If the 6T3 is much like my Conn ST2 model, you first calibrate it
to itself, then check against another known pitch standard.  There
should be a switch on the front labeled "Calibrate," with a spring
return to "Operate" position.  Hold that switch in Calibrate while
the machine is running.  It doesn't matter what note the rotary note
selector is set to, and you don't need the microphone plugged in.

In the deepest bass octave of the strobe disk, next to its center,
you should see a pattern.  Adjust the tuning knob until that pattern
is stationary.  You may have to turn the knob all the way around to get
it still, but don't do that unless you have to (because someone else
turned it clear around, which makes the dial readings meaningless till
you turn it back).  Be patient, since the pattern (a dark spot) moves
very slowly.

The pointer on the tuning knob is not firmly attached, but it can be
slid around to line up at 0 once you have the pattern stationary.  You
needn't hold the switch down any longer while adjusting the pointer.
Just make sure the knob shaft doesn't turn with it.

Now you can check the instrument against a known keyboard, tuning fork,
or other musical instrument.  You can get A=440 Hz by holding the mike
against a telephone to hear the dial tone, though I found this pitch
was a little off.  Again, the pointer can be slid around to read 0 when
you have frozen the strobe pattern.

By the way, the Calibrate function sets the tuner to a 13th pitch, not
in the chromatic scale, but between A and A#/Bb, which is the 60 Hz
power line.  It also bleeds some 60 Hz "hum" into the mike circuit to
form the pattern.  When you've finished the first step, the instrument
will be very close, within a few cents.

These old Conn tuners are very good for organs and instruments, though
maybe not the best for tuning pianos.

Best wishes
Mike Knudsen

 [ They're great for tuning a xylophone, because the sound is so
 [ short that it won't register on other tuning devices.  -- Robbie