Sequential prophet 5 - Trimming and Tuning; General.4-1

Section 4
Trimming and Tuning
4-0 GENERAL
Prophet trimming and tuning is a sensitive procedure which as a rule should not be done more than
necessary. Prophets are carefully adjusted at the factory, where many trimmers are sealed to prevent
misadjustment by vibration or accident. You will rarely be able to make an audible improvement
upon these trims — unless a malfunction has occured or a part has been replaced. If you do try to
achieve all of the ideal settings described below when no repairs have been involved, you may
succeed only in correcting for the difference between our and your DVMs or room temperature.
These procedures are ordered for a Prophet assumed to be 100% electrically functional but com¬
pletely untrimmed. But this does not mean you should completely retrim whenever something
doesn't sound right. Rather, for example, isolate "a tuning problem" to a specific voice (see para. 4-9)
and correct the errant oscillator. Of course you should familiarize yourself with the entire trim pro¬
cedure before attempting it on the Prophet. Make a "dry run", using the schematics and pictorials
in Section 3.
A complete retrim begins with the power supply, then scales the DAC and ADC on PCB 3. PCB 3 VCAs
are balanced (BAL), the LFO triangle symmetry (TRI SYM) checked, and the OSC A and B Master
Summer (MSUM) offset is trimmed. PCB 4 is then installed, the amplifier and filter envelope gen¬
erators (AMP and FILT ENV GEN) timed. VCA BAls and OSC B TRI SYMs set. Then the voices are tuned
filters before oscillators—and the Prophet closed. This raises the oscillators to normal temperature.
The Prophet is again opened for "hot tuning" and the oscillators are fine-tuned as quickly as possible.
Finally, the overall voice levels are matched. When using the Prophet with an external sequencer,
an accessory trimmer prevents detuning of VOICE 5.
Mechanical procedures required for trimming are covered in Section 1. Consult schematics (SD) and
pictorials (PA) in Section 3, as required to locate components mentioned. MA110 shows all controls,
PA310U shows PCB 3 ICs and trimmers, and PA416 shows PCB 4 trimmers and oscillator sawtooth
outputs. Key designators are shown on SD212; CO is the lowest, C5 the highest. Trimming serves as
an initial troubleshooting procedure. When normal indications can't be achieved, see Section 5 for
service information.
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