Offset
Adjustment
1. Disconnect R508 from the wiper of P503 and connect it to ground.
2. Monitor the output voltage of U502A with a multimeter. It will probably
exhibit a tendency to drift positive or negative, and the voltage will
settle at +15V or -15V.
3. Reset the output voltage to zero by discharging C501 through a 1K
resistor.
4. Adjust P504 until the voltage remains stable at zero volts for a
period of several seconds (without the discharge resistor in circuit).
5. Repeat steps (3) and (4), progressively switching the multimeter
to more sensitive ranges, until the drift is only a few hundred millivolts
in several seconds.
LED Adjustment
P505 should be adjusted so that the brightness of the LED follows the
amplitude of the triangle output, i.e. the LED brightness should be
at minimum brightness when the triangle is at its most negative, and
at maximum brightness when the triangle is at its most positive.
P505 should be adjusted so that the LED brightness does not reach maximum before the peak of each triangle, but on the other hand it should not extinguish completely before the trough of the triangle.
Selection of
R507
The only other adjustment possibly required would be an adjustment to
the value of R507. Readers wishing to get very low frequencies from
the VCLFO may wish to experiment with smaller values of R507 however,
making this value very small (or even zero) may result in the VCLFO
`stalling' when the frequency control is set to minimum.