Capacitors: What, Why, How?

What is a capacitor? Why are capacitors used? How does a capacitor work? How to replace capacitors? What are the different types of capacitors? What are different types of capacitors used for? The symbol for a capacitor is two parallel lines. These represent metal conductors. One where electricity goes in. And one where electricity goes out.

The gap represents the insulating space between the conductors. This can be air, paper, plastic or an electrolyte. Electrical charges form in this space and it’s the key to how capacitors do their magic.

Unlike a resistor, which loses voltage instantly when turned off, a capacitor holds its charge even when power is removed, and then slowly releases it.

When a signal enters a capacitor, it is altered because the capacitor holds on to its charge and releases it at a different rate. This allows capacitors to smooth a pulsing signal, and that’s just what C23 and 24 are for.

As voltage leaves our rectifier tube, it’s pulsing because half of the AC cycle has been removed. If the pulsing voltage is introduced into the radio’s signal, it will be heard as a loud hum.

By smoothing the pulses, C23 and 24 filter the hum, so we hear just the radio signal.

Because power filter capacitors need to hold a large charge, electrolytic capacitors are used.

Electrolytics use a paper insulator impregnated with an electrolyte. The electrolyte allows electrolytics to hold a greater charge in a smaller space than other types of capacitors. The conductors in an electrolytic are two pieces of foil rolled around the insulator. An electrolyte separates into positive and negative molecules when charged, so one conductor becomes negative, and the other becomes positive.

Electrolytic capacitors fail over time because the electrolyte eventually dries out. That’s why most un-repaired antique radios have a horrible hum when powered up.

The original C23 and 24 electrolytics are housed in this cylinder which is actually two capacitors in one. Here’s the old capacitor next to the new ones I replaced it with. Capacitors have gotten both smaller and more reliable over the years, but even these new ones will fail someday.

Here’s one of the new capacitors getting a lead soldered to it. And here are the two newly installed capacitors. Electrolytics are polarized, so it’s important to install them with the correct orientation.

To pass through a capacitor, a signal must have a constantly varying AC or pulsed voltage. If voltage is DC and stays steady, the first conductor will charge, but never discharge to the second.

Passing AC while blocking DC is an important characteristic of capacitors. Capacitors C15, 19 and 20 do just that in their jobs as coupling capacitors. Coupling capacitors join the output of one amplification stage, with the input of another. C20, for example, allows the signal to pass from the 6Q7 tube, to the 25L6, while blocking DC voltage that would harm the circuit.

As is the case with most of the capacitors in our old radio, C20 is a wax/paper tubular type.

Wax/paper caps are almost always in some state of failure in antique radios. Unlike electrolytics which fail when they dry out, wax/paper caps fail because they absorb moisture which breaks down the paper dielectric. The waxy coating helps slow this process, but over time the paper breaks down and the conductors short.

Here I’m testing one of the old wax/paper caps. It’s supposed to measure .1 microfarad, but is coming in at a whopping .375. Farads are a unit of measurement for how much charge a capacitor can hold.

And here I’m testing the new replacement capacitor which is measuring correctly at about .1 microfarad.

When I first started restoring antique radios I ordered capacitors as I needed them. After a while I realized it would be easier if I stocked capacitors in my workshop. Now I own bags of them like these. You can also see on the right that I drew diagrams of the capacitors in their correct positions before I removed them. Taking notes and photos helps make sure that new parts are installed correctly.

#radiorepair #radio #electronicsrepair #restoration #capacitor

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