Another old radio
I've just finished restoring back to use a 1936 Invicta AW57 valve radio. I've done a few 1950s valve radios over the last year, but this is my first attempt at something pre-war. It came into my possession from a friend having a clearout, and I spotted it's Art Deco 1930's style and thought it was worth a go.
The radio was in the usual terrible state - scratched, scuffed case with splats of paint, insides full of dust and spiders, mains cable cut off etc. But all the valves were present, and a quick check on t'internet found the service sheet from October 1936 for download. Radios up until the late 1960s were designed to be repaired rather than thrown away, so the circuit diagram and details of voltages etc are readily available. In addition it's relatively easy to remove parts and there's rarely anything unusual or unbotainable. Of course - it was made almost 75 years ago, so I'm not going to be able to pop to the high street for bits any more.
First job was to remove the chassis and speaker from the case. Valve (and early transistor) radios generally have all the components fitted to an aluminium box (or chassis) which serves to provide a bit of electrical screening as well as structure. In common with most pre-60s valve devices the valve sockets are bolted to the chassis along with any transformers or large components such as variable capacitors, and everything else (resistors, capacitors etc) is wired point-to-point between them underneath. The case had to go immediately out to the garage as it had woodworm holes that looked recent. So out with the Cuprinol woodworm killer and that was dealt with. I then left the case out in the garage for a couple of weeks in a bin bag to make sure there was no activity.
With the chassis out, I could give it a clean up. So out with the valves, and in with the vacuum cleaner and brush. Underneath, there was some evidence that the radio had been repaired probably during the 1950s - there were a couple of RadioSpares capacitors, some not in exactly the same places as the diagram in the service sheet. Capacitors until the 1970s were often made of foil and paper sealed in wax. Over time they dry out and don't work properly, so it's common to just replace every one of this construction as a matter of course. The modern plastic film replacements are much smaller and more reliable. I also replaced all the electrolytic capacitors as some had leaked electrolyte through their cardboard tubes. If I was a purist, I'd have carefully opened up the old capacitor's casing, removed the guts and put the smaller modern one inside so it all looks original. But I'm not, so I only did that with the main smoothing capacitors - a waxed cardboard box about the size of a pack of cigarettes which I pulled out the massive roll of foil and paper and replaced with two 10uF capacitors about the size of Wotsits.
The valves in this set are 1930s 'balloon' type valves with metal paint coatings. I have never played with this type of valve before, only the more modern 1950s all glass valves which are much, much smaller and have no coating. These are about 15cm tall, as compared the more usual 5cm for post 1950 valves. There is something attractive about the gold paint and shape of the bulb, but I have to say I prefer the more modern ones where you can see the structure and glowing heaters inside. Only the rectifier valve (far right, equivalent to two diodes the size of a grain of rice these days) is uncoated. The output pentode isn't gold painted, but has a silvery black coating inside the glass that prevents seeing anything.
Next was the wiring. Most of the circuit wiring was solid tinned copper with fabric insulation, which was OK to leave. Some of the longer wires were cotton wrapped stranded copper, again in good condition. But some was rubber insulated, which had dried out and cracked. That had to be replaced with silicone coated, which looks the same but will last a long time. Also the frayed cotton covered rubber mains cable had to be replaced with modern mains cable. Valve radios draw well under 1 Amp, so there's no need for anything special. I tend to fit a 1A fuse in the plug too rather than the 3A or 13A that's usually there.
A good going over with contact cleaner and lubricant brought the variable capacitors, volume, tone and waveband controls back to life. Hopefully eliminating most crackles and giving a nice smooth tuning action. This set is unusual (to me) in that it's got a vernier type tuning control. An outer knob moves the needle quickly, and an inner one moves it at about 1/3 the speed for fine tuning. A bit of WD40 to wash out the old grease and a little bit of new grease got that moving silkily. All the resistors were checked and found to be within 10% of their stated value - which is pretty good going.
The valves were replaced (after a wipe over to remove dirt), and the radio powered up for the first time - through a device called a 'lamp limiter' that is a good idea on things like this. Essentially it's a 100W bulb in the live lead so if the radio draws a lot of current the bulb will drop the voltage, preventing damage. In practice you turn on the radio and watch the bulb as well as a voltmeter on the high voltage part of the circuit. As the valves warm up, the voltage should rise and the bulb will change in brightness. In this case - the HT voltage levelled out at about the right value (220 volts), but the bulb was glowing much more brightly than I'd like - and there was no noise from the speaker.
So I switched off and set about checking the valves. Usually this excessive current draw it down to the valve that drives the speaker, so I removed it and tried again. Still too much current. Then I tried the first valve in the circuit, the mixer/oscillator octode valve. It rattled. Not a good sign! Tried power, and the current draw was much less. In addition, moving the volume control produced a satisfying crackle from the speaker. Excellent news.
Knowing the main valve involved in receiving radio signals was dead wasn't the end of the world. I connected a small battery radio to the gramophone input on the radio and got audio out of the speaker. Very good news indeed, as it meant that the output stages of the radio worked fine. Which gave me confidence for the tuning section.
I managed to get a second hand Mullard FC4 valve from one of the usual vintage wireless suppliers for £7 - not bad for 73 year old valve that would have cost 15/- when new (equivalent to over £100 now). Plugged it in, and the current draw was still good. As the valves heated up, I got a faint sound of music from speaker. A twiddle of the tuning control and I had BBC Radio York belting out nice and loud. Yay!
Now I went round checking the voltages on the valve pins corresponded to those in the service sheet, and they did (more or less). So I was happy that all was well and went to look at the case again. I'm not very good with wood, so decided to just use wire wool to remove the paint and other things spilled on it, and take out some of the smaller scratches, then put a bit of brown wax in the woodworm holes and finally a thin coat of varnish to protect the wood. Maybe one day I'll take it to someone who can strip it back and restore it properly. I managed to clean up the plastic (some sort of cellulose resin I think) window the goes over the tuning dial with a combination of Mr. Muscle, carburettor cleaner and a lot of paper towels.
Reassembled it all this morning, then tuned around the various bands to see what it'd get. Unlike the 1950s sets it doesn't have a built in aerial, as it would have relied on a long wire in the garden that most houses would have had back then. However it has a wander plug on the back panel next to the aerial socket to enable you to use the mains cable as an aerial. Works surprisingly well, unless you've got nasty modern switched mode power supplies nearby which chuck out loads of rubbish onto the mains.
I quite like working with pre-1960s electronics, despite my 1990s degree. Components are bigger and things are designed to be easy to replace and test. Modern things are designed to be made quickly and cheaply by machines and thrown away if they break. Plus you can't beat the sheer warmness of the sound and look of valves.
So now it's taken it's place in our sitting room, with it's Edwardian purple fireplace, bookcases, lights and chairs. If we just removed the TV you could be mistaken for thinking you were in the 1930s in that room. :)
The radio was in the usual terrible state - scratched, scuffed case with splats of paint, insides full of dust and spiders, mains cable cut off etc. But all the valves were present, and a quick check on t'internet found the service sheet from October 1936 for download. Radios up until the late 1960s were designed to be repaired rather than thrown away, so the circuit diagram and details of voltages etc are readily available. In addition it's relatively easy to remove parts and there's rarely anything unusual or unbotainable. Of course - it was made almost 75 years ago, so I'm not going to be able to pop to the high street for bits any more.
First job was to remove the chassis and speaker from the case. Valve (and early transistor) radios generally have all the components fitted to an aluminium box (or chassis) which serves to provide a bit of electrical screening as well as structure. In common with most pre-60s valve devices the valve sockets are bolted to the chassis along with any transformers or large components such as variable capacitors, and everything else (resistors, capacitors etc) is wired point-to-point between them underneath. The case had to go immediately out to the garage as it had woodworm holes that looked recent. So out with the Cuprinol woodworm killer and that was dealt with. I then left the case out in the garage for a couple of weeks in a bin bag to make sure there was no activity.
With the chassis out, I could give it a clean up. So out with the valves, and in with the vacuum cleaner and brush. Underneath, there was some evidence that the radio had been repaired probably during the 1950s - there were a couple of RadioSpares capacitors, some not in exactly the same places as the diagram in the service sheet. Capacitors until the 1970s were often made of foil and paper sealed in wax. Over time they dry out and don't work properly, so it's common to just replace every one of this construction as a matter of course. The modern plastic film replacements are much smaller and more reliable. I also replaced all the electrolytic capacitors as some had leaked electrolyte through their cardboard tubes. If I was a purist, I'd have carefully opened up the old capacitor's casing, removed the guts and put the smaller modern one inside so it all looks original. But I'm not, so I only did that with the main smoothing capacitors - a waxed cardboard box about the size of a pack of cigarettes which I pulled out the massive roll of foil and paper and replaced with two 10uF capacitors about the size of Wotsits.
The valves in this set are 1930s 'balloon' type valves with metal paint coatings. I have never played with this type of valve before, only the more modern 1950s all glass valves which are much, much smaller and have no coating. These are about 15cm tall, as compared the more usual 5cm for post 1950 valves. There is something attractive about the gold paint and shape of the bulb, but I have to say I prefer the more modern ones where you can see the structure and glowing heaters inside. Only the rectifier valve (far right, equivalent to two diodes the size of a grain of rice these days) is uncoated. The output pentode isn't gold painted, but has a silvery black coating inside the glass that prevents seeing anything.
Next was the wiring. Most of the circuit wiring was solid tinned copper with fabric insulation, which was OK to leave. Some of the longer wires were cotton wrapped stranded copper, again in good condition. But some was rubber insulated, which had dried out and cracked. That had to be replaced with silicone coated, which looks the same but will last a long time. Also the frayed cotton covered rubber mains cable had to be replaced with modern mains cable. Valve radios draw well under 1 Amp, so there's no need for anything special. I tend to fit a 1A fuse in the plug too rather than the 3A or 13A that's usually there.
A good going over with contact cleaner and lubricant brought the variable capacitors, volume, tone and waveband controls back to life. Hopefully eliminating most crackles and giving a nice smooth tuning action. This set is unusual (to me) in that it's got a vernier type tuning control. An outer knob moves the needle quickly, and an inner one moves it at about 1/3 the speed for fine tuning. A bit of WD40 to wash out the old grease and a little bit of new grease got that moving silkily. All the resistors were checked and found to be within 10% of their stated value - which is pretty good going.
The valves were replaced (after a wipe over to remove dirt), and the radio powered up for the first time - through a device called a 'lamp limiter' that is a good idea on things like this. Essentially it's a 100W bulb in the live lead so if the radio draws a lot of current the bulb will drop the voltage, preventing damage. In practice you turn on the radio and watch the bulb as well as a voltmeter on the high voltage part of the circuit. As the valves warm up, the voltage should rise and the bulb will change in brightness. In this case - the HT voltage levelled out at about the right value (220 volts), but the bulb was glowing much more brightly than I'd like - and there was no noise from the speaker.
So I switched off and set about checking the valves. Usually this excessive current draw it down to the valve that drives the speaker, so I removed it and tried again. Still too much current. Then I tried the first valve in the circuit, the mixer/oscillator octode valve. It rattled. Not a good sign! Tried power, and the current draw was much less. In addition, moving the volume control produced a satisfying crackle from the speaker. Excellent news.
Knowing the main valve involved in receiving radio signals was dead wasn't the end of the world. I connected a small battery radio to the gramophone input on the radio and got audio out of the speaker. Very good news indeed, as it meant that the output stages of the radio worked fine. Which gave me confidence for the tuning section.
I managed to get a second hand Mullard FC4 valve from one of the usual vintage wireless suppliers for £7 - not bad for 73 year old valve that would have cost 15/- when new (equivalent to over £100 now). Plugged it in, and the current draw was still good. As the valves heated up, I got a faint sound of music from speaker. A twiddle of the tuning control and I had BBC Radio York belting out nice and loud. Yay!
Now I went round checking the voltages on the valve pins corresponded to those in the service sheet, and they did (more or less). So I was happy that all was well and went to look at the case again. I'm not very good with wood, so decided to just use wire wool to remove the paint and other things spilled on it, and take out some of the smaller scratches, then put a bit of brown wax in the woodworm holes and finally a thin coat of varnish to protect the wood. Maybe one day I'll take it to someone who can strip it back and restore it properly. I managed to clean up the plastic (some sort of cellulose resin I think) window the goes over the tuning dial with a combination of Mr. Muscle, carburettor cleaner and a lot of paper towels.
Reassembled it all this morning, then tuned around the various bands to see what it'd get. Unlike the 1950s sets it doesn't have a built in aerial, as it would have relied on a long wire in the garden that most houses would have had back then. However it has a wander plug on the back panel next to the aerial socket to enable you to use the mains cable as an aerial. Works surprisingly well, unless you've got nasty modern switched mode power supplies nearby which chuck out loads of rubbish onto the mains.
I quite like working with pre-1960s electronics, despite my 1990s degree. Components are bigger and things are designed to be easy to replace and test. Modern things are designed to be made quickly and cheaply by machines and thrown away if they break. Plus you can't beat the sheer warmness of the sound and look of valves.
So now it's taken it's place in our sitting room, with it's Edwardian purple fireplace, bookcases, lights and chairs. If we just removed the TV you could be mistaken for thinking you were in the 1930s in that room. :)