QRM Eliminator from eBay

I suffer from significant electrical noise on 40m (7MHz) so I thought I'd try one of the cheap QRM Eliminators available on eBay.
The main antenna is an End Fed Half Wave resonant antenna that works on 40m, 20m, 15m and 10m without an antenna tuning unit. The secondary antenna is a length of insulated wire about 10m long run around the inside of the shack and up against three walls.
The QRM Eliminator works well for me, cancelling out the annoying repetitive noise and making the signal much cleaner and easier to hear. Adjustment is fairly critical and you do need to play around with all three controls for best effect. SWR is unaffected on 40m with the QRM Eliminator in series.
I don't have the noise issue on 20m and up so I don't need the Eliminator connected. However, I did notice that the Eliminator does adversely affect SWR on 15m and 10m if it is left in series even when switched off.