of a fairly heavy filament. Because these valves used an AC supply, a special centretapped cathode resistor was used across the filament to reduce mains hum. The amount of resistance used on a 4V valve would be different to that used on a 2.5V or 6.3V valve and the resistor should be replaced if the filament voltage is changed. Substituting a 4V valve with a 2.5V valve, as was done in the Seyon, would require a centretapped resistor of a slightly lower value. Not having the appropriate resistor meant that it wasn't changed at all and that didn't seem to make the slightest difference. It also meant that no alteration was necessary when the set was converted back to 4V operation. There are numerous situations whereby one can get out of trouble by using a resistor and a valve with a lower filament or heater voltage. Such a method is quick and easy and does not involve separate transformers or other inconveniences. It is also applicable to both AC and DC-powered receivers. Warning signs Some radios, such as this 5-valve Philips mantle set, have a tone switch at the back which cuts in a capacitor that is used as a fixed tone control. Altering the size of the capacitor affects the degree of tonal response. ance. This done, the multimeter (set to AC volts) is connected across the filament of the valve. The rest is straightforward: you simply adjust the rheostat until the meter reads 2.5V. The rheostat resistance is then measured with the multimeter to get the required value for the filament resistor. In the case of my valve substitution in the Sey on, there is a 4V supply with 1.5 amps flowing through the filament resistor and the 2.5V valve. Therefore, the potential across the resistor is 1.5 volts. Since Watts = Volts x Amps, the wattage of the resistor required must be 2.25 watts. A 5W wirewound resistor would do nicely, even if the valve drew two amps. The 10W one I used was more than adequate. Most of the old 2.5V output triodes had directly heated cath- Although this story is about getting out of trouble, ignoring certain warning signs can also get one into trouble. For example, my old Seyon has a potential problem that I have ignored for years - one that could give rise to a serious short circuit. The 240 volt supply to the power transformer is platted rubber covered wire and over the years the insulation has decomposed, merged into a single mass and gone quite brittle. Just how close those wires are to each other is anyone's guess and if there is a big zap one day and a fuse blows then I will know that they were a little closer than I thought. In my defence, there are two reasons why I have not replaced this old wiring. First, it has not been disturbed and is in working order. Second, the wires are not just rubber covered but also cotton covered. That layer of cotton seems to make all the difference and I doubt if it will ever give trouble. Only time will tell. ~ JULY 1990 7