Sequential Pro One - Routine Maintenance

Figure 2-4
VOLTAGE-CONTROL TRIMS
Figure 2-4 graphs the ideal VCO function and the corrections possible with the
trimmers provided. As can be seen, there is considerable overlap between the
adjustments.
(Thus, tuning procedures are written specifically to overcome these
interactions.) Initial frequency, or OFFSET, raises or lowers the function. VlQCT
adjusts the slope of the line, and HI TRIM adjusts the function above about 3 kHz.
OSC A% SAWTOOTH output at pin 8 ranges 0 - 10V. It is switched by 5106 through
summing resistor R172 to summer UE05-7. The PULSE output from pin 4 is pulled
down by R177, and is attenuated (with respect to the sawtooth) by R171.
CVs for OSC A pulse-width modulation are summed and inverted by U104-8 yielding a
basic control range of 0 to +5V into W102-5. R165 OSC A PULSE WIDTH knob connects
through R169. 5109 MOD TO OSC A PW selects the modulation buss summed through
R166. R170 sets the summer gain to 1. Cll8 filters noise which may be generated by
rotating R165.
U102-9, S SYNC is a 5-V source which biases the base of dl01 thmugh R138. C108 is
simply a dc filter. When closed, 5108 OSC A SYNC coup1es"the falling edge of the OSC
B sawtooth through C122 and R188, which turns on Q101. The internal sawtooth buffer
connected
to UlO2-10 is thus pulled-down to -5V, resetting it. This "hard syncs" OSC A
to B, independently of whatever OSC B waveforms are swttched on.
Further details of the CEM 3340 can be found in the Appendix.