[R-390] AGC operation question

[David Wise]

> From: Steve Goode [mailto:goode@tribeam.com]

snip

> of the AGC switch.  In this radio, if I start at the slow AGC 
>  position and
> switch to the medium position, the radio is muted for about 
> two seconds and

You've already been told this is normal.
Here's why it happens:

In SLOW, C551 is between plate and grid of the
AGC time constant tube and gets a big charge.
In MED, the end of C551 that was on the plate
(the positive end) is moved to ground.  The
negative end stays on the grid, which is also
the AGC line.  This pushes it very negative
(20-200V, depending on signal strength),
effectively muting the receiver until it
discharges.  C551 is 2uF so the time constant
is about 0.8 seconds.  If they had used a switch
with one more pole, they could have reversed
the charge, which instead of muting would give
you a short burst of loud sound, like you get
when switching from MED to SLOW.
Either they didn't think it was worth it or
they didn't think it through.  It's not
mentioned in the R-390 Final Engineering
Report or the Cost Reduction Report.

Or, they could have used two 2uF caps, one for
SLOW, one for MED.  This was out of the question
for cost and space reasons, but hmm, I wonder if
we could do it.  The second cap would only have
to be rated around 25V.  Check the schematics...
nope.  Unless you change S107 from one pole,
three position to two pole, three position.
Then it would be trivial.  Who's going to be
the first on the block to have an R-390A that
doesn't observe the "Moment of silence"?

*   *   *

I thought so.

They were right, weren't they?

:-)

Dave Wise