if you decide to copy any of the things I did here, you do so entirely
at your own risk. If you are not completely confident you know what
you are doing consult a qualified electrician.
After we had a new Vaillant 'ecoTEC plus' boiler fitted I discovered I
had a high noise level on 15m. It was at about S4-5, which might not
seem so high, but it's enough to wipe out most of the interesting DX.
Using a cheap portable commercial radio that had shortwave band
capability, I initially tracked this down to the wiring around the timer
which is located in a cupboard upstairs, which also houses the unvented
hot water cylinder. The boiler is in the utility room immediately below.
I could turn off the power to the central heating and hear the noise on
15m disappear both on my amateur rig and on the portable radio brought
up close to the wiring in the cupboard.
My initial thoughts were that the timer was to blame because the
portable radio showed an increase in the noise when it was brought
closer to the timer. However, I then removed the timer for inspection
and turned the power back on to the central heating system. The noise
returned! A bit more detective work showed it was coming from the
boiler. Fortunately, there is a line switch to the boiler, so I was able
to ascertain that the noise disappeared when the boiler alone was
The problem was easily solved using a common-mode choke formed out of
the 5-core power/control cable and a ferrite toroid from Maplin
(order code QT26D), as
Normally, the cable just passes through the two strain relief bushes and
connects to the PCB terminal block. The addition of the connector block
between the two bushes allowed me to make the common-mode choke at the
workbench. You need to get as many turns on the toroid as possible,
which is aided by removing the outer sleeving.