[R-390] FM'ing with solid-stated PS

[Jim Miller]

I di get about 50 Hz FM with BFO turned on when the AGC increases/decreases, 
or on a strong carrier when I turn the RF gain up and down.  This is with 
audio set very low.  I believe it is caused by AGC changing current drain on 
some of the tubes in the RF/IF stages which pulls the B+ as the plates draw 
more r less current.  But in particular I find that the AGC control of the 
1st mixer tube V207 actually pulls the 17 Mhz crystal oscillator V207.  I 
put frequency measuring equipment on all oscillators as an experiment 
andmeasured the frequency chage of every oscillator as a function of AGC 
voltage.  V207 appeared to be the only one that was pulled off significantly 
enough to be audible in the BFO beat note.  Evidentally the AGC control on 
V202 causes some change in impedance or current draw that is "seen" by the 
V207 oscillator and causes the crystal to be pulled slightly.  I have seen 
this on 2 radios.  In one case I was able to reduce the pulling slightly by 
changing V202 (6C4) for a newer tube.  Jim N4BE

----- Original Message ----- 
From: "Tim Shoppa" <tshoppa at wmata.com>
To: <r-390 at mailman.qth.net>
Sent: Wednesday, July 13, 2005 9:13 AM
Subject: [R-390] FM'ing with solid-stated PS


I've heard testimony (I think it was first mentioned in an Electric Radio
from a decade ago but I've seen it repeated here and elsewhere)
that solid-stating the power supply, with a
200-ohm series resistor, can cause FM'ing of various receiver-internal
frequencies as the audio output goes up and down with strong signals
and modulates the B+.

My cursory inspection of the 26Z5W curves (see e.g.

 http://www.mif.pg.gda.pl/homepages/frank/sheets/087/2/26Z5W.pdf

) lead me to believe that a 26Z5W has an effective dynamic impedance
of 200 ohms.  (in particular for 100mA of load current the output drops
20V.)

So my gut feeling is that if there is any FM'ing due to a solid-state-with-
resistor PS mod, it's pretty much the same FM'ing that a PS with real
26Z5W's has.  All my PS's were solid-stated before I ever got them, and
I don't have any 26Z5W's in any event.

Did I make a mistake in my math?  By my reckoning this is a half-wave bridge
rectifier so at any moment only one rectifier is conducting, and there's no 
need
to double or half the 200 ohm impedance.  Are the tube curves not typical 
but
instead worst-case?  I read them as typical.

Tim.
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