Mains suppression caps - UK Vintage Radio Repair and Restoration Discussion Forum

60 oldjohn · 12th Nov 2014, 11:47 pm

Following on from a conversation this afternoon I realise that not all AC sets have a Mains Suppression cap fitted, at least not in the conventional manner being across the mains. Are AC sets less affected by mains borne interference? The set I am referring to is a Murphy A252.
John.

Al (astral highway) · 13th Nov 2014, 12:23 am

Quote:

Originally Posted by 60 oldjohn

Are AC sets less affected by mains borne interference? The set I am referring to is a Murphy A252.
John.

Hi John, this will interest you. When the BBC did their confidential research report on the set, they said it had 'improved AM suppression' compared to the earlier A242 model. It's on the opening summary section in the report here.

Mains borne interference on AM, certainly in this household, is pretty dire!

Radio Wrangler · 13th Nov 2014, 12:31 am

AM suppression means that the receiver, switched to VHF/FM does not respond to any AM present on signals. It's nothing to do with the 'AM' band or with mains interference.

Some FM signals acquire unwanted AM by frequency caused differences in amplitude... EG from multipath and this can be somewhat distorted. Poor transmitters can give amplitude variation as a side-effect of applying FM.

Good limiting in the IF is the main defence.

David

G6Tanuki · 13th Nov 2014, 10:20 am

The mains suppression cap is indeed less important (to the point of irrelevance in some cases) in the case of receivers that have their HT supplied via a transformer.

In a well-designed mains transformer the primary winding and the secondaries are separated by an earthed electrostatic screen which prevents mains-borne 'nasties' jumping across through the inter-winding capacitance and getting into the active electronics.

In an AC/DC-style receiver the HT line is essentially connected direct to the incoming mains (via the rectifier) so nasties coming down the mains have a direct path for them to get on to the HT rail. The usual smoothing-electrolytics won't necessarily have a low-enough self-inductance to bypass such nasties to earth hence the 'suppression' capacitor(s) are needed.

Personally I don't like the usual suppressor-cap approach: I generally fit an outboard mains filter [common-mode choke on phase and neutral, caps betweenphase/earth and neutral/earth] - this setup can sometimes be scavenged ready-assembled and encapsulated in a nice can with 'blade' connectors from things like dead washing-machines.

If you can't find one, have a look at something like

http://export.rsdelivers.com/product...s/2192791.aspx

and put it in a diecast box.

G8HQP Dave · 13th Nov 2014, 1:08 pm

For a valve superhet one of the strongest signals coming in on the mains lead can be the LO - the mains lead acting as an antenna. The rectifier in an AC/DC set then chops this at 50Hz (plus lots of harmonics) - the result is lots of hum all over the band. The mains cap stops this. On an AC set even a cheap mains transformer won't pass much RF so the cap is less necessary.

turretslug · 13th Nov 2014, 2:18 pm

Hence the frequent presence of a capacitor across the rectifier in "AC/DC" technique to short-circuit it at RF. A replacement ought to be of good quality and 1,000V rating would be prudent.

60 oldjohn · 13th Nov 2014, 4:29 pm

Its an AC set, I now understand why no mains suppression is needed.

Thank you to all that replied, John.

12th Nov 2014, 11:47 pm	#1
60 oldjohn Dekatron Join Date: Dec 2004 Location: East Yorkshire, UK. Posts: 3,990	Mains suppression caps Following on from a conversation this afternoon I realise that not all AC sets have a Mains Suppression cap fitted, at least not in the conventional manner being across the mains. Are AC sets less affected by mains borne interference? The set I am referring to is a Murphy A252. John.

13th Nov 2014, 12:31 am	#3
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,905	Re: Mains suppression caps AM suppression means that the receiver, switched to VHF/FM does not respond to any AM present on signals. It's nothing to do with the 'AM' band or with mains interference. Some FM signals acquire unwanted AM by frequency caused differences in amplitude... EG from multipath and this can be somewhat distorted. Poor transmitters can give amplitude variation as a side-effect of applying FM. Good limiting in the IF is the main defence. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

13th Nov 2014, 10:20 am	#4
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,010	Re: Mains suppression caps The mains suppression cap is indeed less important (to the point of irrelevance in some cases) in the case of receivers that have their HT supplied via a transformer. In a well-designed mains transformer the primary winding and the secondaries are separated by an earthed electrostatic screen which prevents mains-borne 'nasties' jumping across through the inter-winding capacitance and getting into the active electronics. In an AC/DC-style receiver the HT line is essentially connected direct to the incoming mains (via the rectifier) so nasties coming down the mains have a direct path for them to get on to the HT rail. The usual smoothing-electrolytics won't necessarily have a low-enough self-inductance to bypass such nasties to earth hence the 'suppression' capacitor(s) are needed. Personally I don't like the usual suppressor-cap approach: I generally fit an outboard mains filter [common-mode choke on phase and neutral, caps betweenphase/earth and neutral/earth] - this setup can sometimes be scavenged ready-assembled and encapsulated in a nice can with 'blade' connectors from things like dead washing-machines. If you can't find one, have a look at something like http://export.rsdelivers.com/product...s/2192791.aspx and put it in a diecast box. Last edited by G6Tanuki; 13th Nov 2014 at 10:26 am.

13th Nov 2014, 1:08 pm	#5
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: Mains suppression caps For a valve superhet one of the strongest signals coming in on the mains lead can be the LO - the mains lead acting as an antenna. The rectifier in an AC/DC set then chops this at 50Hz (plus lots of harmonics) - the result is lots of hum all over the band. The mains cap stops this. On an AC set even a cheap mains transformer won't pass much RF so the cap is less necessary.

13th Nov 2014, 2:18 pm	#6
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,400	Re: Mains suppression caps Hence the frequent presence of a capacitor across the rectifier in "AC/DC" technique to short-circuit it at RF. A replacement ought to be of good quality and 1,000V rating would be prudent.

13th Nov 2014, 4:29 pm	#7
60 oldjohn Dekatron Join Date: Dec 2004 Location: East Yorkshire, UK. Posts: 3,990	Re: Mains suppression caps Its an AC set, I now understand why no mains suppression is needed. Thank you to all that replied, John.