Matinee Organ (Part 5)
A complete electronic organ to build at low cost
PART 5: Cabinet construction; lower manual and rhythm generator circuits
If you wish to make your own cabinet, cutting details and requirements are given in Table 6. Construction details are shown in Figure 36. Fix the swell pedal to the baseboard before starting to put the panels together. The cabinet can now be completed except for the top and the back. The ready-made cabinet comes complete with a roll-top that runs in plastic guide slots fitted into routed out slots in the cabinet sides. The roll-top and guide slots are available separately for those who wish to make their own cabinet but since few constructors have access to routing machinery, it is not easy to fit and we have not shown details in the construction drawing.
If you use the guide slots you will need to cut off the forked end 465mm from the closed end and let them into the sides of the cabinet. Also, the dropped front of the top will need to be shortened. Alternatively, another inner side-piece could be fitted, on whose edge the roll-top would run and in this case it will be necessary to widen the whole cabinet slightly. However, if you feel the roll-top is essential, we strongly recommend using the ready-made cabinet.
Install the trim and housing around the swell pedal, then fix the pedalboard and bolt the loudspeaker to the baffle board. If making your own cabinet, this is a suitable time to fit the grille cloth. Remove the transformer fixing bolts and place the PSU module in position on the base. Mark the four holes and drill through the base using a 4mm or 5/32in. drill. Note that these are the only four fixings that are not provided in the ready-made cabinet due to variations in the transformer frame size.
Screw down the terminal block as shown in Figure 12. Fix the P-clip to the 3-core mains cable and screw down near the terminal block. Fix the reverb spring-line, using the rubber couplings, either to T-nuts fitted in the ready-made cabinet or as shown in Figure 36.
Place the keyboard roughly in position. Remove the screws supplied with the keyboard assembly from where they are fitted in the top of the keyboard cheek mounting supports. Placing the keyboard cheeks in position now determines the exact position for the keyboard. Carefully remove the cheeks, screw down the keyboard and refit the cheeks screwing down from above. Hinge up the top keyboard and remove the screws in the frame. Place the keyboard separator in position and screw up.
Push the mains switch into the pre-cut hole in the side cheek and carefully reconnect the wires. Fix the metalwork (supplied with cabinet kit), as shown in Figure 37. The screws into the sides of the cabinet should not be tightened to allow for adjustment when the PCB and front panel are in position.
Since describing the main PCB construction in Part 3 we have discovered that it is much easier to put the drawbars together before soldering the slide pot to the PCB. Put the M3 bolts through the frame after placing one of the special 4BA washers under the head of the bolt through the slot as shown in Figure 38. Hold the unit upside-down and place four of the special washers on each bolt; then place the slide pot upside-down over the bolts, align and screw up. Now solder the whole assembly to the main PCB.
Snap the latch buttons onto the latchswitches. S16, 24 and 33 are red, S22, 23, 34 and 35 are grey, S26 and 27 are white and the remainder are black. Cut the shafts of the rotary pots to 22mm long. Bolt the PCB onto the metalwork, then pull the covers off the rocker tablets S17 to 21 to enable the front panel to be fitted.
Fit the LED holders to the front panel then snap the LEDs into the clips from behind. The red LED fits in the 'Downbeat' hole and the green LED in 'Tempo'. Push the drawbars fully back, then fit the front panel and bolt up to the metalwork. Carefully realign the metalwork frame then remove the front panel and PCB and tighten up the end screws.
Re-fix the main PCB and reconnect all the cables. Bolt the headphone socket to the front panel, then refit the front panel. Replace the covers on the rocker tablets. S17 and 18 are grey, S19 is orange and S20 and 21 are red. Push the three knobs onto the shafts of the rotary pots. Doublecheck that all plugs and sockets are correctly connected. If using the ready-made cabinet kit, fit the roll-top as described in the instructions supplied with the kit. Fix the top and the back to the cabinet. The Matinee organ is now completed.
Figure 39 is the circuit diagram of the lower manual. In Part 4 we described the circuits around the lower manual M108 and this is now continued. There are eight signal outputs from the M108, all connected to quad op-amps IC5 and 6 that are used to raise the signal levels. The footages are connected through CMOS switches, IC8 and 10 to the appropriate flute and string filters. These each have two inputs mixed together in the manual mode, except for 8-foot from IC4 pin 5 which does not require the CMOS switch. In automatic mode, the footages from the upper half of the keyboard (IC4 pins 16, 17 and 18) are cut off by the CMOS switches, and IC4 pins 3, 4, 5 and 6 now become four notes of an 8-foot chord when one key is pressed in one of the two lower octaves of the keyboard.
The 16-foot and 4-foot from the lower half of the keyboard are disconnected from their filters and the four notes of the chord are combined in R84, 85 and 86 and mixed together in IC9. The output of IC9 in automatic mode is connected through a CMOS switch into the 8-foot string and 8-foot flute filters. The note on the original 8-foot line is mixed with the other three notes at the input to the 8-foot string and 8-foot flute filters.
In the manual mode the lower manual circuits are much the same as for the upper manual.
The 4-foot flute filter is built around IC11a, 8-foot flute around IC11b, 16-foot flute around IC11d, 4-foot string around IC12 and 8-foot string around IC13. In the manual mode the outputs of the flute filters are connected through the mixer IC11c to the flute VCA, IC15. The 8-foot string filter is connected to its VCA IC14b and the 4-foot string filter is connected to its VCA, IC14a. In the automatic mode the 4-foot string VCA becomes the countermelody VCA and the 8-foot string VCA becomes the vamp VCA when the vamp button is pressed.
In the manual mode all three VCAs are controlled by the flute envelope shaper (TR5 and 6 and associated circuitry) which is itself controlled by KPS. If memory is pressed, the KPS signal is overridden by an earth on the cathode of D53 which pulls TR5 on permanently and the last keys pressed sound indefinitely. When the vamp button is pressed, all three VCAs are controlled from the vamp envelope shaper TR9 and 10. However, the vamp only plays when keys are pressed and the rhythm unit is switched on.
When KPS is high (all keys released), both the vamp and countermelody envelope shapers are inhibited to prevent pulses from the rhythm generator triggering them. When KPS goes low or memory button is pressed, the bias is removed from D55 and 66 and the pulses from the rhythm generator are now able to trigger the envelope shapers.
With the "auto" button pressed, the flute envelope shaper is still controlled by KPS as is the 8-foot string envelope shaper. However, the 4-foot string VCA is now controlled from the countermelody envelope shaper, TR7 and 8. The countermelody is generated from the four notes of the chord produced by IC4. These are switched through to the mixing resistors R69 to 72 by IC7 under control individually of four outputs from the rhythm unit.
From the mixing resistors, the countermelody is fed to the 4-foot string filter and thence to the 4-foot string VCA.
If the auto and vamp buttons are both pressed, the flutes and 8-foot string VCAs come under control of the vamp envelope shaper whilst the 4-foot string remains under the control of the counter-melody envelope shaper.
The vamp envelope shaper is triggered by positive-going pulses from the rhythm unit. These pulses turn TR9 on and C41 charges through R169. Whilst C41 is charging, TR10 is turned on. When the input pulse is not present, TR9 turns off, C41 discharges through D62 and then TR10 turns off. Thus we now have a pulse at the junction of D63 and 64 of the length we require and at the required level. C42 now determines the attack and decay of the envelope in the same way as the other envelope shapers we have described previously.
The rhythm unit has four of its outputs connected via R80 to 83 to TR11 to 14 which act as level shifters and inverters. When a negative-going pulse occurs on one or more of these lines, the transistor turns on and drives the appropriate CMOS switch in IC7 which connects one or more of the notes of the automatic chord through to the countermelody VCA.
At the same time, any pulse is also fed through D15 to 18 and D74 to trigger the envelope shaper. The resultant positive going pulse turns TR7 on during the charge time of C38 and the collector of TR7 going low turns on TR8 which charges C39. When the pulse goes off, C39 discharges via R162 which sets the discharge time. This envelope shaper is a high speed version of the others in the organ, designed to respond to rapid input control signals, retriggering each time.
As the rhythm generator can be used in manual mode, the countermelody must be inhibited when auto is not selected and this is achieved by preventing the cathodes of D15 to 18 from going high which stops the CMOS switches in IC7 from turning on. The 'auto/manual' switch S24 when in auto mode presents a -6V level to the junction of R91, 139. In the manual mode, this -6V is not present and R139 pulls the junction of R91, 139 to +6V which turns on the CMOS switches in IC8 and 10a, b and c. This also turns on TR4, via R91 which turns off IC10d and this turns the automatic chord lines off. The collector TR4 being low provides the -6V that allows D88 to conduct and turn off the countermelody also.
The circuit diagram of the rhythm unit is shown in Figure 40. All the rhythms consist of two bars of up to 32 counts total and at each count, any of the 16 outputs can be enabled. Four of these outputs provide a continuous level until changed (countermelody), whilst the remainder simply provide a pulse at the appropriate time. The information is stored in a pre-programmed 16K EPROM. As the EPROM has only 8 output lines, for each count the EPROM is addressed twice.
IC29 is a dual monostable that provides the clock pulses from which the rhythm generator is timed. The timing consists of a fixed duration high period set by C53 and R230 and a variable duration low period set by C53 and R232 and the tempo control RV51. The output is from pin 7. IC27 provides high impedance points to allow the timing circuit to function correctly.
Referring to Figure 41, positive transitions of the clock pulse first enable all the gates in IC21 and 22 during the charge time of C58, thus transferring the information standing on the outputs of the EPROM to the outputs of the gates. Second, they reset the four countermelody latches IC19 and 20 and third, enable IC26b which we shall describe later.
The leading edges are also inverted by IC27c, a Schmitt trigger, and delayed by R224 and C47 to allow time for the above operations to take place. After the delay, IC27b further inverts the signal and the positive levels are applied to IC24 pin 8. This is the least significant address line of the EPROM and allows the second set of 8 bits to be made available at the outputs of the EPROM.
By this time all the gates in IC21 and 22 have been disabled and the positive-going transition at IC27b output has been inverted by IC27a. The signal is now delayed by R225 and C49 and inverted again by IC27f. This positive-going transition enables all the gates in IC18 during the charge time of C44 and clocks the four latches in IC19 and 20. This is also connected to the input of IC25, a 7-stage ripple counter.
The positive-going pulse at the output of IC27f is delayed by R226 and C50 and inverted by IC27d. It is turned into a pulse by R227 and C48 and applied to IC23 pin 8. When the clock goes negative, the least significant address line of the EPROM returns low and this negative transition steps IC25 which moves the EPROM to its next address ready for the next positive-going clock pulse.
IC25 addresses the five next least significant address lines for each of the 32 possible counts. The four next address lines are directly coded from the rhythm selection switches via the diode matrix D19 to 46. These lines are normally held low by R186 to 189, but go high via the diodes when selected. S16 switches the most significant address line to select the second set of 15 rhythms.
Two methods are needed to reset the counter to count one (count one is the downbeat of the first bar, bearing in mind that there are two bars in each rhythm) so that a rhythm can be any length to generate unusual time signatures such as 5/4 time. A reset always occurs if the counter reaches maximum and for rhythms requiring fewer steps a non-valid output option (short and long cymbals together) is coded. This corresponds to pins 16 and 17 of the EPROM being high simultaneously.
Thus the output of IC23a goes low, is inverted by IC23d and applied to IC23c, the other input of which is only high for a short period after the counter has returned the least significant address low and moved the counter to the next EPROM address. So, if a reset is required after count 24, for example, the non-valid output option must be coded in count 25. This all happens very quickly, whilst the delay set by the tempo control occurs and ensures that the next beat is the downbeat.
If the output of IC23c goes low or pin 4 of IC25 goes high, thus indicating the end of 32 counts, then after an inversion by IC26d, the output of IC23b goes high and resets IC25 via D106 and IC28d is enabled.
When S22, the rhythm start/stop switch is pressed, S22B connects a low level via S23C normal, to the cathode of D110 which pulls the junction of R236 and C54 low. The pulse produced by C54 charging pulls pins 5 and 8 of IC28 low, resetting the clock and causing the start/stop latch IC28a and b to change over giving a high on pin 4. This removes the -6V that has been inhibiting the output gates and latches via D102 and 104 and allows the rhythms to sound. It also resets the counter IC25, so that rhythms always start on the downbeat of the first bar.
If S22 is released and S23, the auto start/stop switch is pressed, the -6V is removed from IC28 pins 5 and 8 as before. This allows the clock to run, but as pin 6 of IC28 is held high by KPS from IC4, the start/stop latch remains at stop. When a key on the lower manual is pressed, KPS goes low and the start/stop latch changes over. The pulse produced by C54 in this case, ensures that the clock starts in the right phase.
When the keys are released, KPS goes high which allows the high from R236 via D110 to turn the start/stop latch off. This ensures that the rhythm starts on the downbeat of the first bar when a key is pressed, stops when released, yet allows the tempo and downbeat lamps to flash even when the rhythm is not sounding, so that the tempo may be set prior to playing.
If S22 and S23 are both pressed, the rhythm starts when you press a key, but does not stop when the keys are released until a pulse via C55 from IC28d pulses IC28 pin 1 low and sets the start/stop latch to stop. This occurs at the end of the second bar as previously described.
The downbeat and tempo LEDs are controlled by the output from the counter IC25. The down-beat LED always flashes on the first count. The second flash occurs halfway through the count (except for 5/4 time) which can be either 24 or 32 counts. For unusual time signatures the second flash does not occur in the right place but we considered this was relatively unimportant compared with the facility of being able to generate unusual time signatures. The second flash therefore occurs at count 13 or count 17. This is selected by the strapping behind the first six rhythm switches S1 to 6. The unit is set for 32 counts if no connection is made and for 24 counts if a connection is made.
The count is decoded by D95 to 98 for the 32 count rhythms and D56 and 71 for the 24 count rhythms. The seventeenth count is pins 6, 9, 11 and 12 of IC25 going low together, which allows the input of IC26a to go low via R221. Pin 3 of IC26 now goes high, thus pulsing the input of IC26b high. If this corresponds with a high on pin 6 of IC26, pin 4 goes low thus removing the high from the input of IC26c which produces a high that turns on TR19 via R240 and this turns on D150 via R228. The length of this flash is determined by the charge time of C51 and R239.
The thirteenth count is pins 6 and 9 of IC25 going high simultaneously which means that the cathodes of D56 and 71 are allowed to go high via R37 and the 24-count strap. This positive-going transition is connected via D69 to C46 as in the previous case. In this case, however, the discharge time for C46 exceeds the maximum length of time possible between counts 13 and 17 and thus the flash that would have occurred at count 17 is inhibited.
The tempo LED, D151, is triggered by pins 11 and 12 of IC25 going low together, thus allowing the cathodes of D27 and 28 to be pulled low via R210, thus turning on TR18 which causes the green LED to flash via R163.
The four latches in IC19 and 20 hold the countermelody information for each count of the rhythm. IC18c provides the vamp output to the vamp envelope shaper and IC18a, b and d provides the outputs to control the bass codes of the M108. TR15 to 17 act as inverters and level shifters. The M108 recognises this negative logic 3-bit code and internally latches it and produces a bass note and bass envelope trigger pulse TDB. The automatic bass output from IC4 pin 7 is amplified by IC6a and fed to IC2 pin 11 (see Figure 34).
Since all the CMOS in the rhythm unit is designed to run at +5V relative to -6V it is necessary to generate a -1V rail (which effectively becomes +5V). This is achieved by feeding the 0V through D87 which drops about 1V. This rail is smoothed by C206 and 207.
In the final part, next month, we shall describe the rhythm generator voicing, the audio mixing stages, the wah, rotor sound, reverb and power amp circuits to conclude the circuit descriptions. We shall also list any amendments and errors that have occurred in the series and give some playing instructions.
[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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