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

[Old Sparky]

Sam, this is a portable radio with a built-in linear mains PSU. Trev - I guessed that traffic lights might be responsible, though Iwouldn't do as you suggest (Tongue in Cheek I hope!)

[Refugee]

What is the shortest time it has taken for them to dig a new road surface up again?
They did one for gas about 3 months after about 2 miles from me. It now looks a right mess.
When I lived down south they did much better and again it was gas. They got there before the phantom skid mark man.

[Old Sparky]

I don't know about the shortest, or longest times between a road being resurfaced and it's being dug up again, though with so many services being underground, sometimes the intervals can be quite short. The Gas Mains renewals here in Chippy have been going on since late June, and reportedly will continue for at least another three months. Back to the Radio - I have since found that if it is powered from batteries, there is no interference, so the latter is obviously mains-borne. Probably no cost-effective way of suppressing it, unless a mains plug or adaptor incorporating a built-in RFI filter will help. A now deceased Radio Amateur told me over 40 years ago when I was into SWL-ing, to build a delta filter into the 13Amp plugtop using, IIRC, something like 3 x 0.1uf HV Ceramic Capacitors. In those days neither ready made Filter plugs, not, again, IIRC, Clas X &Y capacitors wrere available, and there was far less interference about, but using a filter plug or adaptor might be worth a try.

[Refugee]

In the 1970s medium wave used to have very loud bursts of noise on it on all mains powered sets. Never on a battery set even if it was indoors. Car radios were obviously not effected.

Tarmac.
When I lived in the south it was in an area close to the center of town.
Within ear shot they resurfaced a shopping street on a Sunday.
On the Monday morning well before the phantom skid marks had appeared and 18 hours from completion of the resurfacing the tell tail sound of the air tools were heard and as soon as I got there to have a look they had already dug a hole three feet deep in the less than a day old tarmac. The tarmac must have still been warm. The phantom skid mark driver must have been devastated.

[Terry]

You've already established that the problem is signal level dependent and you say it has an internal linear PSU.

Do you have a suitable external PSU that you could try in place of the battery? If this works - with the PSU as close to the radio as possible, of course - this would point to a problem with the internal PSU. Check any suppressor capacitors on the mains input, that any electrostatic shield in the transformer is grounded OK. It's a long shot but I wonder of faulty rectifier diodes could cause a problem?

[Murphyv310]

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.

[Old Sparky]

I haven't checked if those caps. are fitted, Tre. As yuou know sometimes manufacturers fit them, sometimes not.. As for an external PSU, no doubt I have somewhere a suitable linear one, or could run it from my variable voltage bench PSU, though I'd have to temporarily 'bodge' a connection to do so.

[Terry]

If you operate the set in battery mode you'd only need a pair of wires with a couple of crocodile clips to attach the PSU to the battery holder - hardly a 'bodge'!

[Murphyv310]

Why would you possibly think a temporary connection of an external PSU is a bodge??

[Old Sparky]

True enough, Terry. I've often powered battery radios from my bench PSU that way in the past, and that certainly isn't a bodge. My bench PSU is a good old-fashioned linear one I built from a PW design around 40 years ago. I seldom, if ever, get any interference when using that.