Replacing Capactiors

[bostonman]

I have a question about replacing old capacitors.

The common practice is to replace electrolytic capacitors in old electronics. What about ceramic and mica (the round brown colored ones that range in the pF range)?

Do these type capacitors not degrade over time or do people avoid replacing them because they are not worth the trouble since many are often used?

[wraper]

What about ceramic and mica (the round brown colored ones that range in the pF range)?

Do these type capacitors not degrade over time or do people avoid replacing them because they are not worth the trouble since many are often used?

There are some types of those which tend to go bad (often due to moisture) but generally you should not touch them. Also you may screw up tuning in radios if you start blindly replacing those. Also if you start replacing such capacitors, don't forget that carbon resistors are more likely to significantly change resistance. At that point you might replace just everything.

[bostonman]

Good point, I wasn't thinking about them being used as tuning.

Also, I was thinking more about ones used next to ICs on the Vcc line.

[SeanB]

Only non electrolytics that are replace on sight are those wax encased paper types, as the wax crazes with time, and the capacitor picks up moisture in the paper. Other types are certain mica capacitors, specifically the very early micamold types, which have a dot code to indicate the value. Other than that, most capacitors are quite reliable, aside from tiny little glass and dipped ceramic capacitors from the 1980's to around 2000, which have a higher failure rate.

[TimFox]

In general, the only paper capacitors in old equipment that may not need replacement are hermetically sealed (metal enclosure with glass seals).  Based on advice, I pulled all the plastic-cased (color-striped) paper capacitors from a Tektronix 130 L,C meter and replaced each with a modern axial polypropylene capacitor to good result.  Each of the pulled units showed a very low Q value.
In old tube radios, the paper screen bypass caps will often have high leakage that pulls down the screen voltage on IF stages ruining the gain.

[David Hess]

Based on advice, I pulled all the plastic-cased (color-striped) paper capacitors from a Tektronix 130 L,C meter and replaced each with a modern axial polypropylene capacitor to good result.  Each of the pulled units showed a very low Q value.
In old tube radios, the paper screen bypass caps will often have high leakage that pulls down the screen voltage on IF stages ruining the gain.

Those are the same as bumble bee or black beauty oil filled paper capacitors.  They tend to develop high leakage and should be replaced with suitable film capacitors.

Some old epoxy dipped tantalum capacitors can develop shorts.  If one fails then all of the similar ones should be replaced with properly derated parts.

And of course there are those Rifa paper safety capacitors which should be replaced.

[Jwillis]

Also consider that Film capacitors have a "polarity so to speak" .Theirs a right and wrong way they are put in circuit. Mr Carlson's Lab explains this better than I could, The marking does not always show this accurately.