I am working on a vintage radio and as usual, it had a 0.1uF capacitor fitted between one of the mains input wires and ground, as well as a 0.1uF between the heater circuit and ground. The other end of the heater circuit is already tied to ground. As we know, in the day, before X2 and Y types, wax paper capacitors were fitted and for obvious reasons these have been removed.
The question now is, do they need replacing and which sort. But first let me explain the reason for my questions.
The chassis on this radio is floating, that is, it wasn't originally connected to earth and is not directly connected to either incoming mains wires which go directly into the transformer, one of them going via the on/off switch. Since the radio is metal cased, it made sense to earth the chassis for safety reasons. In the original circuit, there was a 0.1uF capacitor placed between the lower wire on the diagram which one might consider to be the neutral connection (the live being expected to go via the switch), but in reality, with the original non-polarised two wire lead, either could end up being neutral or live. The original flex has been replaced with a modern 3-core flex and since earth is bonded to neutral, the need for a capacitor between "neutral" and earth seems to have become nugatory. I should also add that the radio has the usual choke after the rectifier with electrolytic smoothing capacitors to ground on the input and output so there is filtering on the B+ rail.
So I have been doing some reading about the purpose of X2 capacitors versus Y capacitors but for all that, the reasons of using one versus the other is still not so clear, although I gather that Y types filter common and X types differential mode interference . I also understand that the need for filtering capacitors is to prevent RFI and EMI interference in either direction across the mains wiring. There is also a compliance requirement, but we are talking vintage equipment here, so I am more interested in the engineering reasons. I am not sure that RFI is likely to leak from the radio since the oscillator kHz or IF frequencies are not likely to traverse the large mains transformer, and there are no motors to generate EMI, though the operation of switches can generate transients. The more likely source is incoming EMI from modern cheap SMPSUs and the radio is likely to pick that up over the air in any case.
So I guess my first question was do I need a filtering capacitor at all? The answer is probably yes, because although interference is more likely to be incoming than outgoing, the interference from modern SMPSUs is far more prevalent today. Someone on another forum mentioned that the components are cheap enough so there is no reason not to and while this belt and braces approach may be a valid economic reason, it doesn't answer the engineering question, but certainly seems worth doing since there would appear to be an advantage.
The second question is what type? Obviously across the mains it has to be a X2 capacitor and Y types are used between mains and ground. Since the original capacitor was placed between one of the mains wires and ground, one might be tempted to just replace this with a Y type. In like vein I have sometimes seen IEC inlets with ceramic Y capacitors (possibly X1 Y1) between both mains wires and ground, presumably to filter out common mode noise. Yet, since in the UK neutral is bonded to ground, then the capacitor on the live side is effectively connected from live to neutral and the one from neutral to ground is effectively shorted, notwithstanding some minimal resistance between the appliance and bonding point. While on the subject, might I ask, since one is designed to fail open and the other short, which way will an "X1 Y1" or "X2 Y1" fail? Open or short?.
So I am tempted to just fit an X2 across the mains but the choke is there to filter differential noise anyway, so does it need both?
Can I use an ordinary foil cap between the filament output and ground? Or should this be a Y type as well?
Well this is not exactly an exciting question 🙂
After some further reading I have put an X2 across the mains input for now.
On the Eddystone it was a bit simpler because it was a straight swap of the two ceramic caps connected to the mains wires with Y caps, but that radio is an AC/DC set connected directly to the mains. It has no transformer and instead uses a restive dropper. With this radio how to proceed is not quite so obvious.
@waveydipole If you earth the chassis I would keep an eye on the mains transformer as earthing may impose extra strain on the windings. I have noticed this on Dansette record players they often weren't earthed and fitting one makes the transformer run hotter in some cases. On Radios I fit a suitable X2 filter capacitor on TV sets I mostly just snip it out.
Thank you for the hint. I will check the transformer to be sure.
