[R-390] 17 MHz osc pull from AGC

[roy.morgan at nist.gov]

.. the 17 Mhz xtal
> oscillator in the first mixer is pulling just under a hundred cps or so
> in step with the AGC voltage.

Conard,

Are you sure it is the crystal pulling, or the PTO?

> ... CW on the bottom 8 bands is chirpy when using the AGC.

Likely it's the crystal oscillator, then.
 
>... The 150 volt line is dropping about a volt between AVC and MVC
> mode with a strong signal applied. I don't think that is enough to cause
> this problem .... is it?

It might be.. have you tried a different (new) VR tube?  That would be my next
step.  (I can't say what variation is expected on the regulated B+ line
normally - do you have another receiver to test?)  See if the VR tube is
starved by a drifted-high dropping resistor. (I can't remember if that is a
wire wound unit or a carbon one.)

Some speculations:
1)You may have leaky or resistive capacitor(s) in the 17 mc oscillator section.
Are tube voltages and resistances normal in that circuit?  For instance, a
leaky screen bypass cap can bring the screen (or plate) voltage down so the
oscillator is running weakly, or at very much changed voltages and currents.
This can get the thing into a state where slight, normal, changes in voltages
(due to the AGC action) cause frequency drift.

2) The 17 mc crystal, or it's oven, may be the trouble.  Try another oven if you
have one.  The oven should be cycling after warmup.  If it's thermostat is
stuck ON the crystal will be baking at a much-too-high temperature and be far
away from it's stable region.  Same if the heater or thermostat is open.

I look forward to hearing what you find. You are right, the receiver should not
act this way.  A hundred cycles' wander in detected tones would wreak havoc
with an RTTY comm channel.

Roy

 
Roy Morgan
7130 Panorama Dr.
Derwood MD 20855