[R-390] R-390 AGC Question, SLOW MED FAST

Dan Merz mdmerz at verizon.net
Fri Apr 7 01:09:47 EDT 2006


Tom,  I assume you're dealing with 390 not 390a.  There's C545 (0.1ufd)
still going from agc line to ground when fast is switched in,  shown in
diagram I'm looking at.  If this is bad (open) then on the fast position
there would be no smoothing of agc because I think this is the only
smoothing capacitor to ground in that position.  When switching to med or
slow the  C546 and C547 caps, each 1 ufd,  would be to ground or to the B+
and agc would be operative if they are good.  My 390 has about the same
level of  agc response on fast and med on a strong am signal but different
response times, as expected if it's working ok.  The "no-cap" is actually
one cap,  C545, I think.  I didn't track the circuit all the way to the
controlled tubes on the agc line.  I see the 390a has two of these 0.1 caps
to ground on the agc line.  Dan.  

-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net]
On Behalf Of Tom M.
Sent: Thursday, April 06, 2006 6:12 AM
To: r-390 at mailman.qth.net
Subject: [R-390] R-390 AGC Question, SLOW MED FAST

My 390 IF deck seems to have an AGC problem in that it has low AGC action on
FAST (when no timing capacitors are engaged) but good AGC on MED and SLOW
(when the timing caps are engaged).

Anyone know what could cause this?  Seems counter intuative.  The old caps
are good, the no-cap is bad?


Thanks
Tom

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