Re: re PC RFI

Conrad, PA5Y

Alan you are absolutely right. The fact that the vertical stripes are 50Hz apart suggests that mains hum is modulating something else. This mains hum must be from a ground loop but I do not hear it on my audio devices, nothing at all. Getting rid of the ground loop maybe all that I need. The radiation may not be from the screen but one thing is absolutely certain the interference comes down the antenna and only on 432 MHz, I do not see any evidence on the 2m EME system nor 50MHz, 70MHz, 1296MHz. I do not have any HF antennas or wires up so cannot check lower than 50 MHz. It is a Philips LED monitor. I don't have time to really experiment until the weekend.

I may have to take the back of and use an oscilloscope :-)

I will try and get a schematic.


Conrad PA5Y

On 2018-03-06 12:18, 'Alan Melia' Alan.Melia@... [ukmicrowaves] wrote:
Hi Conrad I am wondering if you are attacking the right source. Just because
it is still there on a clean external DC power supply does not mean that
there is no SMPSU internally providing various rail voltages from the clean
I have had screen interference on LF but it did not extend to anywhere near
70cms .....the fundamental was 700Hz or thereabouts, and it was rock stable
on a waterfall, not the drifting you report. Drifting lines with mains
modulation has nearly always been traced to an SMPSU where I have
encountered it.....a free running switching oscillator. What does seem clear
is that it is not radiated from the supply lead ......which might be an easy
The best way to cure RFI is to attack it at source rather than trying to
screen it out. What technology is the monitor screen?? the RSGB EMC helpdesk
man (Ken) is an ex TV engineer and has had a lot of success getting some
errant TVs replaced.
Good Luck with it
Alan (G3NYK)
RSGB EMC Committee member
Posted by: "Alan Melia" <alan.melia@...>
Yahoo Groups Links

Join to automatically receive all group messages.