Horrible hum on Marconiphone radiogram - UK Vintage Radio Repair and Restoration Discussion Forum

kestrelmusic · 25th Aug 2019, 9:03 pm

I've nearly finished restoring a nice Marconiphone model 286 radiogram (it's the same as the HMV Model 444). The radio works a treat but I am getting horrible 50 c/s hum on the Gram setting. The original pickup coil was o/c so I rewound it with as much 42SWG as I could get onto the bobbin. The hum is only there when the motor is running, so I can only assume that the pickup coil is getting some magnetic pickup from the motor windings. There's no hum on MW or LW when the motor is running.

I also have a Columbia Model 228 record deck which suffers from the same problem; it seems to be electrically and mechanically identical to the HMV/Marconi apparatus.

Is there any way of screening the pickup coil from external magnetic fields? Alternatively, has anyone ever tried fitting a crystal cartridge into the Bakelite headshell? I'm sure one would not take the weight of a Bakelite arm and a steel needle, but might there be some way of coupling one indirectly?

Heatercathodeshort · 25th Aug 2019, 9:44 pm

Interesting. Something tells me this pick up should have a 'hum bucking' coil. This has the appearance of the pick up coil and is usually mounted just above the assembly and is wired in anti phase. You may have disturbed/miss wired it? Just a thought. John.

25th Aug 2019, 9:03 pm	#1
kestrelmusic Hexode Join Date: Feb 2008 Location: Wrexham, North Wales, UK. Posts: 466	Horrible hum on Marconiphone radiogram I've nearly finished restoring a nice Marconiphone model 286 radiogram (it's the same as the HMV Model 444). The radio works a treat but I am getting horrible 50 c/s hum on the Gram setting. The original pickup coil was o/c so I rewound it with as much 42SWG as I could get onto the bobbin. The hum is only there when the motor is running, so I can only assume that the pickup coil is getting some magnetic pickup from the motor windings. There's no hum on MW or LW when the motor is running. I also have a Columbia Model 228 record deck which suffers from the same problem; it seems to be electrically and mechanically identical to the HMV/Marconi apparatus. Is there any way of screening the pickup coil from external magnetic fields? Alternatively, has anyone ever tried fitting a crystal cartridge into the Bakelite headshell? I'm sure one would not take the weight of a Bakelite arm and a steel needle, but might there be some way of coupling one indirectly?

25th Aug 2019, 9:44 pm	#2
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: Horrible hum on Marconiphone radiogram Interesting. Something tells me this pick up should have a 'hum bucking' coil. This has the appearance of the pick up coil and is usually mounted just above the assembly and is wired in anti phase. You may have disturbed/miss wired it? Just a thought. John.