[BC] exciter problem fix of note

[RichardBJohnson at comcast.net]

ESR sometimes goes up with age, especially if the capacitors are not designed for the job they are doing. Methinks every equipment designer should read what particular kinds of capacitors were designed to do, especially before they are put into broadcast equipment. You just do NOT allow RF currents to circulate in electrolytic capacitors. Because of Lenz's law, RF currents tend to stay on or near the surface of conductors. That is where the electrolyte is in such a capacitor and the resulting heating makes them dry out.

I've heard the argument that the capacitor needs to be in the microfarads to provide the required low impedance for solid-state circuits and so-called RF capacitors do not come in those sizes. The argument is specious. You use two (or more) capacitors, one to handle the high frequencies and the other to handle the low. You choose the values so that the parallel resonance of the high-frequency capacitor and the inductance of the low-frequency one, is well outside any interesting frequencies.

The result being capacitors that last twenty or more years.

Cheers,
Richard B. Johnson
Book: http://www.AbominableFirebug.com/

----- Original Message -----
From: "Michael Patton" <mike at michaelpatton.com>

Yesterday I was going through a BE FX-30 on my bench and noted spurious 
emissions at +/- 30 KHz and +/- 60 KHz, about -50 dBc.  Checking the 
output of the mod osc by itself, I found no spurs.  Hey, I know this 
one: the electrolytic P/S bypass caps in the PA are dead.  I 
disassembled the PA module and changed the caps, using 105° caps instead 
of the original 85° caps (shame on you, BE), and hey, presto! no more spurs.