18 9720 VCO
010321
should be lit, but not very brightly.
If the LED is dark or
seems fairly bright, quickly feel around on the circuit
board to see if any parts are hot or very warm. If you
find any, disconnect the power and carefully
examine the circuitry around the hot part looking for
solder bridges, backward capacitors or other polarized
parts. If nothing seemed warm but the LED is dark and
the panel switch is set to "Cycle" it may be a problem in
the Modulator circuit. Carefully check the modulator
parts (Q4-Q6, IC8, panel controls R200-R202 and
associated components). Check the LED, it may be
in backwards or may be bad. It's human to suspect bad
ICs but most problems relate to the mechanics of
assembly - the wrong part in the wrong place or facing
the wrong way or not soldered well.
3) Rotate the Modulation Attack control to the midpoint of its rotation. The LED should now
increase in intensity over a 2 second period then suddenly go dark and repeat the cycle. Turn
the Modulation Release control to about the midpoint of its rotation and observe that the LED
slowly brightens and slowly dims with about a 5 second period. If that goes well, crank both
Attack and Release controls full ClockWise and observe that the period is about 15 seconds.
If
there are problems here, check the modulator components outlined in step 2, paying particular
attention to correct values. If cycle times are significantly different (half or twice more, for
instance) it may indicate an incorrect component value or power supply lines that are out of
spec - check for hot parts among the power supply components mentioned in step 1, and if a
Volt Meter is available check the regulated supply lines - IC3 pins 4 (+12V) and 11
(-12V) are handy places.
4) Listen to the Ramp outputs (
) of OscA and OscB in turn. Each should be about 120 Hz
and of equal amplitude. If you have a Volt meter handy, use it to measure the voltages on
these outputs. Both outputs should be about the same and when the meter is set to read DC
you should measure 200mVolt (0.2V) or less. With the meter set to AC you should read 1.25V.
If no meter is available do not be concerned with these measurements unless there are
problems in future steps.
While listening to OscA's ramp output, rotate the Pitch control fully CounterClockWise and
observe that the pitch falls by about 3 octaves to 15 Hz. or so. Do the same with OscB. Be
sure to return both Pitch controls to max (fully ClockWise) for future tests.
Problems here can come from a lot of different sources. If there is no audible output from OscA
check IC7:A & :B and IC6:D, Q10 and associated components. Corresponding parts in OscB
are IC3:A &:B IC6:A and Q7. If there is output but it's very high or low frequency or doesn't
change as described above it may be CV summing/current sources. For OscA these comprise
IC4, Q2, Q3 and associated components. If OscB is not functioning properly IC5, Q8, Q9 and
components associated with them should be examined. These ramp outputs must be working
before any of the others will work properly.
Summary of Contents for 9720
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