[R-390] Re. AGC and Stability

[N4be_Jim]

Both of my 390as do the same thing.  I have tried to isolate the cause but all I can suspect so far is that with varying AGC the current drawn by the AGC controlled tubes varies, causing the load on the B+ to vary.  The B+ varying could pull one of the oscillators slightly.  I know there is a regulator tube in there but it seems like one of the crystal oscillators is not driven with regulated voltage (going from memory here).  Another theory I have is that the AGC applied to one of the mixers, which is also fed from a local osc, could change the operating conditions of the tube, again changing the oscillator slightly as it sees a varying load.  If you are running PSK your best bet is a modern PLL controlled receiver.
Jim M. N4BE

Sent from my iPad

On Jun 25, 2012, at 12:03 PM, r-390-request at mailman.qth.net wrote:

> Send R-390 mailing list submissions to
>    r-390 at mailman.qth.net
> 
> To subscribe or unsubscribe via the World Wide Web, visit
>    http://mailman.qth.net/mailman/listinfo/r-390
> or, via email, send a message with subject or body 'help' to
>    r-390-request at mailman.qth.net
> 
> You can reach the person managing the list at
>    r-390-owner at mailman.qth.net
> 
> When replying, please edit your Subject line so it is more specific
> than "Re: Contents of R-390 digest..."
> 
> 
> Today's Topics:
> 
>   1. AGC and Stability (Steve Hobensack)
> 
> 
> ----------------------------------------------------------------------
> 
> Message: 1
> Date: Sun, 24 Jun 2012 17:11:40 -0400
> From: Steve Hobensack <stevehobensack at hotmail.com>
> Subject: [R-390] AGC and Stability
> To: r390 reflector <r-390 at mailman.qth.net>
> Message-ID: <SNT109-W90EA54593D4D6C498AE3CCAE20 at phx.gbl>
> Content-Type: text/plain; charset="iso-8859-1"
> 
> 
> 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? 
> Thanks
> Steve N8YE                         
> 
> ------------------------------
> 
> _______________________________________________
> R-390 mailing list
> R-390 at mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/r-390
> 
> 
> End of R-390 Digest, Vol 98, Issue 24
> *************************************