Murphy B40 Hum cure. - UK Vintage Radio Repair and Restoration Discussion Forum

Don Collie jnr · 29th Dec 2009, 6:05 am

By ensuring that the Heater return wire was *not* earthed to the power supply chassis, but only earthed in the IF chassis, I was able to eliminate the annoying hum that the set designers have gone to great lengths to reduce.
To do this it is only necessary to snip the earthing wire on one of the Jones sockets on the power supply, although a purist would, of course, desolder the wire neatly.

Cheers!
Don.

AlanBeckett · 29th Dec 2009, 11:48 am

Errrrrr. Mine doesn't hum. Are you sure that there isn't another problem?
Alan

Sean Williams · 29th Dec 2009, 3:11 pm

Glad someone else said this before I commented - I have never had aproblem with hum on any of the B40s I have repaired over the years - must be into double figures by now....

How strange.

Sean

Top Cap · 29th Dec 2009, 4:17 pm

Being an ex RN Radio Mech from the B40 era I can second that. I never came across a B40, B41 or 62B that had any hum problems. Just those damn turret contacts!

Les

murphyv310 · 29th Dec 2009, 5:25 pm

Hi.
My B40D doesn't hum, I'm surprised it doesn't, considering it was lying in water for 15 years.
Sounds like you have a heater cathode short/leak and you cleared it by allowing the heaters to "float".
Reconnect the wire and then try substituting valves, also chech valve bases for leaks.
Cheers
Trevor

Don Collie jnr · 30th Dec 2009, 8:31 am

That`s strange guys, I`ve dealt with three that had this problem, perhaps made worse by using a good quality loudspeaker. They were all B40B`s, which have more filtering in the power supply than the "A", if I remember correctly. removing the earth connection completely cured the problem. Thanks for your comments!,.................................Don.

QWENIX · 29th Jan 2010, 11:25 pm

No powersupply capacitors problem ?

The AGC switch sometimes causes extra humming noise but I don't know (for now) in which lever position (on or of).

best regards

Qwenix

29th Dec 2009, 6:05 am	#1
Don Collie jnr Retired Dormant Member Join Date: Jul 2008 Location: Invercargill, New Zealand Posts: 51	Murphy B40 Hum cure. By ensuring that the Heater return wire was not earthed to the power supply chassis, but only earthed in the IF chassis, I was able to eliminate the annoying hum that the set designers have gone to great lengths to reduce. To do this it is only necessary to snip the earthing wire on one of the Jones sockets on the power supply, although a purist would, of course, desolder the wire neatly. Cheers! Don. Last edited by Darren-UK; 29th Dec 2009 at 12:11 pm. Reason: SKB problem fixed.

29th Dec 2009, 11:48 am	#2
AlanBeckett Rest in Peace Join Date: Dec 2008 Location: Burton upon Trent, East Staffordshire, UK. Posts: 1,686	Re: Murphy B40 Hum cure Errrrrr. Mine doesn't hum. Are you sure that there isn't another problem? Alan

29th Dec 2009, 3:11 pm	#3
Sean Williams Dekatron Join Date: Feb 2005 Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW Posts: 3,518	Re: Murphy B40 Hum cure. Glad someone else said this before I commented - I have never had aproblem with hum on any of the B40s I have repaired over the years - must be into double figures by now.... How strange. Sean __________________ Engineers make things work and have spare bits when finished

29th Dec 2009, 4:17 pm	#4
Top Cap Octode Join Date: Mar 2006 Location: Watford, Hertfordshire, UK. Posts: 1,270	Re: Murphy B40 Hum cure. Being an ex RN Radio Mech from the B40 era I can second that. I never came across a B40, B41 or 62B that had any hum problems. Just those damn turret contacts! Les __________________ Whether the Top Cap is Grid or Anode - touching it will give you a buzz either way!

29th Dec 2009, 5:25 pm	#5
murphyv310 Dekatron Join Date: Sep 2006 Location: Kilmarnock, Ayrshire, UK. Posts: 5,422	Re: Murphy B40 Hum cure. Hi. My B40D doesn't hum, I'm surprised it doesn't, considering it was lying in water for 15 years. Sounds like you have a heater cathode short/leak and you cleared it by allowing the heaters to "float". Reconnect the wire and then try substituting valves, also chech valve bases for leaks. Cheers Trevor __________________ Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member Last edited by murphyv310; 29th Dec 2009 at 5:26 pm. Reason: grammar

30th Dec 2009, 8:31 am	#6
Don Collie jnr Retired Dormant Member Join Date: Jul 2008 Location: Invercargill, New Zealand Posts: 51	Re: Murphy B40 Hum cure. That`s strange guys, I`ve dealt with three that had this problem, perhaps made worse by using a good quality loudspeaker. They were all B40B`s, which have more filtering in the power supply than the "A", if I remember correctly. removing the earth connection completely cured the problem. Thanks for your comments!,.................................Don.

29th Jan 2010, 11:25 pm	#7
QWENIX Pentode Join Date: Jan 2006 Location: Amsterdam, The Netherlands Posts: 180	Re: Murphy B40 Hum cure. No powersupply capacitors problem ? The AGC switch sometimes causes extra humming noise but I don't know (for now) in which lever position (on or of). best regards Qwenix