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

[Bill Hawkins]

What are you using to measure this voltage?

If you have a 100 megohm resistor, try measuring the millivolts
across it. It could be the bias current of the DVM's amplifier.
It takes .0002 microamps to develop 20 mv, and that's not
unreasonable for an inexpensive meter.

If it were just the cap, shorting it would reduce the voltage.

And yet, you say that other caps don't do it. When the cap is
standing still, is it standing alone - not in the circuit?

OK, if a 100 or 10 meg resistor across the cap doesn't lower the
voltage, then you have some sort of thermocouple effect. See if
refrigerating it changes anything.

Bill Hawkins


-----Original Message-----
From: Pete Williams
Sent: Tuesday, December 30, 2008 3:08 PM

  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.
Changing to see if the problem in the radio is still there yet to be
done.

Comments please ?
Pete VK3IZ.