Murphy A484 - residual hum level from speakers - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

dazzlevision · 28th Apr 2021, 8:20 pm

Just to update my work on this matter, I tried the effect of reversing the L & N wiring to the set, which made no difference.

I fitted the modified Murphy mains transformer (with a single heater winding, rather than two, but with the same part number - also used in the A674). Mention of this change is made in the Murphy A484 service manual.

I checked the core temperature of the original transformer and it peaked at around 65°C, as did the A674 transformer. The heater and HT voltages were very similar with either transformer fitted, and the measured hum level (oscilloscope across speaker terminals) was virtually the same with either type.

Although I had a spare ex-equipment A484 audio output transformer to hand, it had a significantly lower primary winding resistance than the one in the set (and the values shown in the service manual). I suspect shorted primary turns, so didn’t try it in the radio.

Out of interest, I then tried earthing the chassis and this significantly reduced the overall hum level (after a full warm up) from the speakers.

As the radio has an “Earth” socket (directly connected to the chassis), I made this connection permanent by fitting a three core mains lead (with the old wiring colours) in place of the original PVC two core type. I am now much happier with the background hum level.

Fitting an EM84 (straight line display) in place of the worn out EM85 (fan type) has completed the restoration and the radio does sound rather nice.

28th Apr 2021, 8:20 pm	#21
dazzlevision Dekatron Join Date: Sep 2003 Location: Near Swindon, North Wiltshire, UK. Posts: 3,612	Re: Murphy A484 - residual hum level from speakers Just to update my work on this matter, I tried the effect of reversing the L & N wiring to the set, which made no difference. I fitted the modified Murphy mains transformer (with a single heater winding, rather than two, but with the same part number - also used in the A674). Mention of this change is made in the Murphy A484 service manual. I checked the core temperature of the original transformer and it peaked at around 65°C, as did the A674 transformer. The heater and HT voltages were very similar with either transformer fitted, and the measured hum level (oscilloscope across speaker terminals) was virtually the same with either type. Although I had a spare ex-equipment A484 audio output transformer to hand, it had a significantly lower primary winding resistance than the one in the set (and the values shown in the service manual). I suspect shorted primary turns, so didn’t try it in the radio. Out of interest, I then tried earthing the chassis and this significantly reduced the overall hum level (after a full warm up) from the speakers. As the radio has an “Earth” socket (directly connected to the chassis), I made this connection permanent by fitting a three core mains lead (with the old wiring colours) in place of the original PVC two core type. I am now much happier with the background hum level. Fitting an EM84 (straight line display) in place of the worn out EM85 (fan type) has completed the restoration and the radio does sound rather nice.