Thuuump... when turning off! - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

murphyv310 · 13th Dec 2009, 11:01 pm

Hi.
Ideally you should put a switch supression cap across the switch contacts, this type of cap has a resistor in series, this stops the back EMF destroying the cap, although as Phil suggests a 0.1uf 1000v will at least tell you that this is the problem.
I had the same problem on my Rgd 746 when the turntable switched off the bang from the speaker was scary and the cap was a 100% cure. A cap across the mains will also work, if you use the sets power switch put it across the contacts, if it was to go short the set would just stay on and not blow a fuse.
Cheers
Trevor

kalee20 · 14th Dec 2009, 12:40 am

Quote:

Originally Posted by Billy T

Quote:

Originally Posted by kalee20

Sure, the field coil current will collapse, but that won't make the cone move unless the voice coil also has current through it at the same time.

I don't think that follows; a back emf field could inject a pulse into the voice coil that would be contrary to the energising field polarity.

Good point. The field coil collapsing WILL induce an EMF into the voice coil. However, this won't make a current flow in the voise coil, unless the voice-coil has a lowish-impedance current path connected to its terminals. If the output valve is a triode (low ra) then a current will flow and there'll be a thump. If the output valve is a pentode with a high ra, then less current will flow and there'll be less of a thump.

Of course, other effects may well dominate this one, in practice...

Billy T · 17th Dec 2009, 10:06 am

Quote:

Originally Posted by kalee20

Good point. The field coil collapsing WILL induce an EMF into the voice coil. However, this won't make a current flow in the voice coil, unless the voice-coil has a lowish-impedance current path connected to its terminals.

But it does have such a current path! The speaker voice coil is connected to the low impedance secondary of the audio output transformer.

Any current disturbance in the transformer primary winding will create a sound from the speaker. It's pretty much the sole purpose in life of the whole assembly. So, provided that there is a closed circuit on the primary (i.e. the output valve is still conducting, which it will be at switch off) there is potential for an audible output from the speaker. The potential increases if the design has a cap from the o/p anode to ground or across the o/p tx primary winding.

I wonder if any progress has been made yet?

Cheers

Billy

Paul LS · 17th Dec 2009, 12:43 pm

Quote:

I wonder if any progress has been made yet?

Unfortunately I am travelling with work this week

Gotta feed the family. I wont be home until the weekend so can not take a look until then. First thing I will try is an external speaker... see if this makes a difference.

Cheers,
Paul.

Paul LS · 21st Dec 2009, 3:29 pm

Very strange, I was travelling all last week so this weekend I took a look at the radio and it is no longer making the very loud scary "thump" when I turn off. Previously the speaker cloth would buldge out with the thump and I was concerned it would damage the speaker. Now even with maximum bass when I turn off there is just a very slight "thump". No idea what has changed, just hope it doesn't come back again.

13th Dec 2009, 11:01 pm	#21
murphyv310 Dekatron Join Date: Sep 2006 Location: Kilmarnock, Ayrshire, UK. Posts: 5,422	Re: Thuuump... when turning off! Hi. Ideally you should put a switch supression cap across the switch contacts, this type of cap has a resistor in series, this stops the back EMF destroying the cap, although as Phil suggests a 0.1uf 1000v will at least tell you that this is the problem. I had the same problem on my Rgd 746 when the turntable switched off the bang from the speaker was scary and the cap was a 100% cure. A cap across the mains will also work, if you use the sets power switch put it across the contacts, if it was to go short the set would just stay on and not blow a fuse. Cheers Trevor __________________ Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member

21st Dec 2009, 3:29 pm	#25
Paul LS Retired Dormant Member Join Date: Mar 2009 Location: Southampton, Hampshire, UK. Posts: 196	Re: Thuuump... when turning off! Very strange, I was travelling all last week so this weekend I took a look at the radio and it is no longer making the very loud scary "thump" when I turn off. Previously the speaker cloth would buldge out with the thump and I was concerned it would damage the speaker. Now even with maximum bass when I turn off there is just a very slight "thump". No idea what has changed, just hope it doesn't come back again.