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Old 14th Jun 2021, 8:40 am   #1
David G4EBT
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Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 4,541
Default Murphy A124 Earthing considerations?

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:

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?
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