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

[Old Sparky]

I've been asked if it's possible to suppress interference to FM reception on a Roberts Classic 928 Portable Radio.  The interference is so bad after about 8AM weekdays that listening to any station is almost impossible.  The radio is used in the town centre near to shops, cafes and a pub, and I suspect it to be due to electronic tills and office equipment.  In a quiet location, the radio performs perfectly, so I'm in the process of fitting tubular ferrite 'rings' around the output leads from the internal mains transformer (a conventional, built-in linear PSU is used in this model, connected via a standard 'figure of eight' mains lead) in the hope that this will solve the problem, although I'm not certain if the QRM is mains or aerial borne - I'll try running it on batteries to prove that point.  Has anyone else experienced this problem and found a satisfactory solution? IMHO, the siting of a small PCB containing 4 FM pre-sets close to the PSU wiring doesn't help mattters, but there is no easy way to relocate this PCB and ribbon connecting cable. Adding filter caps. at various points may be the way to go, though this will be experimental, both in the coice of cap. value/type, and where in the circuit they would be needed.

[Murphyv310]

Hi.
A linear mains transformer is a reasonable RFI filter in itself. I'd try making up a filter prior to the radio. To be honest I think it will be a job getting it right.
Perhaps look at erecting an outside FM aerial on the chimney or gable end to get a better signal to noise ratio.

[Nick]

I doubt that ferrite suppressors on the PSU will have any effect, nor will suppression caps.

Is this on all bands? What does the interference sound like? Where is the radio sited?

It's also unlikely that it's tills etc. as they, like all modern equipment, have strict EMI/RFI standards to meet.

[Old Sparky]

Thanks, Trev & Nick. I've been mulling this over and wondering if there is a satisfactory answer to this problem. At my location on a housing estate at least half a mile from any offices or other business premises the radio works perfectly on FM, which is all the owner is concerned about (on AM the usual 'hash' is present at times), but in my cellar workshop in town, there are whistlles, pulsed noises, etc. continuously, and even strong signals are difficult to reesolve, although most FM radios will receive at least two or three national stations on FM there, even portables using only their telescopic whip aerials. The Classic 928 has no provision for an external aerial and I doubt that it's owner would want to go as far as having an outside aerial fitted in any case, not least because it would detract fom the portability of the radio.

[Crackle]

I suspect the problem there is the radio is not receiving a strong enough signal.
I assume the existing telescopic aerial is connected up inside the case, you could try clipping a longer wire to the tip of the telescopic aerial.
Mike

[Murphyv310]

One salient question is........ Has the problem just come to light all of a sudden? If the customer has had the set for a few years and then the problem starts suddenly perhaps ask if he has bought Power line adaptors, or another item that could be at fault.

[Old Sparky]

I was told by the customer that they had recently bought this radio, which appears to be in good order, though I haven't carried out any detailed checks on it. As noted, performance is OK here at home, but not in my workshop. This, plus the fact that the interference only exsists during office hours, or at any rate only after about 8AM suggests that it's caused when and whilst equipment of some kind is in use. Until recently there have been roadworks in connection with the installation of new gas mains in a street parallel to where the owner lives, so maybe that has caused the problem.

[boater sam]

Is there such a thing as frequency modulated interference, or is the set's detector passing Amplitude Modulated interference?
Otherwise it must be from the mains.
When you say mains powered portable, is this a battery set with a mains converter?

[Refugee]

Until recently there have been roadworks in connection with the installation of new gas mains in a street parallel to where the owner lives, so maybe that has caused the problem.

Temporary traffic lights also make RF noise.
If you get fed up with them and want to get up to mischief just jam the RF link between them and they will get stuck on red in all directions.

[Murphyv310]

Until recently there have been roadworks in connection with the installation of new gas mains in a street parallel to where the owner lives, so maybe that has caused the problem.

Temporary traffic lights also make RF noise.
If you get fed up with them and want to get up to mischief just jam the RF link between them and they will get stuck on red in all directions.

Oh you are awful..... but I like you..... ooo err missus