[R-390] Cap Leakage

[Leland Bahr]

Nice post, Bob!  I enjoyed reading it a lot.
Lee, w0vt

>
> Hi
>
> An ideal capacitor would pass no current at DC once it had been 
> charged. In fact it passes no DC current at all, it just stores it up. 
> That's another issue though.
>
> Back to insulation resistance ....
>
> So we put a DC voltage on a real capacitor and *suprise* it has leakage 
> current. It passes some current at DC even though an ideal capacitor 
> should not. This is simply because we can't make an ideal part.
>
> In order to decide just how good a real capacitor is we need to have a 
> measure of how much it leaks. The leakage behaves like a resistor the 
> easy way to describe it is as a resistor across a an ideal capacitor. 
> The bigger this resistor the better the capacitor.
>
> So far so good ...
>
> Unfortunately there are two ways to describe a resistor. One is in ohms 
> (zero is bad in this case) the other is in 1/ohms (conductance or 
> Siemens). In the case of conductance zero is good. You have to be 
> careful to be sure which your machine is calibrated in ...
>
> Just to make things even more complex ....
>
> The specification on capacitors is normally written in terms of megaohm 
> microfarads. The bigger the capacitor the more it leaks. The more it 
> leaks the lower the insulation resistance.
>
> The capacitors in question all should have insulation resistances in 
> the hundreds of megaohms if they are working right. Anything below 
> about 50 megaohms is likely to be a defective part. This of course only 
> applies to the parts that are not electrolytic.
>
> The electrolytic parts often are rated in leakage current. In an R-390 
> anything below a few miliamps is fine. A miliamp at 300 volts gives you 
> a third of a watt in heat in the capacitor. At ten mils you are up to 
> three watts and the capacitor can get a bit warm.
>
> The problem with leakage measurements is that they do not totally 
> represent the situation with the capacitors. On the black beauty caps 
> the leakage does not entirely behave like a resistance. It goes up and 
> down in bursts. The fancy term for this is popcorn noise. When you get 
> a burst of noise (or current) the voltage on the capacitor drops fairly 
> quickly in a typical R-390 circuit. This gives you an intermittent snap 
> crackle pop noise in the background on an otherwise quiet signal. Since 
> atmospheric noise can do the same thing it may take a while to figure 
> out what's going on.
>
> None of this is to suggest that you should replace otherwise good 
> capacitors. The issue is that bad capacitors can, but don't always do 
> cause real problems in a radio.
>
> About the only other point to make is that you do not want to replace 
> the paper or plastic insulated capacitors with electrolytic capacitors. 
> The leakage levels on all of the electrolytics are *much* higher than 
> the leakage of the plastic or paper parts.
>
> Hope that helps some.
>
> 	Take Care!
>
> 		Bob Camp
> 		KB8TQ
>
>