'X' rated capacitors

[ColinK!]

Just wondering if somebody can tell me about 'X' rated caps. I have a Murphy TA160 that I've been left by a chap who's sadly recently passed away. He had got the set up and running again, the rectifier valve was missing when he acquired it, by fitting diodes and a 400 ohm resister to bring the ht back down to spec. He told me that all it needed now was the high voltage caps replacing.

I've found a website, Cricklewood Electronics that have the right caps, all seem to be within spec, voltage, ripple etc. But I've since read some where that 'X' rated caps should be used. Well now I'm lost ! Whats the difference between a normal electrolytic and and 'X' rated Electrolytic ? Cheers.

[paulsherwin]

Class X (actually X2) caps are used across the mains to suppress interference and modulation hum. Class Y are used to provide isolation, such as the aerial or gram sockets on a live chassis set. They have different failure modes. They are not electrolytics and usually have values of 0.1uF and lower

You don't need X2 or Y caps in most of the circuit, just modern plastic film caps of the correct value and working voltage.

Any electrolytics that need replacement can be replaced by modern types, which will be much smaller. You can leave the original can in place but disconnected to preserve appearances.

[julie_m]

Class-X rated capacitors are not electrolytic. They are special film capacitors designed for direct connection across the mains supply with no series fusible element, and under serious fault conditions will not fail explosively nor set anything else on fire.

The only capacitor in a radio that needs to be class-X rated is the one across the Line and Neutral of the mains supply. All the other capacitors are operating in an energy-limited circuit, and some of them at higher than mains voltages.

There is also a safety class Y, which is for capacitors between Line or Neutral and Earth or user-accessible external metalwork (e.g. pick-up inputs on a transformerless set). These capacitors are specially designed so they will never fail low-impedance under serious fault conditions.

To remember the difference, think "X is aCross".

[ColinK!]

Thats great, cheers for the info, somit else ive learnt. So I can get on and find some electrolytics with a little more confidence that they will be ok. Ive got admit that I was a little puzzed, but as ive not had much expirence of high voltage caps id thought id had better check up on them. Im hoping that thier replacement will also help clear the background mains hum.

[ted_ntsx]

Originally Posted by julie_m

Class-X rated capacitors are not electrolytic. They are special film capacitors designed for direct connection across the mains supply with no series fusible element, and under serious fault conditions will not fail explosively nor set anything else on fire.

Don't be too sure...
I have had two X2 capacitors explode quite violently.
One in a ordinary domestic hairdrier, and one in a old radio that someone had fitted across the mains switch. No fire, but a cloud of acrid smoke!

Ted.

[ekjdm14]

Depends on the brand, the only given is that they are *designed* not to explode/burn & won't do in a reasonable working life.

Many modern ones will likely stay safe for decades, but there are well known offenders (I'm looking at you, RIFA!) that do tend to emit stinky smoke if not catch light. The ones that have let the smoke out though are most likely decades old, like the suppression cap in my B&O TV that failed after over 30 years.

X2 caps IMO should either be looked at as a service item and replaced every few years whether showing signs of distress or not, or otherwise one should just be aware of the possibility of them failing in smoky fashion and never leave an appliance unattended while power is applied. The reason I think the RIFA ones in particular go up is due to their shell cracking and allowing moisture ingress. No matter what the brand though I can't see them having a design life of more than 5 years (otherwise the possibilities for legal cases against manufacturers of such items would be wide open).

[Refugee]

They sure do have a do not use after date even if they are NOS.
They are never marked with any date that is obvious.
I had one go off in the vacuum cleaner a couple of years ago and it sure did kick up a stink.

[Maarten]

Originally Posted by ekjdm14

The reason I think the RIFA ones in particular go up is due to their shell cracking and allowing moisture ingress. No matter what the brand though I can't see them having a design life of more than 5 years (otherwise the possibilities for legal cases against manufacturers of such items would be wide open).

Even the ones without the crappy resin casing will fail (white plastic casing, potted from the top side), but will take a few years longer with a bit of luck... The problem apart from the physical construction, is that they're paper capacitors with the dielectric having been outdated for most purposes for several decades now with no new inventions being applied apparently. From around 1980-1985 on, there was no reason whatsoever to not use polypropylene capacitors in places where paper capacitors were necesary before (polyester capacitors for non-pulse applications were even already available in the late 1950's). Polypropylene caps can fail as well, but usually do so in a more friendly manner and will not fail structurally.

[David G4EBT]

The Murphy TA160 data appears not to be on Paul's disc, but I do have a copy of the (36 page!) manual of this complex set.

There are no capacitors across the mains input, therefore Class X caps have no relevance to this set.

A rather poor scan of the mains input is attached.

Hope that helps a bit.

[ColinK!]

There is (was?) a pdf copy of the TA/TU160/R service manual on the murphy-radio.co.uk website. Is there a section of the forum on here that has manuals etc in it, just wondering as I've not come across any yet.

Well I've just had an excellent result. One of my neighbours that has an electronics prototype board production business has just suggested that I make him a list of caps I need. Very nice man !!

[AC/HL]

At the top of every page is a box with red writing, sometimes referred to as "up top". The service data there is what funds the forum, either individually or as a collection. Members who actively discuss many models tend to have the DVD.

[ColinK!]

Ah yes, I've just found the rest of this site. Cheers.

[Mike. Watterson]

Originally Posted by julie_m

Class-X rated capacitors are not electrolytic. They are special film capacitors designed for direct connection across the mains supply with no series fusible element, and under serious fault conditions will not fail explosively nor set anything else on fire.
...
To remember the difference, think "X is aCross".

Yes, though there are Ceramic X rated and Ceramic caps rated for X or/and Y use. They are usually 10nF or less and 1KV or more rated.

Y capacitors are often in modern equipment that has only a two core flex even though the case is metal. They often then are used for RF interference suppression, such as connecting both live and neutral inputs of the SMPSU to the case.
Older USA equipment often running direct from the 110V - 120V line has two capacitors for RF decoupling, one to the chassis (which unlike European models is usually "floating") and one to the metal case. Obviously these need replaced with 250V AC mains Y rated (or 1KV). The other issue is that such vintage US gear needs an isolation transformer as the regular 230V to 115V adaptors are usually autotransformers. Then the -V HT bus can be at UK 240V AC with respect to the metalwork and chassis. There may be inadequate physical separation by modern US standards, never mind CE standards!
Note that capacitor voltage ratings are normally DC. Plastic Film X and Y types actually have a much higher DC rating, the rating isn't even the AC limit, but the mains voltage they are intended for. The Ceramic X Y dual use types are usually marked with a peak voltage instead.
Motor Run caps can be used as large value caps in vintage circuits (not for X or Y), but motor start caps can't.

There are different maximum values of X (across mains) and Y (to earth, lower value) for either power factor (X) or earth current (Y, which with a lot of appliances with too high values can trip the ELCB) or give someone with wet hands a shock.

[ekjdm14]

Originally Posted by Mike. Watterson

"snip"..... or give someone with wet hands a shock.

Well personally, if someone sees fit to start poking round electrics with wet hands they frankly deserve a belt to knock some sense back into them! Don't get me wrong, safety standards are fantastic today compared to live chassis and non-polarised plugs and I wouldn't want to take a step back in that respect but where has the common sense gone?

Maybe there should instead be a minimum rating for class Y caps, whereby they can give a non-lethal nip if someone is pushing the boundaries of intelligence?

[ms660]

You don't have to be poking around inside electrics to get a shock with wet hands (or dry hands even) All you need is a defective earthing system and a path for current to flow, say via some Y Class capacitors for instance.

Lawrence.

[ekjdm14]

This is what I get for posting whilst not fully awake... (pushing the boundaries of my own stupidity, at least my hands were dry I suppose...)

[Mike. Watterson]

Originally Posted by ekjdm14

Well personally, if someone sees fit to start poking round electrics with wet hands they frankly deserve a belt to knock some sense back into them!

I'm talking about plugging audio leads to a TV, or working on a TV aerial or satellite cable. I've had unpleasant nips with dry hands doing that. IMO some modern gear has too high a value of capacitance from Live and Neutral to the case or connectors with only a 2 core flexes.

Shaver outlets are fine. Hairdryers usually have plastic cases. Of course people may be operating their domestic equipment in bare feet and may be damp. People have sat boxes, DTT boxes & TVs in bedrooms. People may connect up an amp to a phone with wet hands. Or plug a USB stick into a Smart TV.

Though using a phone charger in the bath is terminally silly. There is a good reason why bathrooms only have a shaver outlet (2KV isolation, I think, no Y capacitors and often limited by core design and thermistor to 20W).

[G8HQP Dave]

Confusion between X-rated caps and electrolytics may be caused by talk of 'mains filter caps' (which are the X-rated small value caps across the mains switch or mains transformer primary) and 'main filter caps' (which are often electrolytics and mostly reservoir caps not filters at all).

[Station X]

Some posts moved to a new thread here:-

https://www.vintage-radio.net/forum/...d.php?t=155352