MG Car Repair: Power for modern reciever deck in a 74MGB, cigar lighter, purple wire

Question
I have a 74 MGB that I installed a current JVC receiver in last year. It worked fine until two weeks ago when it simply powered off and refused to come back on. I checked the fuses and confirmed that there is voltage to the receiver. I checked the receiver and confirmed that it still functions, in fact it will work if connected directly to the battery so I know the battery is putting out the required voltage. I only have an analogue multimeter so I can't confirm if there is a minute difference between the direct battery and the designated radio leads. I thought I had covered all of the bases but I have obviously missed something.

Thanks

Marc

Answer
Hi Marc.  Sounds like a loose wire to me.  I suggest you pull the radio and replace it with your analogue voltmeter to trace the fault.  

Did you fit an earth wire, or are you relying on the aerial (antenna) to provide the earth?  The aerial is not a reliable earth, and this could give the symptoms that you describe.  

The designated power comes via the ignition switch, so it will not work with the ignition switched off.  I prefer to take the power from the PURPLE wire that goes to the cigar lighter.  This is permanently live.