capacitor substitution

[fanman1980]

replaceing a micamold type c capacitor in my antique radio and need to know if i can replace a 260 pf capacitor with a 220 pf capacitor voltage is higher on the 220.

[da na]

You can... probably. Where is the capacitor? If it's an AM radio you especially don't want to get values too far off

[fanman1980]

i also have this capacitor i cant find value of

[fanman1980]

also if it works it works it is an am set but the am doesnt work and the capacitors are leaky

 

[fanman1980]

4 minutes ago, Mel0nMan said:

You can... probably. Where is the capacitor? If it's an AM radio you especially don't want to get values too far off

i actually have some 560 pf capacitors as well that if i run in series will be 280 if thats better

[da na]

34 minutes ago, fanman1980 said:

i also have this capacitor i cant find value of

Do you have a multimeter, many can test capacitance. 

[fanman1980]

4 minutes ago, Mel0nMan said:

Do you have a multimeter, many can test capacitance. 

actually after looking on the side of it and the schematic i found it out

[RollyShed]

The first capacitor you should be changing is the audio coupling capacitor which if original will be a waxed paper one. Possibly about 0.1uf. It goes to the grid of the audio output valve (tube).

What make and model is the radio?

[fanman1980]

On 4/2/2022 at 5:23 PM, RollyShed said:

The first capacitor you should be changing is the audio coupling capacitor which if original will be a waxed paper one. Possibly about 0.1uf. It goes to the grid of the audio output valve (tube).

What make and model is the radio?

believe me i replace all capacitors always replace the wax capacitors first as i have many on hand

[H713]

Small caps in the <1 nF range are usually a mica or ceramic. Unless you have good reason to believe they're actually faulty (which is very rare), don't replace them.

 

Mica caps in particular are often used in oscillators, and in many cases are cherry-picked at the factory. Replacing them can introduce a world of pain when trying to re-tune the oscillator. 

 

Those "Domino" Caps are usually mica and seldom fail. Don't replace them. 

 

Aside from in very high voltage (> 5 kV) applications, I've only seen one or two mica caps fail in my entire life. I've seen more than a few oscillators or tuned RF filters that were screwed up because some genius decided to "Recap everything!!!11".

 

If you insist on replacing them, they MUST be replaced by another mica, C0G/NP0 ceramic or polystyrene caps. Anything else is completely unacceptable.

 

Even if it isn't part of an oscillator, you have to be careful about small-value caps, because they're inevitably being used at high frequencies - sometimes really high, so little things can bite you. A few nH here or there, or an extra pF here or there can cause a lot of problems. Violent oscillations at 10 MHz, parasitic oscillation at 250 MHz, ringing, vulnerability to EMI, etc.