MastHead

IC-R75, 2LO vs Display freq.


I measured frequency drift of IC-R75 receiver and reported on the page of IC-R75 Modifications.  The graph was this:

Now, this graph does not tell anything about real drift of the receiver.  You may be frustrated.
I calculated the relationship between
   - Reception band frequency (broadcasting
      station's freq.),
   - Reference freq. (2LO), and
   - Display freq, (dial freq.).

Assumptions are:
  - Local oscillator has been calibrated at zero drift.
  - The target station is at Z Hz and stable.
  - Mode is SSB, and objective is to have zero-beat
     to the original carrier.
  - 2LO drifts by deltaX Hz
  - You must re-dial by deltaY Hz on the display to
     compensate.

After long battle with the circuit diagram, I made the equation between Z, deltaX and deltaY, which is:

      deltaY = - Z * { deltaX / ( 60000000 + deltaX ) }         [Hz]


We got the equation, now, let's convert it into a graph.
This is what you wanted.





I challenged further frequency stabilization. Visit here ==> CR282-Stabilize.htm if you are interested. Sorry, it is in Japanese, but Google interpreter may give some help.


[Note for L2 tuning]
If you do not own an 8 digit frequency counter, use this procedure to adjust the L2 for exact 60MHz.
  1. In stable spring or fall season room temperature, leave the R75 powered-on for more than 2 hours.
  2. Display Hz digit.
  3. Change CW tone to 300Hz.
  4. Receive 15MHz WWV/WWVH.  If not possible, use 10MHz WWV.
  5. Dial to 15.000.000 (or 10.000.000).  Press key for CW.
  6. Carefully listen to the tone. Press CW key for 1 sec to change to REV. Repeat several times and find the tone pitch difference.
  7. Shift the frequency by 1 Hz up or down, then repeat the tone test.  Find the exact frequency where there is no difference between REV and Normal.  Memo the display frequency. You may not be able to distinguish 1Hz difference, but 2Hz difference must be noticeable.
  8. If the number is plus side, say 15.000.005, turn the L2 core clockwise.  Do not use metal driver, use plastic one instead.  If minus side, turn the core counter-clockwise. The amount is less than 1 degree for 5Hz. So you must be veeeery careful.
  9. Repeat 5 through 8 to have same pitch at 15.000.000.