[R-390] R-390 PTO calibration

[DJED1@aol.com]

The recent mail on adjusting end points reminded  me of the specifications 
I've got, which give some insight into how the radio should perform:
The original specs (for the R-390 circa August 1950) called for a readout 
accuracy of 500 cycles over the whole tuning range, and 200 cps after 
calibration at the nearest 100 Kc point.  This was astounding for that day, 
and apparently too ambitious.  A later spec, presumably done after they had 
built a few radios, called for 300 cps accuracy after calibration at the 
nearest 100 Kc point.  Interestingly, the end points were allowed to be -1400 
cps or +200 cps at the top of the readout, after setting the bottom at zero.  
Also, a separate tolerance for band change (4 Kc) was included.  So if you're 
getting linearity of a few hundred cps across the entire range, you're doing 
good.  I don't know how well the units with the calibrator stacks hold up.  I 
did adjust one of the Cosmos units to be good to 300 cps over the entire 
range.  I recall I had to allow the end points to be off in order to get no 
more than 300 cps error over all 42 measurements at 25 Kc  spacing.
Ed