[R-390] Changing filters and frequencies

[Scott, Barry (Clyde B)]

I don't think it's mechanical.  It was pretty much repeatable and the chassis is pretty strong with respect to the force it takes to change the bw.

Yes, it's an R390A.  If I were going to a very narrow filter, the B+ theory might be plausible, but I wouldn't think going from 4 to 8 or 8 to 16 would make that much difference AGC-wise.  Don't know, though.  I can do some checking on the AGC and other voltages when switching.  Perhaps I'll see something there.

Thanks,

Barry(III) - N4BUQ

<snip>
>  
>  It's not the filters, they can't change the frequency of
>  the signal.  That small a change, it's more likely that
>  when you turned the BW switch you flexed the IF chassis
>  which shifted the BFO slightly.
>  
>  Hmm.  Another possibility occurs to me, but it's pretty
>  far-fetched.  If your choice of filters affects signal
>  strength, it will change the AGC voltage which will
>  (a) cause a tiny change in the mixers' input capacitance
>  and (b) change several tubes' plate current which will
>  make B+ bob around (provided you're on an R-390A not an
>  R-390), and if there's trouble in a xtal oscillator or
>  the VFO it might manifest as increased B+ sensitivity.
>  I told you it was far-fetched :-)
>  
>  Regards,
>  Dave Wise