When is a capacitor not across the mains.
 

I’m working on a KB10 mark 2. This has a capacitor C21 installed across the mains but has 150 ohms in series with it. Is this still an across the mains capacitor. I wondering if ideally it would be Y rated as a failure would likely light up the 150ohm resistor but wouldn’t initially blow the fuse with perspective fault current of 1.5 amps. I assume the resistor wouldn’t last very long as it tries to dissipate 350 watts but perhaps long enough to start a fire. I am guessing a failed short circuit rectifier would have a similar result etc.

I suggest Y rated as X rated capacitors are designed to fail short circuit and operate the circuit protection.

I had actually ordered a Xrated capacitor as I thought initially I had the mark 1 version and this does have the capacitor directly across the mains supply.

I wouldn’t normally worry about this but this radio is for a friend and so they may not take care in operating the radio as carefully as I would or do not know about the risk associated with old equipment. I suspect I’m asking an impossible question regards risk here.

Thanks Chris

Re: When is a capacitor not across the mains.
 

When I restored my FB10/2, I fitted a 1250V working polyester capacitor in this position. The series resistor should be 75 Ohms (150/2). I always fit a 1A fuse to BS1362 in the plug of any set I restore - assuming surge current conditions permit.

I would not have any problem releasing a set so equipped, but would be mindful of its new owner - no small kids within reach, dry location, correctly wired and polarised mains socket etc.

One dubious aspect of the FB10 is the closeness of the chassis flange cutouts to the base retaining screws. If the chassis is not centralised relative to the cabinet, a screw head could become live with an incorrectly wired supply. It's just about OK with careful assembly - but my set's chassis was bent when I started work on it.

Leon.

Re: When is a capacitor not across the mains.
 

I would say that's an "Across the mains" capacitor since it will be subjected to full mains-voltage despite the surge-limiting resistors.

I'd go with Y-rated; I'm happy to use 440VAC-rated components in such situations to give a bit more 'headroom'.

Re: When is a capacitor not across the mains.
 

Thanks Leon

Can anyone confirm on the resistor. Certainly 2 x 150 is what the the service information says but my radio has 2x 300 fitted so I did wonder if that was a error in the service sheet or if they had been changed out in my radio for the wrong value

Re: When is a capacitor not across the mains.
 

I think I have some 630v available so maybe a suitable compromise. Just checked my Anode and I appear to be spot on voltage for my 235v AC input

Thanks

Re: When is a capacitor not across the mains.
 

Just for confirmation I have 2 x 330 ohms in that position so I do question the KB service sheet.

Re: When is a capacitor not across the mains.
 

If you look at the schematic I referred to earlier on (FB10 Jan 1954) it shows the resistors as 330 ohms a piece on the modified schematic.

Lawrence.

Re: When is a capacitor not across the mains.
 

Thanks Lawrence.

I think my problem is I don’t have that 1954 sheets.

Chris

Re: When is a capacitor not across the mains.
 

Ok Lawrence I have found that service sheet now Thanks

Chris

Re: When is a capacitor not across the mains.
 

Hi Chris, the difficulty of using a Y class across the mains is that they are not easily available in typical X cap values, at least not in reasonable sizes.
In terms of voltage rating of the X Cap when a series resistor is fitted the resistor is ignored (but it may be helpful in suppressing some level of noise

Ed

Re: When is a capacitor not across the mains.
 

Yes, Y class do not come in large values (to avoid excessive earth current).

But Y class is built to be like 2 caps in series so you could always use a pair of X class of double value, with added bonus that you can check it for single failure.

Re: When is a capacitor not across the mains.
 

The sets fitted with an EZ80/6V4 rectifier had a surge limiter of 75 Ohms (2 X 150 in parallel). I have observed that these sets have a higher HT voltage than those fitted with the - rather unreliable - 6X5.

Too many variations....

Leon.

Re: When is a capacitor not across the mains.
 

It's my understanding that 'Y' capacitors are used from L to E & from N to E, and 'X' capacitors are for across the mains. (As in X = 'A Cross').

I'm sure someone will put me right if I'm wrong, as I sometimes am.:-)

See attached sketch.

As to the FB10, the makers data only shows a cap across the mains - the FB10 MK2 circuit doesn't, but as has been said, like most manufacturers, changes are made after the data is produced, so if parts look to be factory fitted original, it's as well to replace them. In this case, a class X, which is what Chris has ordered.

Re: When is a capacitor not across the mains.
 

That capacitor may have been put across the rectifier to eliminate modulation hum.

It will probably reduce mains borne interference as well.

Re: When is a capacitor not across the mains.
 

The manufactures service supplements for the FB10 show that there's no capacitor across the mains, just one from the anode of the rectifier to chassis, that modulation hum modification was from serial number 60,001 onwards according to one of those supplements.

Lawrence.

Re: When is a capacitor not across the mains.
 

Since X capacitors may fail shorted and Y capacitors may not, I doubt this would satisfy all appliccable rules. The testing voltage for Y capacitors is double that of X capacitors, though, so the series circuit would at least satisfy that criterion. Some capacitors are especially manufactured to fulfil both roles, though.

Re: When is a capacitor not across the mains.
 

Thanks for the discussion. My understanding of X rated capacitor is they are actually designed to fail short circuit as the method of protection is to blow the fuse. Although I do enjoy the phrase self healing regards capacitors I.e one cuts of the affected limb. I have gone for a 1000 rated polyester on the bases this is an order of magnitude better than the waxy capacitor that I’m replacing. However always open to being corrected.

Chris

Re: When is a capacitor not across the mains.
 

Hi Chris, most Xcaps use a very thin metal film, so when a short develops in the cap, that area melts and seals off. Generally this short is of limited energy so can happen many times before the fuse will fail. It does however isolate part of the cap and reduce the capacitance as can often be seen if an old X cap is measured.

When used as a capacitive dropper this may ultimately result in the circuit not functioning as the Z of the cap is now much increased. It will not normally allow large currents to pass for a long time period, so can be used with confidence as a capacitive dropper in miniature sets.

Ed

Re: When is a capacitor not across the mains.
 

They may fail either open or short. They may not fail somewhere in between and start a fire. Mind you, some paper caps only fulfil that criterion when they're new...

Y capacitors on the other hand, should only fail open.

Re: When is a capacitor not across the mains.
 

The whole concept of the X cap was that when it goes SC, it is meant to self heal, then go open or have reduced capacity.
The dreaded RIFA types unfortunately did not.

I believe motor start/ run caps are also supposed to self heal

Ed