I have been complaining about sporadic strong SMPS interference for the last
couple of months. And when it came up again the other day I decided to put
up WinRad and connect an untuned 24 turn 80cm diam loop to the sound card
input and see if I could find where it was. I realised I was getting a
signal from my second PC at about 35kHz, this machine has never produced any
in-band (136kHz) interference since new, and the peak looked clean. I
switched it off to make the job easier, and ......yes the interference
disappeared. Thinks.....why.....so I switched it on again and put my hand
round the back at the top. The PSU area was very hot and there was no "blow"
from the fan. Obviously the frequency only drifted into band when the fan
stopped, and the PSU got overheated, so I didn't relate the interference to
putting that PC on.
The PSU was dropped out and opened and sure enough the cheapo fan was locked
solid on seized "bearings". QuicK swap for a spare fan (I keep several lying
around and some old AT PSUs) Switch on ....nice quiet machine....no
groaning...and no QRM in 136. I suppose the fan has done quite well really
that PC is about 3 years old and you are expected to change them every 18
months !! only I am too mean !!
Motto..... Check at home before blaming the neighbours and check the PSU
fans. If I had not spotted that I might have got very overheated (that is
the machine that runs the T/A captures overnight) and possibly corrupted a
disc. The thing was that the signal did not drift into 136 until the P had
been on for some time and heated up.....so I didnt think it was causing a
problem. I still have the weaker TV qrm to deal with and the one FMing its
PSU but I can hear signals again now. The noise even overpowered Rugby
Loran-C.
Interestingly the ATX PSU did not have an encapsulated filtered IEC input
connector but did have a filer on a small pcb mounted over the plug, which
probably accounts for its generally acceptable behaviour.
Cheers de Alan G3NYK
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