Odd PSU behavior
 

I have a little FM transmitter here which is used to rebroadcast TV sound to my hifi so I can enjoy the better quality and stereo offered by modern TV its powered by a switch mode PSU I got from a charity shop the supply is way over rated current wise so is not strained any way. However there is a 50 cycle hum on the audio which I've noticed disappears when I put y hand on the PSU to unplug it. I noticed that it disappeared like that when I went to move it a while ago its plugged into a short extension lead what do you think why the hum disappear when I put my hand on it??

Re: Odd PSU behavior
 

RF induced hum can be a burger to cure, but filtering the supply feed on both sides of the Transmitter supply line (at the transmitter) will help immensely. You will need to put in a pi filter on both the + and - side of the supply lines.

Placing the transmitter and/or antenna as far away from the PSU will help too, or changing their position relative to the supply cable..

I would use a mirror type (on both + and - supply lines) pi filter with 1 nF and 10 nF caps with any largish value choke of say 5-10 uH or any value that has a high reactance at 100 MHz.

You could try just inserting a toroid choke at the transmit end to see if that works by wrapping some turns round the toroid core, and if you have more than one, insert another at the PSU end too.
An even easier way is to use those clip on ferrite cores (see linked article)
This article may help
http://www.audiosystemsgroup.com/RFI-Ham.pdf
:beer:

Re: Odd PSU behavior
 

Could it be 100Hz hum? More likely with a SMPS which usually have a bridge rectifier to generate the 350V which is then switched. The hand effect probably shows you are diverting some of the Transmitter RF which otherwise passes through the bridge on its way to ground/metalwork and gets modulated at 100Hz

Re: Odd PSU behavior
 

Thanks I will look into repositioning the TX and PSU first then il try some filters on the DC cable il let you know how it goes

Re: Odd PSU behavior
 

In addition to the comments above, hum such as this can also be caused by mains wiring directly picking up the RF and effectively re-radiating it. Try various positions for the TX, away from walls and all cables. Can be a difficult problem.

Re: Odd PSU behavior
 

A big problem these days is that everything is "double insulated" which means that any wired connections see a capacitor to an average of the L and N voltages, so any such connections for simple audio have to face potentially quite a lot of common-mode mains hum, and this may include higher frequencies modulated with 50 or 100Hz too if SM PSU is involved.

I have this problem myself but haven't got round to trying what I would consider the obvious fix - transformer isolation of the audio, with an interwinding earthed screen. Or perhaps an optical isolator.

Powering your rebro from the TV would be the other fix of course.

Re: Odd PSU behavior
 

Powering the FM TX with batteries would be another, as would, dare I suggest, using a direct connection between the hi-fi and TV.

Re: Odd PSU behavior
 

This sort of thing is also an issue with "Direct Conversion" HF receivers - basically, if any RF signal flows back through the diodes in the PSU [whether it's a switcher or a more-traditional transformer-and-diodes type] the RF component can get impressed upon it either the mains-ripple-frequency or the SMPS oscillator-frequency - and this remodulated component then gets transmitted/received along with the intended content.

ARRL handbooks in the 1980s had some good advice on counteracting this.

Re: Odd PSU behavior
 

The idea of trying the TX on batteries is a useful test - definitely worth a go.
I doubt that a direct connection to a HiFi is going to help - most likely to be worse.

Re: Odd PSU behavior
 

I have had a similar problem. I made a unit in an old small speaker box which uses a cheap imported MP3 player, a kitset FM transmitter and a 5v regulator - it all works very well - a 4Gb SD card lays well over 600 oldies - BUT back to the question. When first built there was a lot of hum on the received signal. I was using transformer wall-warts, not switch mode PSUs and was getting to the point where I thought of dumping the whole project - until purely by chance I used a different PSU! Presto - no hum!

Re: Odd PSU behavior
 

A vote for this solution. The rebro takes very little current and problem solved instantly!

Re: Odd PSU behavior
 

I did originally run it from batteries but using ordinary batteries got expensive as the device is never completely off but just sleeping I tried rechargeable which was great for a while but eventually of course they wear out and getting replacements has been difficult so that's why I tried a mains PSU After all mains never runs out and if it does the telly wouldn't work. I've now decided what to do im going to do il change the power supply to a big old linear supply I've got that gives silent working so problem solved thankyou for all your helpful reply