[R-390] R-390A frequency drift stability specs

[Larry H]

I read the stability specs in the Y2KR3 tech ref on the bottom of page 5 in chapter 1 some time ago, but there's more to the spec than that.  I found the original spec on the 
r-390a.net (thanks to Al Tirevold and Tom Marcotte) and read through it (link follows): 
http://www.r-390a.net/MIL-R-13947B.pdf  

Here's some of what I found for frequency stability:

VFO
   Temperature:
       300 cycles as the temperature is changed from +104°F to +140°F
       70  cycles as the temperature is changed from +86°F  to +176°F
   Time:
       With ambient temperature held constant: 
       not to exceed 10 cycles per day (for 10 days starting 5 days after temperature               stability)
Overall
   With ovens on, shall be within 300 cycles over the temperature range of -40°F to +149°F,     as the temperature is varied in steps of -40°F to -4°F to +32°F to +68°F to +104°F to       +149°F. 

Line voltage

   Not more than 30 cps, with a change in line voltage from 105 to 125 volts.


That was most of the frequency stability related info that would be of interest to us.  But 2 important items were not covered there, and are: 1. BFO and 2. Overall with ovens 'off'.
  1. From experience, most of the BFO's I have seen these days have a large warm up drift issue.  After .5 hour they've drifted +/- 200 cycles, for the next 2 hours +/- 1 or 2 cycles every 15 minutes, and then +/- 1 or 2 cycles every hour until 6 hours, when they hold at about +/- 1 cycle every hour.  I'm sure when these were new, the drift was less as age seems to change the temperature coefficients.  This can usually be corrected with new NTC caps, but a very stable receiver or counter must be used to measure it.
2. With the ovens off, the VFO is much more stable as temperature fluctuations are greatly reduced in our environments. Using the above VFO 'time' spec of '10 cycles per day', that divides out to be .4 cycle per hour.  As with the BFO, that's with new components that have the correct temperature coefficient.  

As for the line voltage effect, I've found it to be less than the spec on todays 390s, I measured a few and found it to be less than 1 cycle drift for voltage between 110 and 120 volts.  The 4 places I've lived at in the last 20 years have had fairly stable power, so this is not a big issue for me.  

I have improved the warm up drift of the VFO and BFO in my R-390A daily driver so that I can copy SSB after it's on for a few minutes with very little drift.  Of course it's much better after a half hour.

Regards, Larry
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