Fwd: Re: [R-390] r-390a chirping CW

[JMILLER1706 at cfl.rr.com]

This has taken on an interest to me because I have two 390a's here, one is a Stewart-Warner, the other a Collins (almost).  I notice a very slight "chirp" barely noticable in the SW as AGC changes.  However, it was much more noticable in the Collins until I changed a mixer tube, ...it is lessened, but still noticable, maybe a tad more than the SW now (judging by ear).  My theory was that the AGC was pulling on a mixer tube, which then pulled more on one of the oscillators causing it to "chirp".  Changing the tube reduced it.  I also observed the +250 volt line which drives the xtal oscillators and other stages, and noted a moderate change in its level as AGC changed (a change in AGC would cause all controlled tubes to pull a varying plate load from that line).  The 250V line in both radios seemed to pull the same amount, but the Collins still show a little more "chirp" as AGC changed.  I would guess by ear to be 20-40 Hz.  Is this normal?  Would be interesting to hear from ot

hers... turn your BFO onto a strong, stable signal and run RF Gain up and down. ... you may notice a slight pulling in beat note.  Do others note this also?

----- Original Message -----
From: Forrest Myers <femyers at attglobal.net>
Date: Monday, June 14, 2004 8:04 am
Subject: Re: [R-390] r-390a chirping CW

> Hello James,
> I checked the 150 volt line with my Fluke DMM and it seems to be very
> stable. Will check it more closely with my old analog VTVM and see 
> how it
> looks.
> Thanks for the help.
> 
> Later,
> Forrest