Replacing paper capacitors

[tryppyr]

I feel certain this has been asked before, but I didn't find it after several searches.

I am about to start working on some old radios with paper capacitors. Obviously, the first thing to do will be replace the paper capacitors with comparable ceramics, micas and electrolitics. Since electrolitic capacitors are aligned, I am wondering how you determine the correct alignment when installing them.

 

[KrisBlueNZ]

Electrolytics are polarised. It's unlikely that you will be replacing a non-polarised capacitor with an electrolytic, because their value ranges don't overlap much. So you would normally replace a non-polarised capacitor with a non-polarised capacitor, and a polarised capacitor with a polarised capacitor.

If the original capacitor is an electrolytic, its polarity will be marked, so you know how to connect the replacement electrolytic. If the original is non-polarised, you would normally replace it with a non-polarised capacitor, and polarity doesn't matter.

If for some reason you want to replace a non-polarised capacitor with an electrolytic, you need to be sure that voltage only ever appears one way round across it. This will usually be true, especially in old radios which usually have pretty basic design. You may be able to tell the right polarity by figuring out the circuit function. For example if one side is grounded and the other side is a positive supply rail, the right polarity is obvious. Actually, if one side is grounded, it will normally be the negative side. Also you could try powering up the radio with the old part in there and measuring the voltage - carefully! But it's unlikely that you'll want to do this.

 

[duke37]

There has been much discussion on this topic on vintage-radio.net/forum/

Paper capacitors would be replaced by polyester or polypropylene capacitors, just make sure the voltage rating is adequate.

New capacitors will have different preferred values from the old ones. Just chose the nearest one unless it is in a tuned circuit. You should not need to replace ceramics or mica caps.

 

[shrtrnd]

What duke37 said. I had a similar issue question last April. I used metallized polyester replacements and they worked fine. 1970's era equipment.

 

[BR-549]

Depending on what you have there, you might have a negative supply for tube bias. The electrolytics in this circuit will be reversed from the norm.