The Matinee Organ (Part 3)
A complete electronic organ to build at low cost
PART 3: Construction of the Power Supply Unit and Main PCB
In this part we shall describe how to make the other two printed circuit boards in the Matinee: the power supply and the main PCB. Once these are complete, there are only 40 to 50 wires to be connected and the electronic construction is finished. First we shall look at the power supply which drives the whole organ.
Fit the twelve Veropins in the positions marked by the large circles around the holes on the PCB. Push the pins firmly into the PCB from the track side and solder them. Insert the resistors, capacitors, presets and bridge rectifiers referring to the parts list and legend on the PCB. Ensure that C1,2,3,7,8 and BR1 and 2 are put in the right way round. The five capacitors have an indent around one end of the body and this is the positive end. Solder all these parts to the PCB. Do not mount the regulators at this stage.
The power supply unit (PSU) bracket should now be made as shown in Figure 11. This is supplied ready-made in the kit but may be bought as a separate item. Cut about 10cm off the piece of 32-strand brown wire and strip a piece of sheath off each end. Twist the strands tightly together at each end, apply the soldering iron and quickly run solder onto the strands to hold them together. All the wires used in the Matinee should be prepared first in this way.
With no fuse in the fuseholder, wrap one end of the brown wire around the end tag and solder. Fit the fuseholder to the PSU bracket, then fit the grommet in the end hole as shown in the photograph. Take the piece of two-core mains cable and strip off about 10cm of the outer sheath. Strip and tin the blue and brown wires as described earlier then push the cable through the grommet and solder the brown wire to the other tag on the fuseholder.
Cut two 30cm lengths off the 32-strand green wire and 20cm off the black, blue, brown, red and white wires. With reference to Figures 12 and 13 connect and solder the two green wires to the tag marked 'SCR' on the transformer and the other five wires to the five tags on the other side of the transformer. Bolt the transformer to the PSU bracket using 4BA nuts, bolts and washers as shown in the photograph.
Take one of the green wires out of the bracket through the grommet then solder the blue wire from the mains cable to the tag marked '0' next to 'SCR' on the transformer. Solder the brown wire from the fuseholder to the tag marked '240' on the transformer. Twist the two wires from the 0 and 22V tags together and twist the other three wires together.
Fit the three regulators to the PCB; note that the type numbers are very similar, so take care to fit the right one in the right position. (The pin out details are shown in Figure 14). The leads of the regulators have to be bent at 90° (see Figure 15) then soldered to the PCB so that the holes in the metal tabs are directly above the holes in the PCB. The metal back of the regulator needs to be about 2mm from the board. Smear the mica washers with Thermpath or any silicone grease, then fix the PCB and regulators to the PSU bracket as shown in Figure 15. Bolt the PCB to the PSU bracket using two 6BA nuts, bolts and washers in the two outer holes. One of these bolts connects the 0V on the PCB to the chassis.
Connect the green wire from 'Scr' on the transformer to the top left-hand pin on the PCB. Note that the white wire from the other side of the transformer also connects to this pin. Connect the other four wires as shown in Figure 13. Finally strip and prepare one end of the remaining piece of each colour of the 32-strand wire and connect to the remaining seven pins on the PCB as shown in Figure 16.
Now check all the connections, referring to the diagrams. Ensure that no strands of wire are sticking out anywhere. It is most important that all wires are connected as described. Do not attempt to cut down on the numbers of wires by connecting 0V points together. All the wires shown are essential. It is equally important not to add additional wires. Do not link 0V points together unless the diagrams show that a wire is fitted there. It is particularly important to note that the two points marked '0V' on the secondary of the transformer must not under any circumstances be connected together.
We have deliberately designed the PSU bracket so that the connections to the transformer primary and the fuseholder are rather inaccessible. All the dangerous voltages in the PSU are in this area and although this unit will normally be inside the organ cabinet and out of the way, whilst testing is in progress it will be accessible and we have therefore taken precautions. Using the folded PSU bracket, all the tags at mains potential are both tucked away and close to the earthed chassis.
The circuit shown in Figure 16 operates as follows. Mains (live and neutral) via the mains switch S37 are connected to the primary of T1 which is protected by a 1 amp fuse (FS1). The transformer has two secondaries, isolated from the primary by an electrostatic screen.
One secondary is 22V AC that is full-wave rectified by BR1 and smoothed by C1. This produces approximately 30V, DC. An unregulated output from here supplies the audio power amplifier on the main PCB, and the input to REG 1 is also taken from this point. REG 1 is a 15V, fixed voltage regulator and C4 is provided to inhibit high frequency oscillation and promote stability. This 15V supply is used in the reverberation spring driver circuit on the main PCB.
The other winding on the transformer provides an 18V, AC centre-tapped supply. The 18V is full-wave rectified by BR2 and the positive output is smoothed by C2 and fed to REG 2. This is a variable, positive-voltage regulator whose output voltage is set by the voltage on its control pin. This can be adjusted by RV1 whose range is limited by R1 and R2. The +6V output is smoothed and stabilised by capacitors C5 and C7.
The negative output of BR2 is smoothed by C3 and fed into REG 3. This is a variable, negative voltage regulator, otherwise similar to REG 2. Its voltage is adjusted by RV2 and R3 and R4 limit the range. C6 and C8 smooth and stabilise the -6V output. The 6-0-6V output from the PSU form the main voltage rails for the organ, driving the vast majority of the circuits on the main PCB.
Well over 90% of the circuits and components in the Matinee are located on the main PCB. This very large board measures over 3ft by 10in and has track on both sides. The board is printed with a solder resist which will help constructors keep solder off parts that should not be soldered. The component side of the PCB, which we shall call the topside, has most of the component designations marked on it and the switch side, which we shall call the underside, has a few switch designations and many pin circles marked on it.
In many places the track on one side of the PCB has to be connected through the board to the track on the other side. The board could have been made with plated-through holes but this is a very expensive process unless the board is manufactured in vast quantities. This board would have cost over £30 more than it does, had it used plated-through holes. In our case the tracks have to be linked with track pins. These pins are supplied in strips and are inserted from the underside as shown in Figure 17. After snapping off the strip (17b) push the pin firmly into the PCB. Insert ten pins, then solder on both sides. By inserting and soldering in batches of ten a balance is achieved between constantly changing jobs and doing so many at once that one pin somewhere misses being soldered. The positions for all the track pins are marked with circles on the PCB.
A few holes are marked on the underside of the PCB with little squares. Veropins are fitted in these holes. Ensure that they are pushed firmly into the PCB then solder on the underside only. Four Veropins have to be put in the PCB from the topside. Again they are marked on the PCB with little squares, but are soldered on the topside only.
We are now ready to start fitting the components to the PCB. It is advisable at this stage to sort out all the components, first to check against the master component list (supplied with the kit) that you have received the correct quantities of all the parts, and second, to help as a double-check that you have put the correct parts in the correct places. The resistors are the most difficult things to sort, but this may be speeded up by sorting first into piles of resistors having the same colour third band. This is the band next to the gold tolerance band that is on one end of every resistor. Before you start, make sure that you can differentiate between the gold, yellow and orange colours printed on the resistors. An identification chart is supplied with the kit to make it as easy as possible for you.
All the following components are fitted to the topside of the PCB and soldered underneath unless stated. Fit all the resistors, double-checking continually that you are putting the right values in the right places. Fit all the diodes ensuring that the end with the band around the body or, if there are several bands, the end with the thicker band, is inserted next to the '+' marked on the PCB.
Next fit all the capacitors. The tantalum types have a '+' marked on the body above one lead and are inserted so that this lead is next to the '+' on the PCB.
The electrolytics are fitted so that the lead at the end having the identation around the body is next to the '+' on the PCB.
The transistors a look very much alike so make sure you have sorted them correctly before starting to fit them. It is most important particularly with the BC212L's that, when viewed from above, the D-shaped top of the transistor corresponds with the 'D' shape printed on the PCB. This may necessitate bending the centre lead of the BC212L forwards or backwards to suit.
Fit the four integrated circuit sockets in the positions marked IC1, IC4, IC24 and IC44. Solder chokes L2 and L3 to the PCB, then bolt the rest of the chokes to the PCB as shown in Figure 18. The two flying-leads must be soldered to the adjacent Veropins but it does not matter which lead is connected to which pin. Fit the presets, then mount the tablet rocker-switches. The two on the right are red, the two on the left are grey and the centre one is orange.
There are three switch-banks to make up: a 16-way; a 9-way and a 5-way. First, assemble the 16-way bracket. Hold the bracket as shown in Figure 19 and place a 2-pole latchswitch (12 tags on each side) in the extreme left-hand position with the printed circuit tags downwards and the solder tags upwards. Bend over three of the four lugs on the mounting frame (leave the front-left one) to hold the switch in position. Fit another 4-pole latchswitch in the next position in the same way, then slide the 15-way return bar into the frame so that the bent-up lug slides into the slot in the side of this switch. Now fit a further 13 identical switches. Before fitting the last switch, carefully press the return spring between the two plastic mouldings as shown in Figure 19. After fitting this switch, carefully lift the rear of the detent spring with a small screwdriver and hook it over the left-hand side of the switch. Do not remove the detent spring altogether. Repeat this on all the switches except the extreme left-hand switch.
Test the switchbank. Press any one of the right-hand fifteen switches. It should lock in and at the same time release any other of these fifteen switches previously pressed. Check all fifteen switches. The extreme left-hand switch however, should lock in when pressed once and release when pressed again without affecting, or being affected by, any other switch in the bank.
The 5-way switch bank does not require a return bar or return spring; each switch locks and releases independently. Fit two 2-pole latchswitches (6 tags on each side) to the left of the bracket and 4-pole latchswitches in the three positions to the right. Do not move the detent spring on these switches.
Assemble the 9-way switch-bank with a 4-pole switch in the most left-hand position, then a 6-pole switch (18 tags on each side), then a 2-pole switch, then another 2-pole switch. Now slide in the 6-way return bar and continue fitting the switches; a 6-pole next and then four 2-pole switches. On the most right-hand switch, fit the return spring as before. Move the detent springs to the left on the six most right-hand switches then test the switch-bank. These six switches should interlock whilst the other three should lock and release independently.
The three assemblies should now be carefully fitted to the PCB ensuring that all the pins seat tightly down on the PCB. These switchbanks mount on the underside of the PCB and are soldered on the topside.
Remove the slide-pots from the drawbar assemblies and fit the pots to the PCB. Ensure that they seat tightly on the PCB and are perfectly upright when soldering.
The pot. mounting bracket should now be made as shown in Figure 20. This is supplied readymade in the kit but can be bought separately. Fit the three rotary pots to the pot mounting bracket with the tags down and the spindles on the same side as the feet. The dual-gang pot mounts in the centre, the 10k log above the larger foot and the 470k lin above the smaller foot.
Cut twelve pieces of hook-up wire (7-strand) each 36mm long and strip 10mm of sheath off each end, so that the insulated piece is 16mm long. Solder one piece to each of the twelve tags on the three pots. Bolt the bracket to the PCB using 6BA nuts, bolts and shakeproof washers through the two end holes. Connect the twelve wires to the twelve pins on the PCB. Each pin is directly beneath the tag on the pot to which it is to be linked.
Fit IC16 and align it so that the hole in the tab is directly above the remaining hole on the pot bracket. Smear the back of the IC with Thermpath or any silicone grease then bolt down using a 6BA nut, bolt and shakeproof washer. Note that no mounting kit is required with this IC.
Fit IC38 noting that no heatsink is required. Now solder the rest of the ICs to the PCB. At this stage, do not unpack the four ICs that plug in the IC holders. Finally, fix the nine plugs to the PCB taking care to ensure that they are the right way round as shown on the legend and in the photographs.
One connection now has to be made on the underside of the PCB by a piece of screened cable. Each end of the cable is connected to one of the pairs of pins on the underside of the PCB as shown on the legend and in Figure 21.
We shall describe the rest of the interwiring of the Matinee in Part 4 but for experienced constructors we include in this issue the interwiring diagrams, Figures 22 and 23, so that virtually all the construction may be completed in one go if desired.
The Banjo Repeat Circuit is part of the upper manual circuit, and is shown in Figure 24.
The input to the banjo envelope shaper goes via the banjo repeat circuit and is triggered by TR43 under the control of the unijunction oscillator TR42. The rate of oscillation depends on the charge on C130 and this is determined by the setting of the banjo repeat rate drawbar. The range of this drawbar is from no repeat to about twenty repeats per second. When a key is depressed KPS goes negative and TR41 turns off.
C130 now begins to charge via R386, R387 and R390. TR40 is momentarily switched on and forces C130 to charge more quickly through D139 and R389 allowing the oscillation to begin. Otherwise there would be a noticeable decay before the first note was heard after a key had been pressed. The oscillator is now under the control of C130, R386, R387, R390 and RV33. If RV33 is off then TR42 fires just once because of the pulse from TR40, but is then held just off by the values of the resistances in the normal charging chain. The output of the preset voices VCA goes to the effects volume drawbar: RV17.
The preset voice cancel switch SW27, in addition to inhibiting the control voltage to the upper manual organ voices when a preset voice is selected, also prevents the preset voices VCA from operating when SW27 is operated i.e. when no preset voice is selected.
In Part4, we shall describe the interwiring, and setting-up of the electronics. We shall also describe the pedalboard circuits and begin to describe the circuits of the lower manual and rhythm generator.
[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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