[R-390] The dreaded C-553

[Charles P. Steinmetz]

Jim wrote:

>in mine it [C553] is a ceramic disc, and the parts list shows it as 
>a paper type.  Does that mean mine has already been 
>replaced?  Should I replace it?

I have never seen a 390A with a ceramic C553 from the factory, so I 
suspect it has been changed.  And yes, I recommend changing 
it.  Ceramics do not make good coupling capacitors for at least two 
reasons: (1) they have higher leakage current than film caps, and (2) 
their high dielectric absorption and voltage coefficient of 
capacitance can create significant distortion.  Neither of these is 
desirable when you are trying to protect your mechanical filters and 
recover undistorted audio.

Of the available choices, the Sprague/Vishay Orange Drop series 225P 
polyester dielectric (or the 715P or 716P polypropylene dielectric) 
film-and-foil 600 volt caps are the caps of choice for C553.  Because 
of their "self healing" feature, metallized film caps are not a good choice.

Best regards,

Charles