This build-it-yourself Heathkit GR-91 is like my first receiver.  It was a very basic, 4
tube radio, and it had a poor power supply which caused annoying AC hum in the
audio.  The GR-91, like most cheaper radios from that era, had Slide-Rule tuning where
the white dial indicator slides left or right under the glass with the frequency markers.
 Example- Above, the GR-91 shows to be on Band-D and it reads halfway between 19
and 20 megacycles. You might assume that you are tuned to 19.5 megacycles but
electrically the actual frequency could be OFF by PLUS or MINUS several
100KC(kilocycles).  If you could tune in radio station
WWV then you'd be on 20MC.  
Otherwise you'd need to use a
1 megacycle crystal calibrator to determine either 19 or
20 megacycles
and then another calibrator for finer frequency resolution.  Back then,
most radio enthusiasts had to live with this kind of inaccurate tuning.  The best
operating strategy was to actively tune back and forth, slowly, across the frequency of
interest.  Tuning the GR-91 to a precise frequency was almost a black art, even if you
had a crystal calibrator.  By comparison, you could simply crank in the desired
frequency if you were fortunate enough to have the Collins R-390 or
51S-1 receiver.