Suppressing Intereference to FM on a mains-powered portale radio.

[Nick]

Hi.
The rectifier diodes are a very good point. It might be an idea to bridge each one with a 1nf cap or in the case of a bridge across each leg in a criss cross and AC in and DC out = 4 caps in total.

Highly unlikely really - the caps were there to stop switching noise from the rectifiers, mostly were added in HiFi systems, however they are out of favour ad they provide a direct route for HF into the DC supply.

Normally folk now don't fit them (if they know what they're doing) as some good solid research has been done by the likes of Morgan Jones and Mark Johnson to show the real cause of post rectifier noise is ringing from the transformer (initiated by the diodes switching) and that a quick, well-designed, snubber can remove almost all the rubbish.

It's been discussed on some of the more technical audio forums and two excellent and detailed reports were published in the "Linear Audio" journal in recent years. The references are available in a post I made on Audio-Talk earlier this year - http://www.audio-talk.co.uk/phpBB3/viewt...76#p141173

Regardless of that, it's a massive red-herring in this case. The caps were originally designed to shunt HF switching noise from the diodes but were found to actually cause more problems by allowing raw HF from the AC side into the DC side. Yes, the XFMR is a LPF, but an awful lot of stuff gets through, especially harmonic-rich spikes from inductive stuff turning on and off.

Real answer is to put the radio in a Faraday Cage

EDIT: Post edited to repair the URL-mangling that the forum WYSIWYG editor did.

[Murphyv310]

Thanks for the info Nick. I also was lead to believe that a linear mains TX is a fairly reasonable RFI filter on its own. I also wonder if making up an RFI filter at or just post the 13 amp plug would be effective? The reasoning being you are now adding in the mains earth and who knows if that is clean?

[Refugee]

I have always been convinced that the noise is on the mains cable in equal amounts on both of the wires so that anything across them does nothing.
Batteries brake the link and the radio is clear.
Go too far and you get the noise back again. Just put the speaker cables on little pylons and go hunting for noise pick up.

[Murphyv310]

I agree that noise will be on live and neutral. Adding an RFI filter and a separate earth rod to the RFI earth connection would be better than utilising the mains earth.

[Refugee]

With those expensive feed thru capacitors.

[Murphyv310]

I've a RFI filter sitting here rated at 20 amps, its about 4 inches square and built in a metal can. I've a good mind to fit it to the incoming mains to the shed and fit a separate earth via a suitable rod.

[Refugee]

That may help. You will need to make sure all unfiltered cable is well away from the filtered area including lighting wiring.

[Old Sparky]

I've been reading with interest all the ideas and suggestions, for which many thanks, but I have to bear in mind two things, firstly that it isn't my radio, and secondly that it has to remain a portable, so, AFAIK, the only two options are to either run it from batteries, or to plug it into the mains, using a RFI equipped 13Amp adaptor. There is little room inside the radio for internal modifications, besides the foregoing requirements.

[Nick]

I've a RFI filter sitting here rated at 20 amps,  its about 4 inches square and built in a metal can. I've a good mind to fit it to the incoming mains to the shed and fit a separate earth via a suitable rod.

I have two power circuits in my workshops - clean power & dirty power.

The main 60A feed in goes into an industrial lightning barrier, then the supply splits. Both arms have industrial RFI filters but the rings are kept separate and the sockets are also marked. There are also separate earth rods.

[Terry]

I've been reading with interest all the ideas and suggestions, for which many thanks, but I have to bear in mind two things, firstly that it isn't my radio ...

No, but that doesn't stop you repairing the PSU or, at least, checking the effect of diode replacement and all the other suggestions that have been made here.

If you are not intended to repair it, why else have you got the set - if you had a magic wand, surely you would have used it by now?