Sequential PROPHET-600 - LOADING PROGRAMS OR SEQUENCES FROM TAPE

There is an interesting way the 3372 can fail which makes a simple situation look like
a major calamity. All the MIX A and B CVs are common. Therefore if one of these
inputs shorts internally to ground, all the OSC As or Bs (or both) will be disabled. To
isolate the bad 3372 in this case you would probably measure the CV while alternately
removing and reinstalling 3372s. (When the CV rises to normal you've found the bad
IC.)
The filters are scaled at l/4V/octave, to enable the various filter control sources—
keyboard, LFO, ENV AMOUNT, CUTOFF to have full range. Filter frequency CV input
is treated as a summing node. R463 FILTER 1 SCALE trims the input dividing
resistors. R468 sets the basic filter range. R470 contributes any external voltage.
When POLY-MOD FILT (R462) is off, U430 connects its output (pin 4) to ground (pin
The FIL 1 CV from the S/H undergoes some local filtering for digitization noise. D401,
D402, R417 and C447 are chosen such that as long as the difference between the S/H
and the charge on C447 is less than 600 mV, C447 will charge or discharge through
R417. When the difference is greater than this, it will be conducted through the diodes
(which takes less time). This two-speed filtering allows enough static filtering for
acceptable noise without excessively distorting shorter envelopes.
CAPS 1 through 4 (C458-61) determine the frequency of each filter pole. The output
from the fourth pole (pin 17) is ac-coupled to both a resonance VCA (pin 11) and the
final VCA (pin 12). Pin 10 is the RESONANCE CV input. The resonance amplifier is
internally compensated to provide a fuller sound than our previous filters.
Finally, there is the amplifier CV. The network D410/D411, R444/R445 and C462 are
another filter for digitizing noise which basically offers two different slew rates,
depending on the level of the change. The Zener diodes are rated at 2.4V. When the
amplifier envelope is moving slowly, the difference between the S/H output and pin 13
will rarely exceed 2.4V. The diodes do not conduct, so the CV is slewed by R445/C462.
When the difference exceeds 2.4V, one of the diodes switches on R444 in parallel,
which considerably accelerates the slewing.
2-15 AUDIO OUTPUT
Referring to Sheet E, the voice outputs are summed by U467-7, which drives the Final
VCA and Tune Comparator.
The VOLUME CV to U458-2 is divided by 2 by R4221/R4220. Pull-down R4222 insures
the CV goes below 0 for complete shutoff. C4146 provides filtering for digization noise
on the MVOL CV.
U452 is our standard 600-Ohm output circuit.
2-16 TUNE
The Tune system is responsible for the entire performance for the instrument in the
sense that without its corrective influence, the twelve oscillators could not by
themselves be expected to constantly stay in tune, due to temperature drift and
component aging. Basically, each time the TUNE switch is pressed the system
TM600A 7/83
2-17