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

[Graham Baxter]

Hi

I have certainly learnt something. I had not recognised the very neat
use of the suppressor grids as AGC clamp diodes.

Graham

On 29/08/2007, Leigh Sedgwick <bipi at comcast.net> wrote:
> 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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