[R-390] R-390 AGC/ Time constant

[2002tii]

Pete wrote:

>In trying to sort an obscure fault in a R-390/URR agc /TC problem in a IF
>module, I've discovered that the 2 mfd TC capacitor appears to be generating
>a 20+ mV charge that won't go away.
>It's  still there  after standing still for  hours and not  affected after
>shorting--- the charge returns.

As others have pointed out, this is due to dielectric absorbtion 
("DA") in the capacitor dielectric (assuming it is not a measurement 
artifact).  In essence, all dielectrics exhibit, to a greater or 
lesser degree, a temporary electret effect.  If you leave the cap 
shorted for a long time -- days to months -- the offset voltage 
should go away or at least be reduced greatly.  20 mV of offset 
should not have much effect on the AGC circuitry, which operates with 
volts on the cap and is inside the AGC feedback loop in any case, if 
it is otherwise still a functioning capacitor of approximately the 
correct value.

Polystyrene and polypropylene capacitors have among the lowest DAs of 
common dielectrics.

Best regards,

Don