[OKDXA] Reference oscillators

[Nelson Derks]

In the case of an Icom 756 (original version and possibly the subsequent
variations) the radio relies on one master oscillator for just about
everything including the internal marker generator. If that oscillator is on
the money, everything else pretty much falls into place.

Here's how I check mine:

Let the rig idle for 30 minutes or better, then set the dial to 10.000 Mhz
and the mode to AM. Pick a time when WWV at 10 MHz is coming in fairly well
(not much QSB) and you hear the 400 Hz background tone between clock ticks.
Turn on the internal marker generator. You'll probably hear a low tone near
zero-beat. Adjust the oscillator through the hole in the lower right side
panel (near the front) until the zero-beat goes sub-audible. Set the AGC to
fast or mid level response and watch the S-Meter 'wobble'. If you have a
steady hand, you can tweak the trimmer cap until the S-Meter holds almost
steady. At that point you are within a fraction of a Hz relative to WWV.

As a secondary check of the alignment, turn off the internal marker
generator and flip between LSB and USB. The pitch of the background tone
from WWV should be the same.

I haven't tried this at 5 or 15 MHz, but I suspect it will work. This is a
variation of the old freq counter tweak for counters that use a 10 Mhz
clock... Plug in a few feet of wire as an RX antenna on any AM SW receiver,
tune WWV at 10 MHz, lay the wire over the freq counter case until you hear
the two signals beat against each other, then adjust the frequency counter
oscillator for zero-beat.

- AC5UP






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