[R-390] R-390A frequency stability

[David Olean]

I am working on an R-390A  and have the Cosmos PTO out of the unit for 
repair. The end point was off by 17 kHz! Yes the R-390A is old (52 
years) but I was surprised to see such a variation.  In line with 
Larry's questions, I checked the VFO stability from a cold start and it 
started at 2.455193. In a few minutes it had drifted up to 2.455218 or 
so, then drifted back down to the starting point at 193.  I left it for 
a few hours and it is now at 2.455228. So I have seen a drift of under 
40 Hz from a cold start. The VFO oven is off, BTW. I view this as rather FB!

As for the end point fix, I messed around with the end point coil and 
have the end point at about 1 kHz error, with no room for any more 
adjustment. The Cosmos end point coil is only 3 turns on my unit and I 
have removed two of them! I feel happy to reduce the error from 17 to 1 
but I am wondering if the problem is not within the coils, but possibly 
a capacitor problem.  The linearity screws look like they could use 
adjusting at the high end  (3.155-3.455 MHz)  All the other points are 
within +/- a few hundred Hz.  I am boning up on  the best way to proceed 
with adjustments. For my tests, I have the PTO outboard in a test 
fixture with a vernier dial driving the PTO shaft.  My freq counter is a 
high stability HP type.  This sure is fun!

Dave K1WHS

On 10/30/2019 8:47 AM, Larry H wrote:
> It was finally time to check the frequency stability of the R-390A that I
> am working on and not that it is bad, but was disheartened to find that it
> took 6 hours to stabilize.  I was checking the frequency stability against
> the 5 MH WWV, so was using all 3 converters and found that it had drifted
> about 300 hertz.  Although this is the allowable limit, I decided to see
> what's going on.  What I saw was all 3 oscillators were about equally
> responsible.
>
> The problem with the 1st osc was that the oven was not working.  I replaced
> it with a functional one and within 7 minutes it had stabilized to a 10
> hertz range (controlled by the thermostat).  It was easy to tell that it
> was not working as it was chassis temperature, not the 75 degrees
> centigrade on the label.
>
> Now the 2nd crystal oscillator situation becomes a little more of a
> problem.  It has the same symptoms as the 1st oscillator - as one would
> expect, it took a very long time to stabilize (about 5 hours).  So, I did a
> little test - I disconnected the heater oven from the VFO (the 2nd crystal
> oscillator and the VFO's heater are on the same line) and turned the 'oven'
> switch on the back to on.  Of course, I measured the circuit first to be
> sure there would not be any big surprises.  Well, it seems to be working as
> desired - 10 hertz stability after 7 minutes.  The oven heats up to about
> 75 degrees centigrade and stays there.
>
> I plan to improve the VFO stability with an appropriate NTC cap, as I have
> done this in the past.  I expect to end up with a fairly stable VFO without
> using it's oven (as that is too risky).  I'll let you know how it all turns
> out.
>
> What do you think about turning on the oven for the 2nd crystal oscillator
> deck?
>
> Regards, Larry
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