More QRM on 4M

[G3LVP]

A new source of QRM has appeared on 4m in the past few days, this sounds like the sort of noise radiated by "Powerline Adapters" and takes the form of a 'chattering' noise which extends from 69.5 - 70.3MHz peaking to S9+10dB when I beam SE. I've also noted this noise on 39, 44, 48 & 78 MHz.
This is in addition to the 'Sweepers' which I've reported before (which appear to be be the second harmonic from something radiating on 35 MHz) and a weak (but annoying) carrier which sits on 70.2MHz h24. I have a suspicion where the new QRM originates but am going to investigate further before referring the matter with OFCOM.
Any other reports of this type of QRM on 4m would be appreciated as I wasn't aware that these devices would affect frequencies above the HF range but in this case I can't hear the noise on any lower frequencies.

73...

Ken

G3LVP

[G3LVP]

I contacted OFCOM regarding this interference, this resulted in a visit by a couple of their engineers who located the source. This turned out to be the control unit/PSU of a 'Stair Lift' in a neighboring house about 60m away from my antennas.
When this unit was switched off the interference disappeared however it didn't reappear when the unit was switched back on, I have a feeling that this might not be the end of the story.
At least for the moment the 4m is usable again just in time for the Trophy contest as well!
Whilst this investigation was going on I took the opportunity to demonstrate the 'Sweepers', whilst these don't make the band unusable getting them silenced would be a bonus.
So far no one else seems to be reporting this form of interference or the similar noise which appears on 21MHz.

73...

Ken

G3LVP

[G3PTU]

I have observed the type of problem you quote with Switch mode PSU's.
I think it might be due to shortcomings in the design or old age, that only show up when the input voltage has wandered outside the norm for some reason or other. When powered from 'cold' the starter arangments work OK, but while working (summise) if the mains volts fall, the feedback loop becomes unstable and rather than behaving in the way it was meant to, maintains the output voltage by switching on for long bursts - thus the chuff chuff.
What was really required was to try this device off a variac, pity this was not done. I have seen Saterlitte receivers PSU do this sort of thing (and seen it on the picture), the cure being " new electrolytics" all round. Which brings in another aspect of this thread, whereby , it apears because of finacial considerations that poor grade Electyrolytics in these things are the norm. Have you noticed that computer PSU's last longer than say 'Pace' PSU's. they are cooler and use 'better grade stuff inside'.
Switch mode PSU's if built corectly acording to the rules are OK, I have some that you cannot hardly hear on my QRM rx, even close up to. In these days of expensive 50hz they represent a considerable saving over the 'dead loss' types.
Insedently the rx I use ( it will only work close up and along cables) is on the WWW " All band rx" Ask Mr Google or Pm me
David G3PTU

[G3LVP]

David,

Thanks for your excellent idea of powering the Stairlift unit from a Variac. I suspect that what may have happened is that there was an interruption to the mains supply and then when it came back on it may have been at a reduced voltage which caused the switched mode PSU in the chairlift battery charger to start up in the 'squegging' mode.
Switching the unit on & off at normal mains voltage as was done the other day didn't cause this to happen.
It's now been almost a week since the OFCOM investigation and so far the QRM has not reappeared. If it does I may see if I can try running it off a variac.

73....

Ken

G3LVP

[G4MDC]

i understand the pro's in SMPSU design have a term for this = "burst firing". Might come in handy in future discussions perhaps? 73

[G3PTU]

Yes I agree its worth remembering and more disscusion fodder as it where. Thinking back over the years (oh hell that makes me old!) I have met Switch mode PSU's that never put cycle wrong as it where, yet others that where bags of trouble. Perticully bad beasts would blow 60A mains fuses randomly. The circuit had a SCR across the mains to do this, but suposadly only when something went badly wrong and the output volts went high, the PA transistor Thunderbird circuit. A simple comparitor saw to this action. On test the thing never did the fault, only in use it happened sometimes twice a week. The problem turned out to be that the mains in rural places is subject to things like milking machines going on and off, this SCR circuit was fast, in fact faster than the normal voltage loop, so it was possible to fire the thing on these voltage blips. A 1000pfd cap was pressed into service across the Gate.
Really designers should have special dirty mains feed unit to simulate these potential problems. We made simple gadget with a 555 and a relay that banged a mains supply off and back on at the press of a button. It did not half find some problems.
David g3PTU

[OZ2M]

Hi

Feel free to submit RFI here: www.70mhz.org/uss.htm

73
Bo, OZ2M - Looking for DX? Try VUSHF DX-peditions www.rudius.net/dxp