Re-capping a Crosley 628-B

[dlinford]

Hello everyone!

I am new to the forum. I've been an electronics hobbyist for years and have a degree in physics, but have only recently turned to vintage electronics. I restored an HP signal generator and now I'm working on a radio -- a Crosley 628-B. I plugged it into my dim bulb tester, used a scrap piece of wire as an antenna, and confirmed that it works! I can pick up two of my local radio stations.

In my next step, I'd like to recap the radio so that the electrolytic and paper capacitors don't go bad and damage other components. I have a few questions about doing this.

First, I know that the electrolytic capacitors are polarized. Can they be replaced with non-polarized capacitors?

Second, I've seen some capacitors being sold that have similar specs as the ones I need for replacing the paper capacitors, but they are "film capacitors". I'm not familiar with film capacitors. Can they be used to replace paper capacitors?

[Radio Wrangler]

Hello and welcome to the forum!

You've committed one common sin, but it looks like you've got away with it, for the time being at least. Paper capacitors use waxy or oily paper as their dielectric. Their sealing is never perfect and water gets in as humidity from the air. The dielectric insulating properties degrade and they start to leak DC current. In many places, this is not a terrible problem, but there is one very common place where they are used where the damage they can cause is serious. It is common for any set with an audio output stage for driving a loudspeaker to have a power type of valve, driven by a lower power audio amplifier type to get the required amount of gain. Coupling between these two stages is usually via a blocking capacitor. One end of this capacitor goes to the control grid of the power output valve with a high value resistor to chassis from that grid. This sets the mean voltage on the grid to zero volts. A resistor in the power output valve's cathode to chassis allows it to self-bias.

The other end of the capacitor driving theat control grid comes from the anode of the previous stage, and has a couple of hundred volts DC bias on it.

Any leakage in this capacitor drives the power valve control grid positive. A lot too far positive, and the output valve is driven to pass a lot too much current. This doesn't do the valve any good and it can use up its remaining life expectancy a lot faster than you want. Worse still, the heavy anode current in the power valve can burn out the primary winding of the output transformer and it's not unknown for power supply transformers to be destroyed.

Replacements for the DC blocking capacitor are easily available and cost pennies. The things they can damage are a lot more expensive and can be unobtainable. You may need to find a specialist to re-wind transformers.

So the optimum game plan on acquiring a new set is to find and test for leakage through the audio stage grid blocking capacitor. Most people just replace it and don't bother testing. With the age of classic sets, nearly all the paper capacitors are getting marginal.

Ah, you've restored an HP sig gen. I wonder which? In a previous life I worked designing a number of HP products, including a couple of areas of sig gens.

Oh, that destructive capacitor is such a common trouble maker, that hereabouts it just gets called "That capacitor" often with a choice adjective attached.

Electrolytic capacitors are indeed polarised. Non-polarised ones can be used to replace them in most circumstances. There are a couple of obscure gotchas. Electrolytic capacitors use a conductive gel as one electrode (Aluminium oxide is the dielectric) and the process which makes them is reversible and the oxide can degrade. The conductive gel is resistive and can act to dampen resonances in power distribution wiring. Sometimes fitting a lower loss non-polar capacitor can stir some trouble. It's not common, but it can catch people out.

Film capacitors just mean that the dielectric is a plastic film. Several different plastic families are available, with different properties. Electrodes may be foils wound up with strips of dielectric film, or the electrodes can be vapour deposited onto the surface of the film. At RF, polystyrene film works well, but other plastics are best used for less critical applications.

When I get round to rebuilding my RCA AR88, I have film capacitors ready to go into the big smoothing capacitor block.

Cheap film capacitors tend to be Mylar film (Brand name for mechanically-stressed PET plastic) They are fine for most applications except RF and the integrator capacitors of DVMs ADCs etc. Charge one up. Discharge it right to 0v. Remove the discharge load and watch it partially charge itself back up! It's called dielectric absorption.

Hope that's enough to get you going.

David