[R-390] Question on COSMOS PTO

[Kenneth G. Gordon]

> Hello to the Group,
> 
> I'm continuing my saga of just zero beating against the calibrator
> mark and have notice the following observations which I think are
> cause to be concerned.
> 
> 1. Zero beating against the calibration 100Khz signal at x.000 after
> using the zero adjust knob to be exactly at x.000, I noticed that if I
> either dial up or down from this calibration spot, coming back to this
> mark, I'm always either +/-100Hz off depending on the direction I'm
> turning. Is this what you call backlash or hysterisis?

Yes, most probably. We call it back-lash.

> 
> 2. Doing the same experiment as described above only this time using a
> counter and taking note of the frequency, I observed that starting
> again at 3.455000Mhz with the dial set to x.000 and dialing up or down
> the frequency can be off as much as 80Hz i.e. 3.45508 in returning
> again to x.000. I also noticed that by pressing on the KHz knob with
> just pressure from my hand I can cause the frequency to be off again
> as much as 80Hz. And since the 390a is on its back I can touch the PTO
> and cause 10s of Hz of be off.
> 
> This is my 1st 390a and I don't have any experience in the feel for
> calibration. Am I approaching the design limit being off +/- 100Hz on
> the calibration mark? It seems to me something is either up with the
> PTO or the mechanical linkage to the PTO. The oldham coupler is
> aligned as best I can with a new backlash spring and I'm also keeping
> to the .032 spacing as best I can. The ground strap is present and
> looks good against the shaft. I'm really kinda of frustrated at this
> point and any help would greatly be appreciated.

Jon:

Are you joking, or are you serious? I can't believe you're serious... 

+/- 100 Hz is, IMHO, infinitesimal and not worth worrying about.  As far 
as I'm concerned, any practical receiver that holds its calibration within 
100 Hz is an absolute marvel.  These receivers were never meant to be 
laboratory grade frequency meters: they are radio receivers, designed 
to help a hummin' bean hear radio signals that are very seldom stable 
enough to be within 100 Hz.

Ken Gordon W7EKB