Murphy A3A TRF radio set.

[Geordie McBoyne]

The A3 was Murphy's first all mains radio receiver.
Was it a good set?   Let's find if it is any good.   One thing is certain it is possibly the most over engineered TRF receiver ever made. Everything is made to a grand scale.                                   Four valves are employed. The RF amplifier is a Mazda AC/SG. The detector is a Mazda AC/HL and the output valve is another Mazda, an AC/pen.   The HT rectifier in this set is a Mazda UU4, although I believe some sets were supplied with a Philips 1821 as original fitment.  

Geordie McBoyne.

[Geordie McBoyne]

This trick might work. To reform the HT electrolytics I've started by supplying them from the mains through a BY105 diode and a series high value resistor starting with 390Kohms. As the voltage gradually increases to a certain level the resistor is reduced in stages until the highest possible voltage is achieved.
The process can take a long time, several days.

Geordie McBoyne.

[Murphyv310]

Hi.
Yes I have been reforming caps this way for some time too.
I do it as a bit of a lash up though with it just connected to the mains What I will do though is to use an old transformer from a scrap radio and use the HT secondary and fit a multi position switch with various values of resistors that can be switched into circuit.
I could also add in a relay fed from the heater supply so it closes when the reformer is off to discharge the cap through a resistor.
Another project for the next few years!

[Geordie McBoyne]

The HT capacitor reforming process was a success. There is 300 volts at the reservoir capacitor and the supply to the detector and RF amplifier is about 210 volts.
Performance is disappointing, I'd expect a lot better from a Murphy receiver. There is a nice loud hum when the grid of the leaky grid detector is touched, so we know that all is well in that part of the set. The set is fitted with a Mazda AC/VP1 vari-mu pentode as the RF amplifier, it's certainly not the original, it's more likely the A3A was supplied with an RF tetrode such as the MS4 or AC/SG. An MSP4 was tried and the results were even worse.
Of course these sets were intended to work with a long wire aerial, perhaps I'm expecting too much from ten feet of wire.

Geordie McBoyne.

[Geordie McBoyne]

Circuit diagtam of the A3A.   The 1931 A3 is almost most identical except it does not have a gram pick-up socket.

Geordie McBoyne.

[pwdrive]

I would have thought it would work quite well with doubled tuned band pass front end, RF amp and regeneration given a decent antenna and earth.

Lawrence.

[Yorkie]

Good news that the set is basically working, meaning that the mains transformer, field coil, speech coil and OPT are all sound, any of which could have proved quite a hurdle had they been O/C.

 

[Crackle]

I made up the following circuit into a Capacitor reformer. The values around the meter circuit are only approximate. The idea of the zener diode was to protect the meter if the capacitor was shorted. It seems to work quite well.
Mike

[Geordie McBoyne]

The chassis has been refitted into the cabinet. I've managed to squeeze a liitle more performance from the set. The tuning capacitor trimmers have all been readjusted for max output at the HF end of the medium waveband. The two band pass trimmers proved very effective for improving sensitivity and selectivity.
The detector trimmer response is very flat because of the damping effect of the detector. It can be sharpened up with careful adjustment of the reaction control.
Nevertheless, for all these inititaives this set will need an efficient aerial system to get any decent results.
Just accept that the way it is.

Geordie McBoyne.

[Murphyv310]

Hi David.
How does the set perform on your 1 metre frame aerial?