The Matinee Organ (Part 4)
A complete electronic organ to build at low cost
PART 4: Interwiring, setting-up and Pedal board circuitry
Last month we showed in Figure 22 the interwiring details. This month we describe how to achieve the results shown.
Take the seven wires coming from the power supply PCB, cut the blue and white wires to a length of 1.2m and the remaining five wires to 1m. Strip and tin the wires, terminate to the socket contacts and fit into the five- and six-way housings as shown in Figure 25. Check again that the wires are connected to the correct pins on the power supply.
Cut 2m from the length of yellow wire, then cut that in half, and twist the two lengths together. Cut 80cm off the length of screened cable and terminate these three wires on the socket contacts as shown in Figure 26. Push the contacts intothe six-way housings as shown in Figure 25. Connect the free ends of the two yellow wires to the two tags on the spring line marked "input" (it does not matter which way round they are connected). Prepare the other end of the screened cable and connect it to the tags on the spring line marked "output". The screen must be connected to the terminal that is already connected to the metal case.
Cut two 1.5m lengths of violet hook-up wire and 1.5m of screened cable and connect to the four-way housing as shown in Figure 27. Connect the other ends to the swell pedal and glide switch as shown in Figure 28.
Cut a 50cm length from the orange, yellow and violet wires and connect them to one of the three-way housings and the two LEDs as shown in Figure 29.
Cut 30cm off the end of the 4m length of 2-core mains cable (at present connected to the PSU), then cut off a further 70cm and connect one end of each length to the headphone socket as shown in Figure 30. Connect the two resistors R604 and R605. To the other end of the 30cm piece connect the two-way housing. The remaining cable is connected to the loudspeaker with the brown wire connected to the tag nearest the red dot on the speaker chassis.
Cut 1.5m off the 3m length of 2-core mains cable (still connected to the PSU) and connect one end of this piece and the end of the piece from the PSU to push-on receptacles, after sliding the covers back over the wires, as shown in Figure 31. Temporarily connect these to the mains switch. Refering to Figure 12, connect the other end of the cable to the 3-way terminal block, connecting the green wire from the PSU to the third terminal. Connect the 3-core mains cable to the other side of the block as shown in Figure 12. Do not connect to the mains yet.
With reference to Figure 23, link together all the top, left-hand tags of the pedalboard switches with the strapping wire. Then link together all the lower, left-hand tags of the pedalboard switches. Referring now to Figure 32, identify which wire number is which in the 15-way jumper that has a plug already connected at one end. Separate wire number 1 from the ribbon by carefully cutting along the groove between that wire and the next. Once started, it can be pulled back by hand. Pull back for exactly 50cm then cut 45cm off this wire and terminate at the tag shown in Figure 23. Separate wire 2 and then wire 3 to the same distance as wire 1 and terminate, then work through the remaining twelve wires separating back each wire only as far as required for each connection.
Now connect the remaining lengths (just over 1.3m each) of the orange, yellow and violet wires to the remaining three-way housing as shown in Figure 32 and connect the other ends to the pedalboard as shown in Figure 23. Connect the orange wire to the centre, right-hand tag of the extreme right-hand switch then cut 15cm off one of the wires cut from the ribbon jumper and link that tag to the centre right-hand tag of the third switch from the right. This completes the wiring.
Ensure that the covers are completely covering the tags on the mains switch and take care with the mains terminal block,the fuseholder and the transformer primary tags whilst testing is in progress. Insert the fuse FS1 into the fuseholder in the power supply chassis. Ensure that the two sockets on the ends of the wires from the power supply are NOT connected to the main PCB. Connect the 13A plug to the other end of the 3-core mains cable. Plug into the mains supply and switch on.
Set a multimeter to a DC voltage range higher than 40V, take the five-way socket and place it with the locating lugs down and the wires entering from the rear. (The brown wire should be on the left). Switch on the organ mains switch; the neon in the switch should light. With thin-tipped probes or with pieces of wire connected to the probes, connect the meter positive probe to the left-hand socket and the negative to the socket next to it. The meter should read between 30V and 35V. If this voltage is not correct, switch off immediately and re-check all wiring and the polarity of all relative components.
If all is well, move the meter positive lead to the right-hand socket and the negative lead to the socket next to it. Switch the meter to the 10V DC range (or next highest range) and check that the meter reading is between 5 and 8 volts. Adjust RV1 on the PSU PCB with the trimming tool until the meter reads 6.5V. Remove the negative lead and replace it with the positive lead. Put the negative lead in the centre socket. The meter should read between 5V and 8V. Adjust RV2 on the PSU PCB until the meter reads 6.5V.
Take the six-way socket and place it with the locating lugs down and the wires entering from the rear. (The centre wire of the screened lead should be on the left). Switch the meter to a DC voltage range above 15V and connect the positive lead to the third socket from the left and the negative one to the third from the right. The meter reading should be between 14V and 16V. Switch off.
Note that in the following instructions the front of the PCB is the edge with the latchswitches under it. Connect the two sockets from the PSU to their plugs on the main PCB, SK8 with its locating lugs towards the centre of the PCB and SK5 with the locating lugs downwards. At this stage there should be no other sockets connected to the main PCB and ICs 1, 4, 24 and 44 should not be plugged in.
Switch on and switch the meter to the 10V DC range (or next highest range). Connect the meter negative lead to the metal of the pot mounting bracket and the positive, to the positive end (nearest front of board) of C216 on the extreme left-hand edge of main PCB. The meter should indicate between 5.5V and 6.5V. Connect the meter positive lead to the pot mounting bracket and the negative one to the negative end (nearest the rear of the board) of C216. The meter should indicate between 5.5V and 6.5V. Switch off.
Unpack the remaining four ICs taking care to avoid touching the pins. Plug them in as shown on the legend on the PCB and on the identification chart supplied with the kit. Note that all the ICs on the main PCB have pin 1 either to the rear of the board or to the left-hand side (except IC24). Ensure that all drawbar slide pots are at minimum i.e. towards the rear of the board and that all switches are released or off, except any one rhythm switch and the presets cancel switch.
Connect the two jumper cables to the keyboard PCBs, ensuring that the cable leaves PLA towards the centre of the PCB, not as shown in Figure 22. Connect the other ends to the main PCB making sure that the cables come off the sockets towards the rear of the PCB as shown in Figure 22. Plug in the 15-way connector on the end of the cable from the pedalboard with the cable coming off downwards. Plug in the other four sockets. When you are certain that they are all correctly connected, switch on.
Connect the meter negative lead to the pot mounting bracket and the positive to the positive end of C216. Adjust RV1 on the PSU PCB until the meter reads exactly 6V. Connect the meter positive lead to the pot mounting bracket and the negative to the negative end of C216. Adjust RV2 on the PSU PCB until the meter reads exactly 6V.
Leave all the drawbar slide pots at minimum and set the meter range to 2.5V DC (or next highest above). With reference to Table 4, adjust each preset shown as follows. Connect the meter positive lead to the pot mounting bracket and the negative one to the test point shown in the table. With no keys pressed note the voltage reading as accurately as possible. Press any key on the manual indicated in the table and adjust the preset shown in the table until the voltage is identical to the one noted previously. Set all seven presets in turn.
Advance the Bass Guitar drawbar RV12 and advance the master volume RV50. Press any pedal on the pedalboard and adjust RV1 on the main PCB for the cleanest attack that gives the most realistic sound.
Reset RV12 and advance the pedals 8ft. Flute drawbar (RV10). Play a note repeatedly on the pedalboard and adjust RV2 on the main PCB to set a fast or slow attack for the most pleasing sound.
Reset RV10 and advance the 8ft. Flute drawbar for the upper manual (RV38). Adjust the manual balance control (RV46, 47) if necessary, then play middle A. RV32 should now be adjusted to set the pitch of the entire organ. The frequency for middle A should be exactly 440Hz and may be set with pitch pipes available from most music shops or from the tone on E&MM demo cassette No. 1. Alternatively, the preset may simply be set to its centre position. Another alternative for early risers is the 440Hz tone broadcast by Radio 4 before programmes start each day (about 5.50 a.m. except Sundays).
Play A again and adjust RV31 so that when the glide switch S36 is pressed, the tone drops to Ab.
Select Vibrato (S20) and Vibrato Delay (S21), pull vibrato depth (RV30) fully forward and vibrato rate (RV28) half way. Adjust RV29 to give the delay you prefer. Reset S20, S21, RV28 and RV30.
Set the swell pedal roughly central, play a chord on the upper manual then press manual wah (S35) and wah on/off (S34); adjust RV48 for the same volume. Reset S34 and S35.
Press rotor on/off (S17), play a chord and adjust RV52 slowly until the best effect is achieved. Switch to fast rotor (S18) and the effect will accelerate slowly just like a real "Leslie". Reset S18 and the rotor will decelerate slowly. Reset S17.
Set RV19, 20, 21, 22, 23, 24 and 25 fully clockwise and RV26 and 27 fully anticlockwise. Advance the rhythm level RV18, select Waltz, rhythm start S22 and adjust tempo RV51 for a reasonable speed. Adjust RV19 for the best sounding bass drum. Adjust RV25 to a point just before it starts to oscillate. Set RV27 half way. Adjust RV26 for the best snare drum sound, then re-adjust RV27 for the best cymbal and snare sound.
Select Reggae, set RV21 fully anti-clockwise and adjust RV20 until the best low bongo sound is achieved. Re-adjust RV21 to improve the tone, then slowly adjust RV20, 21 relative to each other for the best sound.
Select Disco, set RV22 fully anticlockwise and adjust RV23 until the best conga sound is achieved. Then re-adjust RV22 to improve the tone. RV22 and 23 should then be slowly adjusted relative to each other to obtain the best sound.
Select Cha-cha and adjust RV24 for the best clave sound. Note that in this rhythm, high bongo (RV25), low bongo (RV20) and conga (RV23) form a run and may be re-adjusted to obtain a pleasing downward frequency progression. The long and short cymbals do not require adjustment and the high bongo is set when the snare drum is set. The organ is now completely set-up.
The heart of the pedalboard circuit is the M147 integrated circuit which provides five octave-related outputs for any one of 13 different notes. As with all 13-note pedalboards, the output is monophonic, that is only one note can be played at any one time. However many pedals are pressed, the M147 only outputs the frequencies appropriate to the lowest pedal pressed — in other words it gives priority to the left.
The master frequency 500,060Hz from IC37 pin 13 (Figure 2) is connected to pin 17 of the M147. Since this pin can also accept 2MHz, the mode switch in the IC is set to accept 500kHz by connecting pin 19 to +6V.
When the pedals are normal, the thirteen key line inputs to the IC are connected to +6V, but when depressed, that pedal input is connected to -6V. The M147 detects the transition and if the pedal is to the left of any previously pressed pedal or if releasing a most left pedal makes a higher one the new most left pedal, the frequencies appropriate to that pedal are output on pins 20 to 24. The output of pin 23 is exactly double the frequency (or one octave higher) than pin 24, whilst the output of pin 22 is one octave higher than pin 23 and so on, though the 1ft and 2ft outputs are not used in the Matinee.
This output now remains on even if the pedal is released, unless a new pedal now becomes the most left pedal pressed or until a new pedal is pressed. Each output has a pull-down resistor to -6V (R4, 5 and 6) and is then fed through IC2, the auto/manual switch. When in manual mode, the three footages are connected straight through to the filters but in automatic mode, they are disconnected and the automatic bass output from the lower manual, M108, is connected through instead.
The square wave from the 16ft output is connected through the low-pass filter formed by R17,18, 19, C6 and 7 to form a flute sound. The output is taken via C5 to the level control drawbar RV9. The square wave from the 8ft output of the M147 is connected through the low-pass filter formed by R20, 21, 22, C8 and 9 to form a flute sound also. The output is taken via C10 to the level control drawbar RV10. There is also an input to this filter coming via R215 from the automatic bass when IC2 is switched to automatic.
The outputs of RV9 and 10 are mixed together by R141 and 142 and fed to the signal input, pin 14, of IC3, the bass VCA. The control input, pin 16 of this IC, is fed from the bass envelope shaper. Pin 16 of the M147 provides a low DC level whenever any pedal is pressed and this level is fed to the input of the bass envelope shaper when the organ is in manual mode. In automatic mode the input comes from the TDB output of the M108 and this changeover is carried out by S24A.
In manual mode, the level from the M147 switches TR3 on and C13 now charges via D12 and RV2 at the rate set by RV2. This provides a positive going output from the envelope shaper which turns up the gain of IC3b. When all pedals are released, pin 16 of the M147 goes high turning TR3 off. C13 now discharges, via R30, 31, RV11 and D11 at a rate set by the sustain drawbar RV11. The output of the envelope shaper is now negative-going and this gradually closes the VCA until there is no further sound (remember that the tone outputs of the M147 remain on even when no pedal is pressed). As in all VCAs in the organ, a preset (in this case RV13) is connected via a resistor to pin 13(4) of the VCA to remove any DC offset present at the output when the VCA is keyed.
All three footages from IC2 are connected via D1, 2 and 3 to the bass guitar filter, a complex low-pass filter-mixer consisting of R11 to 16 and R38 and C2, 3 and 4. This produces a characteristic bass guitar sound. The output of this filter is fed direct to pin 3, the signal input of IC3a, the bass guitar VCA. The control voltage to pin 1 comes from the bass guitar envelope shaper.
When any pedal is pressed, a negative pulse around 10ms long, from pin 2 of IC1, is connected via R24 to TR2 causing it to switch off momentarily which charges C11 via R25 and D10, and C12 via R26. C12 rapidly discharges again whilst C11, whose charge does not rise as high as C12 because of D9, discharges more slowly. This produces the early "thud" and longer sustain characteristic of a bass guitar. The output of the VCA is fed via RV12, the bass guitar level control.
The M108 in the lower manual is used in both the "single" and "split" modes. In future we shall refer to the "single" mode as the manual mode and the "split" mode as the automatic mode. The keyboard connections are identical to the upper manual, but the functions associated with octave bar 6 differ as follows. Pin 24 of the M108 (IC4) is not connected and this allows the outputs of the rhythm generator to be active low. Pin 26 and 27 go via D83, 84 and S24B, the auto/manual switch. When this switch is not operated, the manual and 49-note keyboard modes are selected. When the switch is operated, automatic and split-key-board modes are selected. Pin 28 is permanently locked on by D78 so that when a one-fingerchord is released, the whole chord remains on in order to provide an output during sustain time.
Pins 30 and 31 are connected via D72 and 73 to the pedalboard keys low C and D. In automatic mode, depressing low C changes a major chord to a minor chord and depressing D changes a major chord to a 7th chord. Pressing both together would generate a minor 7th chord. The countermelodies and bass runs are also changed accordingly.
The manual mode functions of the M108 in the lower manual are the same as those for the upper manual. However, in the lower manual the M108 is used in its automatic mode as well and the following pins are used. In the manual mode, pins 4, 5 and 6 are the footage outputs for the lower half of the lower manual but in the automatic mode they and pin 3 become the four notes of an 8ft chord.
Pins 8, 9 and 10 are the input from the rhythm generator. After decoding the M108 generates a bass line which is output on pin 7. Pin 11 is not used in the Matinee. The output from pin 12 changes level whenever the code into pins 8, 9 and 10 changes and is used to initiate the bass envelope shaper.
In manual mode pin 15 gives a DC level when any key is pressed, but in automatic mode any key pressed on the lower half of the manual gives a DC level from pin 14. In the Matinee we do not need these two signals separately so these two pins are linked together via D47 and D48.
In Part 5, we shall describe the construction of the cabinet and explain how everything fits in. Ready-made cabinets (in flat-pack form) will also be available at this time from Maplin Electronics Supplies Ltd. We shall also continue our description of the lower manual and rhythm generator circuits.
[Please check the corrections listed in part 6 of this series - where possible, corrections have been applied to the text but in some cases there are circuit diagram amendments.]
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