[R-390] AGC and Stability

[Drew P.]

On Sun, 24 Jun 2012 17:11:40 -0400, Steve wrote:

> I was recently running psk31 and using the r-390a as a
> receiver. I got some garble in the copy and noticed there is
> a slight trace wobble in the waterfall that moves with agc
> action. It is quite stable in MGC. I have changed bfo and
> pto tubes, the VR tube glows. I do use audio derived agc
> that is tapped off the line gain but just a smidgen. It
> still doest it when the line output is off. Where should I
> look next?
 
Hi Steve,

Some time ago, Dave Wise quantified this effect and isolated it to, in his case, varying loading on the oscillators presented by the AGC-controlled mixers. The archived postings will be found in Wei-i Li's "Pearls of Wisdom" at r-390a.net.  If you will read the "Engineering Report" also found at the aforementioned site, you will learn that AGC was applied to the mixers of the R-390a to make its AGC control range more like that of the predecessor R-390, which did not have AGC on the mixers, but instead had a larger number of IF stages, all of them AGC-controlled.

I've wondered if that much AGC authority is really necessary in the R-390a, and if we couldn't just remove AGC from the mixers, live with a slightly less effective AGC, and eliminate the problem of the AGC-induced frequency shift.  It would seem that the audio-derived AGC which you have applied would make up the difference if you were to remove AGC from the mixers, and thereby control just the RF and IF amplifiers.

Drew