Murphy A124 Earthing considerations?

[David G4EBT]

I'm about to start restoring one of these radios, which have a number of design shortcomings, reflecting perhaps the limitations of the austere times in which they were produced,(1948) but even in those less safety conscious times the shortcomings seem to have been recognised if only to a small degree. It's a 'short superhet' 3 Valves plus rectifier, and no built-in aerial so won't set the airwaves humming. It uses an auto transformer, so the chassis is connected the one side of the mains. It uses an odd assortment of valves - a pre-war Octal Pen45DD and three post war B9A valves - 10C1, 10F9 and a U404 rectifier.

The Pen45DD has a 4V heater, and the two dial lights are in parallel with a 4V tap on the auto-transformer. The 10C1, 10F9 and U404 heater voltages are respectively 28V, 13V and 40V, 81V total, and are in series from another tapping.

Of particular concern to me is that the datasheet says the radio 'must have an earth connection'. Clearly the chassis can't be earthed and the only reason and earth seems to be called for is the rather dodgy dial lights and speaker socket arrangements, which it seems to me, are quite a serious safety shortcoming. The Perspex dial is supported by two dial light bulb carriers, which if the MES bulb-holders (which push into the holders and are only insulated by thin rubber tubing), were to make contact the the dial lamp sockets, they would be live, and so would the metal parts on the dial.

Likewise, the internal speaker is connected by uninsulated brass plugs which plug into brass sockets on the rear chassis apron. This would enable the internal speaker to be disconnected and an extension speaker plugged in if desired. Though the speaker sockets aren't connected to the chassis, maybe Murphy got the jitters that should one of the sockets short to the chassis it would become live.

The dial light sockets and speaker sockets go directly to the earth socket, and C19 & C2 (both 0.01uF) isolate the earth socket from the chassis.

The speaker sockets can be seen in the second pic at the first post and the dial lights in post #3 in this excellent detailed 'Success Story' by Howard Craven back in 2008:

https://www.vintage-radio.net/forum/...ad.php?t=27804

The priorities I've always applied when restoring vintage radios or test gear have always been:

1) Safety.
2) Functionality.
3) Originality.

Clearly it isn't practicable to provide an earth connection in the traditional way, which leaves two option:

1) Fit a small isolating transformer to power the dial lights rather than from the 4V tapping on the auto transformer just for the PEN45DD heater. The wire the speaker directly to the output of the output transformer secondary winding, and either leave the LS socket disconnected, or remove them entirely.

Those modifications would satisfy the criteria and 1 & 2 above and would be reversible.

2) Given that C2 and C19 insulate the earth from the chassis, if those caps were Class Y, then perhaps the mains earth could be used, wired only to the Earth socket. That would ensure an effective earth at all times in the unlikely event of the dial bulb-holders shorting or the speaker sockets becoming live.

(There is a built-in fuse-holder with 0.5A fuse in the mains input neutral line).

I've attached a truncated clip of the circuit with the earthed components shown.

I'm not content with simply leaving it as it is, with no earth connection.

As I said, originality is way down the list below safety and functionality.

Just wondered if anyone had any thoughts please?

[David G4EBT]

Forget to add the part circuit with the earthed components highlighted in red.

[ms660]

It's just another live chassis receiver with provision for an Earth connection.

Lawrence.

[murphyv310]

Hi.
I agree with Lawrence. If it's your set you should be aware of the issues and make sure its chassis is connected to mains neutral with a non reversible plug. It's no worse than I believe PME earthing that is if I'm not mistaken is tied to mains neutral and can float above earth anyway. As a Radio Amateur you will realise that when using an RF ground with PME it is your obligation to get clearance from your electricity "board" to coin an old phrase.

You must never tie a true earth to PME unless it's checked by engineers.

[David G4EBT]

Thanks for reading and for your comments Trevor and Lawrence.

I was prompted to mention it because the Murphy Datasheet firstly states the obvious:

"Both the A124 and U124 receiver chassis are connected directly to the mains supply, therefore earthed equipment such as signal generators etc should not be used without adequate precautions being taken".

(In any event, while doing any life testing on the radio it will be via an isolating transformer).

The datasheet goes on the state:

"An efficient earth is required for satisfactory operation of the mains filter and as a safety precaution against electric shock if the scale lamps are touched by the hands".

Not altogether sure why the mains filter needs the radio to be earthed, but certainly (in the eyes of the Murphy designers back in the 1940s, the dial light bulb-holders, which also secure the tuning dial, are accessible on the front of the radio to hands and could, in some circumstances, pose a safety risk. It occurred to me that if the lights were powered from a small transformer instead of the auto-transformer, that risk would be eliminated. Likewise, there is no reason to have exposed brass speaker terminals which are only there for the provision of an extension speaker. If the spekaer is wired directly to the OPT, an earth then ceases to be relevant.

It says much about the socio-economic conditions of the 40s and 50s, coupled with labour intensive production techniques, that many - if not most households - could only afford one radio, which stayed put where an aerial (and earth) could be provided, and extension speakers used in other rooms. Despite the evident corner cutting and compromises of this radio, back in 1948 it cost £16.10s0d plus PT, so in excess of £20, which today equates to over £800. (In contrast, a 55" LED smart TV with Apple TV app and a 6-year warranty from Richer Sounds currently costs £549).

How times change.

A couple of pics are attached showing the radio in its unrestored state. It's now dismantled and I'll first be re-veneering the cabinet in Sapele mahogany and the end panels with sycamore. (As the veneer at the top of the front panel is a tight radius against the grain, I'll steam and clamp the veneer as it's too much to bend it and expect the glue to hold it in place. (That's why the veneer lifted and splintered at the top edge during its long life).

Pic 1 shows the metal bulb-holders from the front, which also secure the dial.

Pic 2 shows the rear of the bulb-holders with the earthing point.

The perished rubber insulated wiring is evident in many areas of the radio so will be replaced with silicone insulated flex.

[ms660]

With an Earth connected both filter chokes are in effect connected to Earth so far as RF goes.

Operate via a suitable mains isolating transformer is the best guarantee, don't rely on "Neutral runs through to chassis"....nature conspires.

Lawrence.

[David G4EBT]

Thanks for your additional comments Lawrence - points well made.

The reality is that once restored, this radio won't be in regular use in any event.

For now, I'll focus on the veneering!

What about an RF isolating transformer for the (required) aerial and earth, a few 10's of turns of each primary and secondary on a ferrite toroid or the like would do the job. At MW/LW frequencies a scrap SMPS (Switched Mode Power Supply) core will do nicely. IT would fit inside and be reversible. Rather a splendid set too.

[ms660]

Originally Posted by merlinmaxwell

What about an RF isolating transformer for the (required) aerial and earth, a few 10's of turns of each primary and secondary on a ferrite toroid or the like would do the job. At MW/LW frequencies a scrap SMPS (Switched Mode Power Supply) core will do nicely. IT would fit inside and be reversible. Rather a splendid set too.

That might affect the resonant frequency of the existing antenna primary.

Lawrence.