[R-390] Call for measurements - 100dB carrier reading

[Leigh Sedgwick]

Hi David,

I disagree with one earlier comment kinda ripping this AGC discussion.  I 
find this much more interesting than many of the topics that appear through 
the R390 threads....however, I do have a rather simplifying question:  Why 
not just use a 10-turn precision pot to solve the carrier meter zero issue? 
I made that change and it is now easy has heck to zero and the adjustment 
has not moved since.

Keep up the great tech discussions...I learn a lot from them all!

73 de Mike
K7PI
Mercer Island, WA
www.k7pi.com

----- Original Message ----- 
From: "David Wise" <David_Wise at Phoenix.com>
To: "Graham Baxter" <graham at delphe.co.uk>
Cc: "r-390 at mailman.qth.net" <R-390 at mailman.qth.net>
Sent: Tuesday, August 28, 2007 10:08 AM
Subject: RE: [R-390] Call for measurements - 100dB carrier reading


(There's a question for all of you at the bottom.)

Thanks for all the info, Graham.  Except for R504, all my resistors
are within tolerance.  I was able to duplicate your AGC voltage
by substituting tubes.  It appears that my gain-controlled
IF signal chain (V501, V502, and V503) has a wider cutoff
characteristic than yours.  (For the record, they are JAN Sylvania
5749W's.)  I substituted 6BA6's that have sharper cutoff
and about the same gain.  This, plus a new 6AK6 (so my IF gain
is normalized lower) got my 100mV AGC down from 14-somthing to
12-something.  My AGC amp, V508, has unusually high gain.  After
replacing it with my weakest spare, and putting in my sharpest 6DC6,
I got -11.8V .  (I also tried substituting mixers; they affect the
gain curve very little.)  So there you have it - it was just normal
variation.

Quick recap of factors that can contribute to low AGC when the
circuit tests okay and the IF gain is normal.

1. Low RF deck gain: weak 6DC6, 6C4's, or 6AK5's, or misalignment.
   There's less output than normal which results in less AGC.
2. Sharper than normal cutoff 6DC6 or V501-V503 6BA6.
   A given AGC is unusually effective in reducing the gain.
3. Weak V508 6BA6 AGC amp.
   For a given output, less AGC is developed.

Diagnostic tip: for a given 6AK6 and AGC voltage,
unusually high diode load voltage points to a weak V508.

The manual's procedure for setting the IF gain depends on
uniform 6AK6 gain.  Since 6AK6's are not all alike, every
deck will in fact be different, as far as the gain control
loop is concerned.  If the goal is to achieve uniform AGC
action, it would be better to normalize the IF OUTPUT level.
(No doubt this was not done because (a) it would require more
test equipment, and (b) it wasn't worth the effort.)
But this doens't help; since 6BA6's vary in their cutoff
characteristic and (V508) gain, every deck would be different
even if the 6AK6's weren't.  The R-390x is not a voltmeter;
we should not expect every set to read the same.  The
carrier meter is an approximation only.

With that in mind, how many of you would adopt an extremely
simple (no module removal) carrier zero mod that, in the worst
case, drops the reading by 3dB?  Doesn't change the gain or S/N,
just the meter.

How many would adopt a more complicated mod (change a couple
resistors under the IF deck) that doesn't drop the meter ever,
and in addition makes nonstandard substitute meters read right?

Regards,
Dave Wise

> -----Original Message-----
> From: grahambaxterdelphe at googlemail.com
> [mailto:grahambaxterdelphe at googlemail.com]On Behalf Of Graham Baxter
> Sent: Saturday, August 25, 2007 4:47 AM
> To: David Wise
> Cc: r-390 at mailman.qth.net
> Subject: Re: [R-390] Call for measurements - 100dB carrier reading
>
>
> Hi Dave,
>
> Just to clarify, all my openloop AGC voltages were negative. I omitted
> the - sign on the first two measurements.
>
> OK, I did put the covers on. I swapped radios; I am now on as close to
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