You will not see a 50Hz ripple on a simple voltmeter. You need a oscilloscope that shows the DC output on a CRT screen. In effect you zoom in to a point where you can actually see the 50Hz ripple.
What is oscilloscope? - Definition from WhatIs.com
You can buy a Sky-HD box for next to nothing.
eg
SKY HD BOX AMSTRAD DRX890 500gb Inc Remote And Power Cable | eBay
...
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You can measure stray voltages with even a farely basic digital test meter although they dont give you all info & I do have Oscilloscopes they are useful to show the voltage if it is at 50Hz. Stray voltages on AV equipment in households today are very common where they are typucally extensively interconnected & especially where aerial distribution &/or multi output lnbs on satellire dishes are included.
As most know on here I install loads of motorised satellite systems, it's the 22kHz diseqc signals to the motor that can serve as a unique but very useful indicator of stray high AV voltages on household AV setups because uniquely those signals can get affected by stray AC voltages even if they are of low power as they usually are. The motor also can act as an earth path. On almost every occasion this happens a fault can be measured as spurious AC usually on the common chassis ground but can be on the 'core' too.
Often stray voltages faults are 1st highlighted by actually receiving a shock when disconnecting / connecting leads & definitely imo should not be solved by attempts to circumvent them but tracked down to their source.
The most common devices to have such faults are old Sky boxes number 1#, old tvs, any older AV gear and some poorly designed / cheap PSUS.
Find the fault and remove it or fix it properly, not just a couple of capacitors but replace whole aging PSUs and these sorts of faults happily track over any interonnecting cables.
Only if it's say a genuine incompatibility issue or a build up of touch voltages & not a fault should a 'fix' be used imo, otherwise you can be masking a potentially serious fault that can cause damage to other newer equipment, or worse, a fire.
I've seen too many of these, it's an issue that now needs addressing, the CAI did offer a bonding solution but I think a device similar to an RCD but designed for this lower power issue and I'm not 100% it should be called an earth loop issue, it's more likely stray voltages.