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

[Tim Shoppa]

VK3IZ writes:
>  G'day...when does a capacitor  become a battery?
> 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.
> Anyone else seen this?-Other 2 mfd caps don't show this effect.

It's called "soakage":

  http://www.national.com/rap/Application/0,1570,28,00.html 

I'm not sure 20mV itself is the cause of a problem in the AGC system, but the AGC system is an integrator and soakage means dielectric absorption can mess up integrators more than just their voltage offset.

20mV is pretty small for soakage. I'm used to old big oil filled capacitors which would "bite" (must be > 100V) after sitting on the shelf :-).

Tim N3QE