[R-390] AGC Cap

[David Wise]

> From: Gord Hayward [mailto:ghayward at uoguelph.ca]
> 
> I wouldn't use an electrolytic.  At the best the ones I've 

What Gord said.  Even a new electrolytic will leak about as
much as your bad old oil/paper cap.  The new cap *must*
be "no"-leakage, for example any of the plastic film varieties.
I suppose ceramic would be up to the job, but I've never seen
one at 2uF/400V.  It would be quite a lump!

I replaced mine with a small AC motor cap, again with
a plastic dielectric.  It fitted easily in the
original can after gutting.  That's not a pleasant job,
but since it wouldn't fit under the chassis it was
my only choice.

When cruising, the cap won't see more than about 200V,
but if you have a SS power supply there will be more during
warmup but not more than about 350 for sure.  Since cap life
depends more on cruising voltage than transients, even a
400V cap will last for generations.

On a related note, I breadboarded my anti-moment-of-silence mod,
and it looks good.  As expected, 0.15uF is the optimum value
if you like the old time constant.  0.1 and 0.22 are perceptibly
off.  Next week I'll do quantitative measurements.  I'll modulate
the signal generator with a slow square wave, and watch the AGC
line on a scope.

73,
Dave Wise (SWL in Portland OR)