Sequential prophet 5 - Hardware Description; Bd001 Interconnection

During the TUNE routine, the computer Initializes all S/Hs that affect tuning (and defeats any external
CV). It has no control over the MTUN or P-BND, so these must be centered and not mov«?d. The
computer then sets OSC 1A BIAS CV to 2V, and the OSC A INIT FREQ and KEY 1 CVs to cause a p.tch
near 300 Hz. It then selects OSC 1A's sawtooth output through the TUNE MUX. (If for some reason
an oscillator has failed, the TUNE routine cannot execute.) Counter 0is programmed to count 1 cycle.
Gating counter 1. the REF CNT will be approximately 9000. In other words the period of one cycle at
300 Hz is approximately equal to 9000 CPU clock cydes.
The CPU now sets all BIAS CVs to OV, leaving the INIT FREQs and KEY CVs unchanged. Reigning to
OSC 1A it again counts one cycle period, and compares it to the REF CNT. The period will almost
always be too long with 0 BIAS. It then increases the BIAS one step (83 mV), and measures the period
again. The count-and-compare process continues until the osdllator count equals the REF CNT. at
which point the computer saves the BIAS 1A value. This repeats for the nine remaining oscillators, so
that each has a BIAS CV in memory which matches its pitch to the others at 300 Hz. The tuning routine
ignores badly mistuned oscillators. If BIAS increases 100 steps without tuning the oscillator, the CPU
resets it to 2V and goes on to the next oscillator.
Next, the computer obtains another set of ten biases to match the oscillators at 600 Hz, by program¬
ming counter 0 to count two cycles, and raising the INIT FREQ and KEY CVs by IV. (Two cycles at
600 Hz should take exactly the same time as one cycle at 300 Hz). The process continues at 1200 Hz for
four cycles, and at 2400 Hz for eight cycles. When done, the computer has four bias values for each
oscillator at four different frequencies (40 total).
Now, when playing the computer determines the pitch each oscillator is to sound and continually
corrects by applying one of the four biases. It uses the 300-Hz BIAS up to about 450 Hz, where the
600 Hz BIAS takes over. The 1200 Hz BIAS CV is used between about 900-1800 Hz, and 2400 Hz BIAS CV
corrects the rest of the frequency range.
2-16 HARDWARE DESCRIPTION
The remainder of this section comments on circuitry which may not be dear from either the sche¬
matics or the more general description, above. Logic or analog drcuits which should be apparent to
anyone qualified to work on the Prophet, are not discussed. It is assumed throughout that the reader
will follow signals continued on other sheets as explained in paragraph 3-1, DOCUMENT NOTES.
2-17 BD001 INTERCONNECTION
The diagram depicts the physical organization of the Prophet's assemblies. Symbols show on which
schematics components on each PCB or assembly can be found.
The Prophet is designed for compactness and light weight. It separates into a TOP and a BOTTOM
PANEL ASSEMBLY. The BACK PANEL CABLE connects PCB6. POWER SUPPLY BOARD, and the BACK
PANEL INTERFACE jacks to PCB3. COMPUTER BOARD, at P301. The only other connection between
the main assemblies is the AUDIO OUTPUT cable from PCB4. VOICE BOARD. This quick-disconnects
from the BACK PANEL at P402. The TOP PANEL ASSEMBLY of course holds the KEYBOARD and
WHEELS. PCBs 1 and 2 mount directly to the CONTROL PANEL. And PCBs 3 and 4 mount on PCBs 1
and 2 with standoffs.
PCB1, RIGHT CONTROL PANEL, contains potentiometers, switches and LEDs wired into the POT
MUX and SWITCH/LED matrices on PCB2 through W701. Also through W701 passes MTUN signal
which terminates on PCB3 and the VOL CV signal to the AUD OUT circuit on PCB4.
PCB2, LEFT CONTROL PANEL, contains the POT MUX and SWITCH/LED matrices. The KEYBOARD
plugs in to the SWITCH MATRIX at 1201. Through P201 passes the DATA BUS which conveys switch
data from the matrix to the LED data and POT MUX addresses are sent to PCB2 over the DATA BUS.