About The Pro-One Basic Operation Front Panel Back Panel Routine Maintenance Service

Section 5: Service

5—0 Introduction

The information in this section is provided for reference by qualified service technicians only. We recognize that circumstances may arise in which this data may have critical value for the player. We are making these procedures available with the understanding that our warranty terms are not affected. In short, as an owner you void your warranty if you attempt these procedures on the Pro-One.

5—1 Removing PCB

  1. Open the Pro-One as directed in paragraph 4-3.
  2. Pull off all knobs.
  3. Disconnect wheel connector at left of PCB. (When reconnecting, orient connector so that the side in which you can see the metal contacts is visible.)
  4. Remove 9 screws holding PCB to top panel (there are two screws near the transformer).
  5. The PCB should now lift away from the top panel. If not, check that the GATE LED is not stuck or cemented.
  6. Set the PCB on insulation on the bottom panel, and reconnect AC and keyboard. Switch power on, and allow the instrument to warm up for at least ten minutes before proceeding.



5—2 DAC Scale

This adjustment should not be performed as part of the tuning procedure. In fact, readjustment may only be necessary if a repair has been made. Whenever this adjustment is made the Pro-One will have to be completely retuned. A 3-½ digit voltmeter (DVM) is required.

  1. Connect DVM to back-panel CV OUT jack.
  2. Hit CO and note reading. The reading will be approximately 0.083V. (The absolute value of this reading is not critical).
  3. Hit C3 and note reading, which should be the value read in step 2, plus exactly 3V. If necessary, adjust R1181 DAC SCALE for the exact reading; e.g. 3.083V.
  4. Repeat steps 2 and 3 as required to establish an exact 3-V difference between CV OUT played by C0 and C3.

5—3 Tuning

figure 5-0

Figure 5-0


  1. Patch according to Figure 5-0. For component locations, see Figure 5-1
  2. Hit C0 and adjust OSC A FREQUENCY for zero-beat.
  3. Hit C3 and adjust R128 OSC A SCALE for less than 1 Hz beating.
  4. Repeat steps 2 and 3 until no further improvement can be made.
  5. _OSC A HI TRIM_

  6. Switch OSC A OCTAVE to 3.
  7. Switch OSC B OCTAVE to 3.
  8. Hit C0 and adjust OSC A FREQUENCY for zero-beat.
  9. Hit C3. Adjust R135 OSC A Hl TRIM for zero-beat.
  10. Repeat steps 7 and 8 until no further improvement can be made.
  11. Figure 5-1

    Figure 5-1

    _-1V TRIM_

  12. Switch OSC A OCTAVE to 0.
  13. Switch OSC B OCTAVE to 1.
  14. Hit C0 and adjust OSC A FREQUENCY for zero-beat.
  15. Switch OSC A OCTAVE to 3 and adjust Rllll -1V TRIM for zero-beat.
  16. Switch OSC A OCTAVE to 1 and 2 and check for less than 2 Hz beating.
  17. _OSC B SCALE_

  18. Switch OSC A OCTAVE to 1.
  19. Switch OSC B OCTAVE to 1.
  20. Turn OSC A FREQUENCY to 5.
  21. Turn OSC B FREQUENCY to 5.
  22. Switch OSC B KYBD on.
  23. Hit C0 and adjust OSC B FREQUENCY for zero-beat.
  24. Hit C3 and adjust R148 OSC B SCALE trimmer for less than 1 Hz beating.
  25. Repeat steps 20 and 21 until no futher improvement can be made.
  26. _OSC B HI TRIM_

  27. Switch OSC A OCTAVE to 3.
  28. Switch OSC B OCTAVE to 3.
  29. Hit C0 and adjust OSC B FREQUENCY for zero-beat.
  30. Hit C3 and adjust R157 OSC B HI TRIM for zero-beats.
  31. Repeat steps 25 and 26 until no further improvement can be made.
  32. _OSC A/ S OFFSET_

  33. Check that PITCH WHEEL and MASTER TUNE are centered.
  34. Switch OSC A OCTAVE to 1.
  35. Turn OSC A FREQUENCY to 0.
  36. Switch OSC B sawtooth off.
  37. Using an external reference (tuning fork, piano, or Prophet), adjust R120 OSC A OFFSET so key A2 pitch is A-440.
  38. Switch OSC A OCTAVE to 3.
  39. Switch OSC B OCTAVE to 3.
  40. Turn OSC B FREQUENCY to 0.
  41. Switch on OSC B sawtooth.
  42. Hit Cl and adjust R140 OSC B OFFSET for zero beats.

5—4 Amplifier Balance

figure 5-2

Figure 5-2

  1. Patch according to Figure 5-2.
  2. Increase system volume to fairly high level. Adjust R1138 VCA BALANCE for minimum envelope bleed-through.

5—5 Key Contacts

After the top panel is removed, the keyboard can be detached from the bottom panel for cleaning by simply removing four screws.

The keyboard uses self-cleaning, gold-plated contacts which will provide many years of reliable service. Oxidation, particularly of lesser-used keys, may be aggravated by ambient conditions or handling. If this occurs, causing "dead" keys, wipe the contacts carefully with an alcohol- or freon-wetted cotton swab. Don't use any abrasive techniques, as this will simply remove the gold plating.

5—6 Digital Interface

Use of the digital interface for external computer control requires both some simple hardware modifications to the Pro-One and the creation of software for your system. If you doubt your technical ability to install the interface, please see an SCI Authorized Service Center or contact our Service Department. However please note that we must leave all questions of programming your system to you.

  1. Remove U113 (see Figure 5-1) 8021 microcomputer from its socket. Keep it safe. Note that with the 8021 removed, the Pro-One's keyboard cannot operate.
  2. Jumper pin 2 of the 8021's socket to J101-7. Pads are provided on the printed circuit board (PCB) for this purpose.
  3. Add R 1182, 10K-ohm, ¼W, 5% resistor where shown on Figure 5-1.
  4. Table 5-0 lists pin assignments. Mate J101 with a 14-wire ribbon cable terminated in a DIP plug. (There are six spare lines).
  5. Table 5-0

















    -DAC EN



    The interface is compatible with standard TTL logic, where a 0 is signified by less than 0.8V and a 1 by greater than 2.5V. It accepts six data bits (DO through D5). This allows the numbers 0 through 63 (decimal) to convert to over five octaves of "keyboard" control when latched to the internal digital-to-analog converter (DAC) by the active low -DAC EN pulse. The GATE is exercised via a bit at pin 7.

  6. To play notes on the Pro-One, the system driving it will have to do something like the following. (The way in which these events happen is not so important as their sequence.) At the start, the analog KYBD CV output from the DAC (which controls the oscillators and filter) is unknown. Nothing is heard from the Pro-One because the envelope generators are not being gated. First the system somehow decides what key (out of 63) it wants to play. It then sets up the key number in binary form on D0-D5. (This might be done with a six-bit output port.) -DAC EN is normally high. To latch the key data to the DAC, -DAC EN is momentarily forced low. Data is then latched on the rising edge of this pulse. For practical purposes a delay of 20 microseconds will allow the DAC time to "settle" (worst-case). At this point, GATE can go high, triggering the envelope generators. When the note is to be turned off, it is likewise advisable to turn the GATE off (low) before again strobing the DAC.
  7. To return the Pro-One to normal operation, unplug your system from J101, remove R1182, cut the jumper installed in step 3 and carefully re-install U113, 8021.
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