Murphy B143 Refurbishment

 I don't seem to have photographed this old portable receiver since I bought it many years ago. Partly this was because it had suffered an attack of woodworm and it had been consigned to a plastic bag and hidden away. The finish is a thin crackle coated with nicotine.

 When I'd spotted it at a car boot sale it had seemed innocent enough because the wooden parts were not obvious. The set uses a wooden panel, on which is mounted its sizeable loudspeaker, hinged to a wooden base although the set is predominantly made of aluminium and plastic.

The two wooden sections which, despite appearing pretty solid, are almost entirely eaten away.

 When I first saw the extent of the damage I'd put it away to check if the infestation had finished. Opening the plastic bag the other day revealed that there was no obviously fresh wood dust so the occupants had long gone. I decided the time had come to clean up the set and take a view on what was required to restore it.

The set is held together with screws and, as there are only two relatively simple wooden parts, I reckoned it was quite feasible to replace these and get the old set working.

 After unscrewing the loudspeaker I could see that the main panel is made from a piece of 10mm thick plywood embelished with curved wooden sections on which are glued pieces of felt against which the aluminium outer case rests. A section of felt is glued along the top of the panel. Probably the felt is used to prevent resonances and vibration from the loudspeaker.

 The base panel is used to hold the plastic sides in place and holds a small paxolin panel for the external aerial/earth sockets (accessed through the hole on the left) and to hold the two batteries. There are also a pair of holes which are almost visible to the left of the aperure for access to a tuning coil and a trimmer capacitor for resonating the inbuilt frame aerial. I've removed a pair of canvas webbing straps used to secure the batteries.. A couple of pieces of thin wood are used to mount one of the batteries under which was threaded one strap. The second strap was fitted under a metal plate to the right of the label.

Both pieces of wood are now extremely fragile and totally beyond treatment.

I'll take a moment or two to describe the set. First the circuit diagram where you can see the valve line-up. You can also see that the set uses an RF amplifier which is not too common for a domestic portable. As the design pre-dates the ferrite rod the built-in aerial marked "L1" comprises a loop carried around one of the plastic sides. Because the design incorporates an RF amplifier the tuning condenser has three gangs, again very unusual in a domestic set.

It's pretty obvious when you see the interior of this set that the design makes use of many components used during WW2 in the manufacture of Murphy-supplied products.

 The set was released in December 1949 at a price of £14:10:6d (plus a horrendous amount of purchase tax) and has two wavebands, Medium Waves 188-560 metres and Long Waves 970-2,050 metres. Going by the date on the electrolytic condenser my set was made around June 1952 and would have cost typically a fortnight's wages so it was not cheap!

The specified valves are all-glass B7G types using 1.4 volt filaments and are fairly uncommon. I haven't checked other than to confirm all are present, but these Mazda-coded types are equivalent to the more common Mullard-coded varieties as follows.

1F3 = DF91 pentode; 1C1 = DK91 heptode; 1FD9 = DAF91 diode pentode; 1P10 = DL92 output pentode

I found a piece of plywood about 5mm thick which I cut to shape then cut a hole for the loudspeaker. I then fitted the three curved pieces and two long pieces of wood salvaged from the old panel. These had been tacked into place and came off easily, being pressed onto the new panel using the old pins and fillets of glue applied to keep them secure. Fortunately four of these pieces were clear of woodworm holes and the last only had a couple. I sprayed it with aerosol paint to give it a finished look.

 Then fitted the loudspeaker whose cone had completely escaped the notice of the woodworms.

 The new base panel was made from a piece of 10mm pine and the holes for the aerial/earth connections and adjusters were cut out.

 Next I'll fit some wood fillets to match the original, stick on some felt pieces then fix the makers' plate and aerial/earth terminals. Finally I'll copy the interior label, PhotoShop out the holes and print a new one which I'll glue into place. The larger wood panel has hinges at the corners and these slot into the plastic end panels. I refitted the hinges using 6BA screws and nuts in place of the original rivets.

Before I proceeded to refit all the parts I checked the set for problems. Some time ago I made a universal power supply which can deliver several voltages (see below). I set this to deliver 1.4 volts of LT and 100 volts HT. Before connecting the power I checked the valves. The frequency changer had a whitish top indicating that air had got into the envelope. Sure enough the base had a tiny crack in the glass. The other valves appear to be OK. I found an unmarked but possible DK91 in my valve collection and fitted this in place of the u/s 1R5 fitted in the position of the original 1C1. Turning on the power revealed two problems. The audio output transformer had an open circuit primary winding and the set consumed around 65mA of HT current. The latter was due to a leaky capacitor. This was marked 8uF 200VW and tested OK but clearly was breaking down when HT was applied as disconnecting it reduced the HT consumption to a few milliamps. I removed the transformer and detached the outer layer of insulating material and found the break in the winding was buried in the winding so not readily repairable. Sometimes the link from the thin enamelled wire and the terminating wire corrodes and can be repaired, but unfortunately not in this case.

Click the picture above to see more

 The cause of the failed transformer was possibly due to a breakdown in the wiring. The designers had fitted an inch or two of screened wire between the anode of the output valve and the transformer and the insulation in this may have failed, putting the HT battery voltage across the primary winding of the transformer. Just in case the fault had been caused by the tone control capacitor connected between the anode and earth I cut this off and I'll fit a modern replacement. Finding a suitable output transformer isn't too difficult as I find that a small mains transformer will perform tolerably well. My junk box has a lot of these and one with a 1kohm primary worked OK. As the replacement worked I removed the metal fixing band from the old transformer and after some squashing and bending I fitted it to the replacement which had been a circuit board type and scewed it in place using the original screws and mounting holes.

 Above is the faulty output transformer and the leaky capacitor.

I tried to get the receiver working and after a lot of fiddling I managed to hear Radio 4 on long waves. The signal was very weak and I worked out that the likely reason was a bad frequency changer. After checking my valve collection I ended up using a new DK92 (my records show that I have a spare DK91 but I couldn't find it). Now, the difference between the DK91 and the DK92, which are both heptodes, is the arrangement of the grids and their connections. The DK92 has G4 connected to Pin 5 but this pin on the B143 frequency changer valve socket is grounded, conforming to the requirements of the DK91. I cut the ground connection to Pin 5 and Radio 4 shot up in volume. I then connected G2 to Pin 5 (ie connecting G2 and G4) and Radio 4 shot up in volume once more. A twiddle of the tuning on long and medium waves showed all was now tolerably well.

 Above is the receiver chassis shown during alignment. The two knobs on the left are connected together for volume adjustment, but the two on the right are separate; the inner working the wavechange switch via a mechanical linkage and the outer driving the tuning condenser via a long cord connected to the dial pointer.

Now that the receiver was working properly I commenced to align it. The IF is 465KHz and I found three of the four IF coils were OK but the last was out of tune. After tweaking with a plastic trimming tool the IF tuning responded OK. A word of advice here. As one's ear is relatively insensitive to changes of audio volume it's best to use a meter across the loudspeaker when aligning a set. An old AVO set to AC volts is OK, but I now use an audio wattmeter which can be adjusted for impedance and sensitivity. An analogue meter is much better than a digital meter for this sort of work.

Turning to the RF alignment. I found some very odd things and the tuning slugs for the coils were peculiar. More investigation is needed as one of the capacitors may have drifted so far that it's upsetting alignment, however it's possible I've overlooked something. To save on complexity the designers have wired the oscillator trimmer capacitors so that C31 will affect both wave ranges, so it's important to follow the manufacturers alignment instructions to the letter and do not mix up the indentification of the correct trimmer and their adjustment sequence. Also, the oscillator and RF amplifier coils for the wavebands share the same formers meaning that their two slugs will interact somewhat. Savings are also apparent in the RF tuning circuits, where C22 tunes both medium and long waves, meaning that long wave tuning is somewhat compromised. This weakness is also seen in the frame aerial and RF input circuit still, this set is a domestic portable receiver and was never intended as a communications receiver.... It's possible to correctly adjust the medium waveband, then the long waveband but, in doing the latter, completely mess up the settings for the medium waveband.

A second session at alignment revealed that resistor feeding G2 and G4 of the frequency changer (which also feeds the screen grid of the RF amplifier) had gone slightly high and read 59Kohm instead of 47Kohm and I found that decreasing its value increased the gain by quite a bit. I fitted around 15Kohm and the volume of Radio 4 doubled. In fact the DK92 favours a resistor of around 15Kohm so changing the valve type would point to what I found in practice.

As the volume is still not as loud as I'd have expected either the output transformer wasn't ideal, or another fault is present. I could try swapping valves in case one has low emission, but first I'll change the capacitor between the audio amplifier and the grid of the output valve in case this has gone leaky and biased the grid too high. I could also swap over the filament power leads in case the overall bias for the valves is wrong, as with a directly heated cathode self bias is not a simple matter as it is with indirectly heated cathode. I could also look at the AGC line.

At this point I decided to continue restoration of the case. The front and back panels are linked to the narrow top panel, which is under the handle, by full width pins. The pins are pushed out releasing the panels. Before you can do this the top has to be unbolted from the aluminium stretcher. The two bolts also hold the plastic handle in place. At either side are polished aluminium covers which slide away once the securing nuts are released. Once the securing nuts are removed the cover assembly can be lifted off. The covers were painted in a thin black crackle finish which is now in poor condition. I removed the speaker fabric from the front panel. After filling areas of missing paint I used wet and dry to give a keyed surface for spray paint. Rather than using a crackle finish I used a shiny finish black paint.

Below shows the speaker panel refitted. This has a pair of pins on the lower edge which engage with holes in the end panels so that after removing two fixing screws the panel can be hinged downwards for access to the front of the chassis.

 The repainted front and rear panels linked to the top panel ready for slipping over the set. The fabric speaker material was glued back to the front panel. A small hole at the top was easily dealt with by moving the fabric up slightly so it wasn't visible. The fabric isn't glued to the back of the louvres but rests on the shaped wood nailed to the speaker panel.

 The transparent dial cover was detached by slackening the three small screws under the aluminium stretcher and cleaned. The dial was also cleaned, both parts being treated with care to avoid losing their markings. The grooved knobs were cleaned using a toothbrush moistened with a spray furniture polish. The plastic end pieces were cleaned using the spray polish and kitchen tissues. From the deposits removed the set must have been owned by a heavy smoker.

Below, the cover loosely refitted to check it fits OK.

 The rear of the case reminds one of an old fashioned leather briefcase.

continues

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