Replacement capacitor polarity

[teedeeb]

Hi,

Recently, I've acquired a vintage radio, which only produces humming.
I'm willing to give it a try to replace its capacitors, likely to be the cause that the radio isn't working properly.
I've read about it online, but one thing isn't clear to me.

From what I can see, most paper and electrolytic capacitors inside don't seem to indicate polarity.
Some of the replacement capacitors I'm buying online however are polarised.

Is it okay to just install the replacement capacitors without taking polarity into account? Or do I need unpolarized replacement capacitors to replace unpolarized original capacitors?

[peter_scott]

You really need to look at the schematic to determine whether polarised capacitors can replace non-polar types. You need to be sure that the rated voltage is not exceeded and will not be reversed.

Peter

['LIVEWIRE?']

Paper, Polystyrene, and other , normally low value (below 1uf) capacitors are not polarised, though they may have a ring around one end, whereas Electrolytic capacitors are polarised, and therefore must be connected the right way round. AS others have said, blanket replacement of capacitors is not really the way to go, but a logical approach to fault finding is. Do you have any test equipment, such as a multimeter, and what knowledge of electronics do you have. Before going any further, if this is an old valve radio, it may not have any form of internal aerial, in which case an external aerial ( a few feet of wire) will be needed. The other, more important point is that the radio may have a live chassis, meaning that one side, which should, for the sake of safety be the Neutral, of the chassis is directly connected to the mains. Some pictures, and/or the make and model of the radio concerned will help us identify it, so that we can better advise you how to proceed.

[Oldcodger]

No bearing on polarity issues, but from dim & distant memory , I seem to recall that the end of the cap with the ring meant that the outer of the roll ( most of these caps are wound as a toilet roll ,with dialectric in middle ) was connected to ( again from memory) the ring end. In certain circumstances it might be advantageous to connect this end to ground to aid in screening .
As to correct connection.Be aware that polarised capacitors ( electrolytic as used in valve radios ( and Tantalum as used in more modern stuff) ) will react violently to in correct polarity, and the innards of an electrolytic capacitor can be toxic, when the cap explodes ,as it will on exposure to incorrect polarity, with shards of hot metal flying about. Not said to scare, but as a warning.
As Livewire said- SOME chassis can be live ,intentionally or not. So my main rule before laying fingers on any conductor or anything connected to/potentially connected to mains is to apply a Neon test screwdriver to metal etc.
Then when set is switched off, bear in mind that capacitors retain charge. 250/220 v DC ones will retain sufficient to give you a nasty jolt.Even 100v dc ones can pack a nasty.

[Diabolical Artificer]

As LW says, it may just need an arial or bit of wire plugging in. Check first, recapping a radio may not be needed, can be a nightmare and full of traps for a beginner.

You can soon loose your way inside a radio ( a lot of radio chassis's are a complete jumble) or any bit of gear when replacing caps. It's helpful to mark capacitors with a marker pen dot in one orientation, to the the front say. Then take lot's of good quality pics from all angles. This way you have a permanent record of what cap goes where and it's polarity if any.

If you post any details, make,model and any pics here, we can better advise you.

Andy.

[G8HQP Dave]

It is likely that many of the capacitors will not need to be replaced, so replacing them will be a waste of time and money and carries the significant risk of introducing new faults. Replacing some capacitors will then require a re-alignment; fortunately these capacitors rarely need replacing - but you need to be able to recognise them so you know not to change them.

It is likely (if the radio is old) that a few capacitors will need replacing: the reservoir capacitor (usually an electrolytic, so polarity matters), the coupling capacitor to the audio output stage grid (almost certainly unpolarised). Check these first, then see what else needs doing.

Humming does not necessarily indicate a capacitor fault. A faulty valve can cause hum too.

[teedeeb]

Thank you for the replies.

The radio is a Radiobell Scherzo. Unforunately I can't seem to find a schematic for it, so I made a list of capacitors that are present inside.

I also took some photos of the different types of capacitors I can find. Please correct me if I'm wrong in any assumptions.

---


This seems to be a paper capacitor, burst on one left end on the photo. I think it's polarised because I can see some ++++ on one side.
Can I safely replace this by a polarized electrolytic capacitor?


I'm not sure about this type of capacitor. These don't seem to be polarised. All the ones I saw inside seem to be intact and working, but I don't know the inside conditions, since I don't know what they are.


From what I've read on the internet, this would be a polystyrene capacitor, inscribed "MIAL". Seems intact so I don't think it needs replacing.


This is a large tube on top containing 2x32µF capacitors. I don't know what's in there, I don't know if they need replacing. Also, there are just two equally coloured wires coming out of the bottom, so I don't know about polarity.


Seems a pretty modern capacitor. I think it's polarised because it says 30V - +, which makes me think the positive side is on the right hand side in the photo. Not sure if it needs replacing.

Are my assumptions correct? Which ones should I replace?

[julie_m]

As a rule, 1μF and larger values tend to be polarised; smaller values tend to be non-polarised.

The 2*32μF is two capacitors in one can, which is the common negative; the connections to it are the two positives.

The capacitor that most desperately needs changing is the one that goes to the first grid of the output valve. Search online for that valve, find which pin is labelled G1 and trace the wiring to the capacitor. It most probably will be 10nF, 400V. If it is faulty (and they do die of old age) it can cause the output valve to pass too much current, with consequent damage to other components -- most often the loudspeaker transformer.

[TrevorG3VLF]

The first one is an electrolytic, 8 microF, 450V and needs changing. Sometimes they are packaged in metal, this one is paper.
Electrolyic capacitors in aluminium cases have the case as a negative connection even if there is a separate tag.

The other capacitors look to be fine but change the one which Julie has nominated.

[ms660]

Schematic and components list:

http://85.144.174.218/nvhr/Radiobell_Scherzo.pdf

Lawrence.

[julie_m]

According to the diagram, C38 is the one that needs replacing and it's actually 0.05μF. 47nF is the nearest modern value (1000 nano-anythings make a micro-one of the same thing), 400V will be fine.

EDIT: The burst 8μF capacitor could be C53, that one certainly could cause severe hum. Nearest modern value would be 10μF, 450V.

[duncanlowe]

Interesting thread. Many years ago, a boss of mine who had previously been in the army school of instruction (?) told me of an event he had when a number of electrolytics popped during a lab he was instructing. Initially he thought it was trainees being a bit careless, but so many went gnnnnn bang that he started to be suspicious the lab technician had been careless when fitting the new parts to the holders. Turned out that RS's suppliers had fitted the sleeves back to front which was obvious when you remember which polarity the can is! He also referred to its as 'the sound of amps' which so many of us will relate to.

[teedeeb]

Thanks a lot for the schematics. I couldn't seem to find them online... this makes things easier.

I'd replace the 0.05uf output transformer drive valve grid coupling capacitor (what a name, got it from one of the pages here on vintage-radio.net!) C38 with one of these from China. They're 5% and 630V and cheap so I guess they'll be fine?

On this site, I read that electrolytic and paper capacitors should always be replaced. There are about 15 paper/electrolytic capacitors inside. Is this useful? Since the large can on top contains 2x32uf electrolytic capacitors, should these be replaced as well?

Aaaaand, I found some sort of 55pF wax capacitor inside as well, inscribed "L.E.M.". Looks pretty unreliable. What would you say about that? See the picture below.

[julie_m]

A capacitor vendor is always going to tell you to replace capacitors. While it is true that they deteriorate, the effects are more noticeable in some circuit positions than others. Also, there is a danger, if you remove many parts at once, that you could mis-wire one of the replacements and introduce a whole new fault into the set that was not there before.

Change just the minimum necessary at first. And then if you are going to replace any more capacitors, do them one at a time and test the set after replacing each one.

Leave that little capacitor alone -- it looks as though it has something to do with the oscillator circuit, and the set almost certainly would require re-alignment if it was changed.

[kalee20]

The large can electrolytic will almost certainly be OK. My experience is, unless the end seal has cracked or is majorly bulging, it's good. If the equipment has not been powered-up for a few years, it would benefit from being awakened slowly (also known as reforming), but otherwise - leave it alone!

[Craig Sawyers]

Originally Posted by teedeeb

Aaaaand, I found some sort of 55pF wax capacitor inside as well, inscribed "L.E.M.". Looks pretty unreliable. What would you say about that?

That is silvered mica. They were often wax dipped back in the day. It will almost certainly be absolutely fine. I'd leave that one well alone.

Craig

[Radio Wrangler]

Paper capacitors are a sort of swiss roll of two strips of foul separated by two strips of impregnated paper. Once water (as vapour) gets in through whatever outer casing or wax dip there is, it is absorbed by the paper, working its way in and the capacitor gets leaker and leakier. Wax isn't a very good long term seal, and so waxed paper capacitors are now a change-on-sight item.

Silvered mica capacitors are small sheets of the mineral mica with silvering applied to them and leads soldered to the silver. Good mica doesn't absorb water, so the slow leak of atmospheric water vapour doesn't do any harm. Silvered mica capacitors, even wax dipped ones seem to be pretty reliable. Also, in radios, these parts are used in places where their value is critical and other components have been adjusted to accommodate the exact capacitance of the individuals fitted. So, replacing one is usually followed by re-alignng the set.

David

[teedeeb]

Thank you all for your time.

I'll be replacing the burst capacitor and coupling capicator for now. I've ordered the parts, so I should get some result in a week or two.
Anyway, I'll let you know the results