Murphy A92 Receiver

 
 The first job will be to generally clean up this set, take some pictures and fit a new piece of speaker cloth. This receiver was introduced to the UK market in April 1940 and only just made it before economy restrictions and high taxation were imposed to preserve our stocks of raw materials. The set was expensive, being almost £16 plus purchase tax which was introduced in October of 1940 at a third of the manufacturer's published price. This bumped up the price of the radio from £15:15:0d to £21. Note that in those days and for some time after WW2 retailers used the ploy of quoting guineas to make prices seem less than they were. This Murphy was therefore sold for 15 guineas and the buyer would need to dig a little deeper for an extra 15 shillings. A typical weekly wage in 1940 would have been something like £5 so, assuming a family could save say 5 shillings a week for luxury goods it would take 60 weeks to raise the £15 and a further 3 weeks for the "guinea" fiddle. Purchase tax would then add a further 21 weeks to the wait for a new Murphy.

 The fancy bakelite dial surround is typically Murphy, being completely different to other makes of sets of this period. Eight push buttons select the waveband of your choice and the tuner travels vertically up or down across the scale. Where there are only a few stations, on the 13m, 16m and 19m bands the scales are reduced in height and a world map is drawn on the dial which includes many distant station names. For some odd reason, many British station names such as Droitwich National are in a reduced font size. Maybe this set was aimed at the exotic station listener, after all it has 8 wavebands, and as such, with that amount of choice, why would you want to listen to boring local stuff?

 

click to see the Trader Sheet

 The first step was to remove the rear cover and to find out why there was a large hole in the side of the set. Fortunately, all that had happened was the tone control on/off switch had been pushed inwards when the set had been lifted and the glue holding the assembly had failed and the thing had fallen inside. Next to check the valves, or at least at this stage to see if they were all present and plugged into the correct sockets. I found the top cap for the RF stage valve, an SP41 had become detached from the glass envelope. Alas, because of the skirt surrounding the top cap it isn't possible to prise off the clip and if the glue fails it's just tough luck as there's no other way to get it off. Fortunately, I've repaired a similar style of valve and I'm sure I can fix the cap. See here. The original copper lead was completely buried in the glass pip and I very carefully filed away the glass to reveal less than a millimetre of copper which I scraped till it was shiny then tinned it and wound some very thin wire around it leaving enough to poke through the hole in the top cap with five millimetres of slack so soldering the cap wouldn't unsolder the wire inside it. Finally a dab of solder to connect the cap and some superglue to secure the cap to the glass. Although I have plenty of SP61 valves, the SP41with its 4-volt heater is much less common.

The chassis is in first class condition as you can see.. nicely painted in blue. The tall reservoir/smoothing condenser must be a replacement as it has the price 11/6d on the side. The dial cord must be intact as the cursor works freely.

 

 Below, the set of Mazda valves. The printing is in green and is always difficult to read. As with most Murphy sets using 8-pin valves they used Mazda types using an odd size octal base. An owner wishing to replace a dud valve had no choice but to buy a Mazda valve. I don't believe I've ever seen an explanation as to why Mazda didn't fall into line and use an International Octal base, but they didn't so both Mazda and International octal sockets had to be fitted on wartime equipments. One of the most common WW2 valves was the SP61 which used an MO base although, due to the huge numbers needed I'm sure it was made by several valve manufacturers. Because the centre spigot was larger than that of the IO type it was impossible to insert an MO based valve into an IO socket but, with a bit a push, not vice versa. An IO valve wouldn't warm up though because the heater pinning was different. Mazda used pins 1 and 8, quite sensibly for the heater, whilst IO valves usually used pins 2 and 7 for this purpose.

 

 

 

 Above: The mains energised speaker, so called because it does not use a permanent magnet but has a large coil of several Henries inductance first powered via a rectifier from AC mains (click the link to read more) or directly from DC mains. Later an engineer had a brainwave and employed the large coil as a smoothing choke and therefore able to dispense with the latter, thus saving some cash.

This speaker is thoughtfully connected via a plug and socket to the main chassis which is a great boon to the serviceman as he doesn't need to cut the wires or balance the detached speaker on the chassis as dismantling takes place.

Above right is the tone control and on/off switch. My guess is the Murphy engineering team were told not to clutter the attractive front of the set and stick these controls out of the way. The on/off switch still feels very positive. Many from this vintage of set get a lazy operation and eventually burn and fail.

And right the label carried on the back of the set.

 

 

 

Above the damaged then repaired SP41 RF amplifier valve with the glue setting. Evidenced by the solder blob the valve must have already been fixed once before.

 The next job was to replace the speaker cloth. I bought a job lot of this stuff many years ago so went through it and chose a patterned type but rejected this because of the way the old cloth was fitted. It's amazing to see how the colour has changed from grey to brown. The wooden baffle board has a shaped piece glued to it that fits through the hole in the front of the set. You can see this in the second picture below. A circular piece of white rubber edging is nailed and glued over the cloth which itself is glued to the board.

 

 

 I tried for ages to fit the decorative rubber ring in place after fitting a new cloth, but I wasn't happy about the way it looked because the tension in the edge of the cloth pushed the edging out of place. I decided to dispense with the edging and fit the refurbished baffle board without it. It fitted perfectly centrally in the hole and once screwed into place the cloth tightened and looked OK.

 

 As there was now a narrow gap around the shaped wood I tried pushing the rubber edging ring into place. The original rubber had perished all the way round but the rear edge was in perfect condition so I fitted it back-to-front and found it fitted perfectly. Strange that Murphy hadn't discovered this in the set's design phase as it would have simplified production and saved costs.

 

 Below you can see the condition of rubber covered wire, thankfully this type of insulation is only used in two places viz. the tone control wires to the side-mounted potentiometer and the mains wiring between the switch and the transformer, so easily replaced. I fitted a new mains lead with modern colours for live, neutral and ground, connecting the ground wire to the chassis.

 

 

  Next I changed the 0.025uF condenser coupling the AF amp to the output valve. Although the original seemed to be in fair shape using an ohmeter I've found that by putting HT across these old condensers and monitoring current invariably shows up progressively worsening leakage. I fitted a high voltage 0.033uF.

 

 Below, the old receiver powered for the first time in probably 30 or 40 years.

 
 

 

 The set powered up more or less satisfactorily; at least it didn't explode, but there was a constant fizzling sound. At first I thought it was background interference from lighting and my network camera, but it turned out to be dirty contacts in the wavechange switch assembly. This was OK mechanically but of course after 30 or 40 years the contacts had tarnished. This is not too serious as the contacts are self-cleaning and by operating each one in turn progressively improved matters until reception stabilised. Sensitivity is really good because it has an RF amplifier stage and on a few inches of wire several medium wave stations and the stronger long wave broadcasts came in well. Connecting a long wire improved reception dramatically and also proved the set's AVC was working normally. Although I usually align a set like this, performance is so good I decided to leave it as there are a few risks such as dust core damage that can result. Before I refit the chassis securing screws and put the back on I'll replace just one dial lamp which is probably a 4 volt type, but no they are all 6 volt types so I fitted four new ones.
 

click to see the Trader Sheet

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