[R-390] Changing filters and frequencies

[David Wise]

> From: Scott, Barry (Clyde B) [mailto:cbscott@ingr.com]
> 
> Something I noticed (for the first time) last night: when I 
> switch between filters, the received audio changes frequency. 
>  For example, if I switch from 4kc to 8kc, the 
> dweedle-dweedle pitch changed ten or twenty cps.  Is this 
> normal?  If so, what causes it?  Is it the filters themselves 
> or is this an IF alignment issue?

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