Sequential Circuits Prophet-5 Service Manual - Audiofanzine

A)I4-16 VCO SCALE TRIMBE SURE RECORD IS DISABLEDTHE RANGE OF THE TUNE SYSTEM IS LARGE ENOUGH SO IT WILL RARELY BE NECESSARY TO TRIMVCO SCALE. IN FACT VCOS CAN USUALLY BE SUBSTITUTED WITHOUT ANY READJUSTMENT.THEREFORE IF THE PROPHET IS BADLY OUT OF TUNE THERE IS PROBABLY A COMPONENT FAILURE.A SCALE TRIM ISINDICATED FOR OSCILLATORS WHICH DO NOT TRACK THE PITCH WHEEL, ORWHICH ARE DETUNED IN UNISON, ESPECIALLY WITH LONG GLIDE TIMES.This trim uses a special microcomputer subroutine which setsup the VCOs to be scaled in the orderOSC 1A, IB, 2A, 2B, 3A, etc. The lowest key, CO, is used to tell the microcomputer to recompute theVCO SCALE bias. One starts with OSC 1A by trimming for zero-beat with a C-reference which thecomputer also provides (through the A-440 circuit), hitting CO, retrimming, hitting CO, and retrimmingand so on until no further improvement can be made. Then key DO is pressed to bring up the nextoscillator, IB, and the process repeated (trim, CO, trim, CO, etc.).1. Locate TP301 at U324-6 (schematic SD332). Tie this to +5V (Most PCB 3s have a wire loop at the +5Vrail, left of U301-U308).2. Press TUNE. You will now hear OSC lA's sawtooth with a reference tone in the background.3. Trim R4294 OSC 1A SCALE for zero-beat. Hit key CO and retrim, using a good amount of"overshoot." Repeat until the tones are in zero-beat.4. When OSC 1A is done, hit key DO and TRIM OSC IB. Table 4-0 OSC SCALE lists trim designators.5. Repeat step 4 for OSC 2A - 5B. If you lose track of which VCO you are supposed to be trimming,simply run your hand along the two rows of VCOs (see Figure 1-0). You will know when you touch apin of the currently-tuning VCO—they are very sensitive to detuning in this way.6. When done untie TP301 from +5V. The Prophet will enter TUNE mode, then come up to program1-1.Table 4-0OSCILLATOR SCALE TRIMMERSVCOTRIMMER1AIB2A2B3A3B44B5A5BR4294R4186R4300R4190R4306R4194R4312R4198R4318R42034-12 D.2