MOTM-420 User Manual
Contents
1. SSM 2220 E PC BOARD SIDE VIEW g Locate the 5 pieces of RG 174 black coax cable Again note that one end has longer wires stripped than the other The short ends will go in the board in positions J1 J4 to J7 Look at the pc board Notice that in the coax positions there is a large hole pad lower pad and a smaller pad top hole The braided wire is soldered into the larger hole The smaller inner conductor goes in the top hole BE SURE THE SHORTER BRAIDED END GOES INTO THE PC BOARD The 1 LONG coax cable solders into J1 1V OCT IN The 4 SHORT coax cables go into J4 J7 Solder each coax cable into the holes Attach a tie wrap to secure the coax cable flush to the board The tie wrap goes down into the left hole and up through2. the Minimoog has switched tracking intervals The net effect of not tracking is the timbre will change as you play up and down the keyboard This may be a good thing USING THE MOTM 420 WITH OTHER GEAR You can patch into the audio inputs from just about anything The only thing to remember is the end to end gain at zero resonance is about 2 4X Of course this is with the IN pot at 10 The built in audio mixer of the 420 comes in very handy for adjusting gains Unlike other MOTM modules there is no real minimum voltage input requirement The limiting factor is the noise floor of the op amps These op amps used in the 420 are older bipolar types and not as clean as the OP 275s used in other modules Why are they there To get that MS 20 sound Experiment You can spend hour and hours playing with all the possible setting and audio inputs SYNTHESIS TECHNOLOGY PAGE 19 MOTM 420 ASSEMBLY 6 25 99 WWW SYNTHTECH COM SPECIFICATIONS MOTM 420 Voltage Controlled Filter Control Voltage input levels 13V to 13 Cutoff frequency range 1Hz to 28Khz Output impedance Filter modes Audio input level Filter gain CONTROLS FREQ RES FM1 FM2 IN 1 2 3 MODE GENERAL Power Supply Size Depth behind panel SYNTHESIS TECHNOLOGY MOTM 420 ASSEMBLY 6 25 99 WWW SYNTHTECH COM 1000 ohms nom LP and HP 12dB oct Notch 6dB oct 50mv pk pk min 12V pk pk max 2 4 nominal RES 0 Input level 10 se
3. SYNTHESIS TECHNOLOGY PAGE 18 MOTM 420 ASSEMBLY 6 25 99 WWW SYNTHTECH COM USING YOUR MOTM 420 WITH OTHER MOTM MODULES 1 The basic patch The most basic synth patch is as follows a VCO outputs into the filter IN jacks b Keyboard voltage into the 1V OCT jack c MOTM 800 EG output into the FM1 jack d OUT of the 420 into a mixer amplifier 90 of synth sounds are tweaking this one set up can you say Minimoog Set MODE to LP set RES to 4 and start turning up the FREQ Adjusting the FM1 pot set the amout of filter sweep and with the reversing feature you can sweep up or down Experiment with different VCO waveforms adding noise and filtering the output of a MOTM 120 2 Using 2 MOTM 420s You can get very complex and interesting sounds by using 2 4205 a Sweep one filter up while the other sweeps down This is effective in NF mode to get phasing effects Use a LFO or MOTM 300 in low speed and the FM1 pots to set the relative sweeps Use 2 different sweep rates to get even more lush effects this is the basic premise used in the 410 filter b Emulate the MS 20 by setting one filter to HP and one to LP c Audio inputs plugging into the first filter will get attenuated differently that audio inputs plugging into the second filter This can make for some bizarre stuff d You DO NOT have to use the keyboard CV into the filters You can use FM1 or FM2 instead to have variable tracking
4. In order for solder to stick to the copper a chemical called flux is embedded in the solder The flux leaves a residue on the pc board that should be cleaned with warm water DO NOT USE SOAP OR OTHER CLEANSERS Most of the parts in the kits are waterproof and can be washed in the sink The flux is OSHA approved for flushing down the drain so don t worry about that A soft brush is used to gently scrub the board We recommend a fingernail brush which is about 1 x 2 and can be found for about 1 The other type of solder is called No Clean Flux because as the name implies it does not require washing This solder is used for wiring the pots switches jacks etc This solder is harder to use on the pc board because even when melted it is not very fluid about the consistency of toothpaste We will use it VERY SPARINGLY on the pc board OK let s get started on the board PART 1 SOLDERING THE RESISTORS Since there are more resistors than anything else we will start here If you do not know the resistor color code refer to the parts list Resistors are not polarity sensitive but the board will be easier to debug and look nicer if you point the first color band in the same direction for all the parts The color code is also in the README FIRST document that every customer receives with his or her first order There are 2 types of resistors in the MOTM 420 kit standard 5 1 4W parts and special precisi
5. 420 ideal for processing just about any audio signal in your studio The filter sections can be switched to form a lowpass filter a highpass filter or a notch filter Switch SW1 does this by selectively grounding audio signals to the 2 filter sections The theory behind this is beyond the scope of this discussion entire books have been written on this subject I like Analog Filter Design by M E Van Valkenburg PRELIMINARY CHECK OUT amp CALIBRATION The VCF needs two adjustments the 1V Oct tracking and the maximum Q trim Both of these trims are done by ear so an oscilloscope is not required However hours of enjoyment can be had watching the VCF output waveforms SETTING THE MAX Q TRIMMER The MAX Q TRIM sets the point on the dial for the RESONANCE pot that causes the filter to self oscillate a a suggested point on the dial for self oscillation is the 8 or 9 tick mark Set the RES pot to the desired point on the dial b Set the MODE switch to LP c Set the FREQ knob to 5 straight up d Plug an audio mixer or preamp into the OUT jack Be sure your amp is not set too loud The 420 can pop a tweeter without any problems Apply NO audio inputs to the 420 e Apply power to the 420 Slowly adjust the trimmer until you hear the filter oscillate The waveform is very close to a sine wave it s not perfect due to the capacitors not being the exact same value and the OTA s current outputs not exactl
6. 5K violet green orange R22 lea 82K gray red orange R33 lea 12K brown red orange R34 lea 39K orange white orange R35 lea 510 ohms green brown brown R36 g IC bag containing the following 12 parts lea MXL1013CN8 precision op amp U1 lea TLO72 dual op amp U2 lea JRC4558 dual op amp U3 lea JRC4556 dual op amp U4 lea LM13600AN dual OTA U5 lea SSM2220 dual matched PNP Q1 Gea 1N4148 diodes D1 D6 SYNTHESIS TECHNOLOGY PAGE 2 MOTM 420 ASSEMBLY 6 25 99 WWW SYNTHTECH COM o 0 o Misc 1 bag containing the following 6 parts 2ea Axial ferrite beads plain gray things L1 L2 lea MTA 156 power connector JP1 lea SPDT ON OFF ON toggle switch SW1 lea trimmer 20K Bourns 3386P TP1 lea trimmer 2K Bourns 3386P TP2 Knobs 7ea ALCO PKES90B1 4 Jacks 7ea Switchcraft 112A Pots 7ea containing the following lea 100K cermet Spectrol 149 VR7 100K conductive plastic Bourns 95A1 VR1 2 3 100K log conductive plastic Spectrol 148 VR4 5 6 Front panel Mounting bracket Wire bag containing the following 12 wires 4ea RG 174 coax 4 inches lea RG 174 coax 7 inches 3 wire set 22ga 6 1 2 inches white red black 2ea 2 wire set 22ga 7 inches white orange lea 3 wire set 22ga 4 inches white orange gray lea Power Cable 20 Hardware bag containing 4ea 8 32 x 3 8 black screws for mounting module to rack 4 6 32 1 2 zinc screws for attaching pc board to
7. LTER WWW SYNTHTECH COM 2 GW WG 2 GW N 2 GL SL LM13600 01 R9 R18 R34 es 0 IN 1 IN 2 IN 3 AUDIO aa
8. PART 7 FINISHING THE PCB You will now solder in the remaining parts on the pcb in preparation for wiring to the front panel USE THE NO CLEAN SOLDER BE CAREFUL g Locate the Spectrol pots 4 There are 3 marked 148 9609 104 These 3 pots are soldered into VR4 VR5 and VR6 Be SURE the pots are sitting down all the way flat on the pc board with the shafts sticking straight out square to the board Solder the 100K linear pot marked 149 into VR7 O Locate the Tempco resistor This part is mounted in a special way it goes on top of transistor Q1 Why In order to compensate for the temperature drift of Q1 the resistor must be in good thermal contact Refer to the illustration and be careful a Apply a small amount of heat sink compound to the top of Q1 Use just enough to cover the entire top without dripping down over the sides b Bend the leads of R4 about 1 8 inch away from the body at 90 degree angle c Insert the resistor into the 2 holes one to the top of Q1 and one to the bottom of Q1 SYNTHESIS TECHNOLOGY PAGE 9 MOTM 420 ASSEMBLY 6 25 99 WWW SYNTHTECH COM d Push R4 all the way down so that the bottom of the resistor is sitting on top of Q1 The heat sink compound will squish out from under R4 Wipe of any excess WARNING This stuff is next to impossible to wash out of blue jeans e Solder R4 making sure the resistor is level and TOUCHING 91
9. TTENTION AND READ ALL OF IT BEFORE PROCEEDING Note that each of the 4 pots on the board have 2 hex nuts and flat washer Remove the first hex nut and the washer Set aside What you will do now is adjust the remaining hex nuts so that when the bracket is all the way down on the panel s threaded studs all the pot hex nuts touch the rear SYNTHESIS TECHNOLOGY PAGE 11 MOTM 420 ASSEMBLY 6 25 99 WWW SYNTHTECH COM of the panel Screw by hand each hex nut on the pots so that it is all the way on touching the face of the pot Now pick up the pc board bracket assembly and carefully slide it over the 2 threaded studs making sure the pots are aligned in the holes Use 2 6 KEPS nuts and tighten the bracket to the panel Loosen the 4 KEPS nuts on the bottom of the bracket Slide the pcb ALL THE WAY TO THE RIGHT AS FAR AS IT WILL GO so that the 4 pot nuts are all pressing against the panel By hand put hex nuts on the outside threads of VR7 and VR6 to keep the pc board in place Now tighten the 4 KEPS nuts on the bracket The pcb and bracket should be secure with no gaps visible between the panel and the pot nuts You may need to loosen the nuts on the pots so that they are touching the back of the panel Again make sure each pot s nut is touching the back of the panel no gaps There will be a gap from the edge of the pc board to the panel Remove the hex nuts on VR6 amp VR7 For all of the pots first put on the fl
10. amp Solder into U4 Locate the LM13600AN OTA Solder into U5 Locate the SSM2220 looks like an IC Solder into Q1 pointing down as the rest Locate the diodes 6 Note each has a dark band on one end on the body This is the cathode and it must be placed in the holes with the band pointing to the left Solder into D1 D6 SYNTHESIS TECHNOLOGY PAGE 8 MOTM 420 ASSEMBLY 6 25 99 WWW SYNTHTECH COM g Locate the blue trimmer pot marked P 202 on it s side This is a 2K trimmer Solder into TP2 g Locate the trimmer pot marked P 203 This is a 20K trimmer Solder into TP1 Locate the 2 2200pf polystyrene caps Solder into C10 and C11 g Being careful NOT to solder the 2 pads for R4 apply a small bit of solder to the via holes These the small pads that allow traces to change sides of the board DO NOT SOLDER PADS FOR THE REMAINING COMPONENTS PART 6 FINAL BOARD WASH amp INSPECTION g Verify all the parts are in the correct locations Make sure all of the ICs are pointing the same direction and that all of the diodes point to the left g Inspect the solder joints Any solder shorts Too much solder Missing joints g Wash the board under warm water Scrub gently Dry THIS IS A GOOD STOPPING PLACE TO REST OR PUT THE KIT AWAY UNTIL LATER You are now finished with the Organic flux solder All soldering past this point is using the No Clean solder You do not have to wash the board anymore
11. at washer then the hex nut Tighten with a nut driver PART 10 FINISH WIRING TO THE PANEL Please read the following instructions carefully In order to neatly attach the many wires to the front panel components the wires are soldered in a specific order Find the SPDT toggle switch These solder to the 3 wires in SW1 on the pc board To make things go easier first you will solder the wires to the component then attach the component to the front panel Solder the Gray wire to the TOP lug the White wire to the MIDDLE lug and the Orange wire to the Bottom lug it doesn t matter which outside lug you call Now remove the outer hex nut Place the switch through the small hole labeled MODE with the Orange wire on the bottom near the phonejacks Tighten the outside hex nut but DO NOT OVERTIGHTEN Locate the 3 blue Bourns 91A panel pots These solder to the 3 wires in VR1 VR2 and VR3 However be careful because the ordering is different on the actual panel Each pot is soldered the same way Facing the rear of the pot you will see tiny indicators for each lug 3 2 1 lug 3 is to the left First slip a piece of the heat shrink tubing over the wire you may need to un twist some first Bend the wire into a small J shape and insert through the little slot in the lug Solder the wire Repeat for the other 2 wires When all three wires are soldered slide the heat shrink over the lug wire and apply heat a heat
12. ate the non polar electrolytic cap Solder into C5 Locate the 33ufd cap Note that it has a stripe on one side the terminal Solder into C12 with the stripe away from the pad Locate the 10ufd electrolytics 2 Note that there is a stripe on the NEGATIVE terminal The pc board has a on the POSITIVE terminal Carefully stick the capacitors into C14 and C15 with the stripe away from the pad on the board Wash the board again gently scrubbing both sides Use ONLY warm water PART 5 MISC and IC STUFF Almost done with the parts on the pc board This will finish up the soldering with the organic flux o OUUOU UYU UYU OU Locate the MISC 1 bag and the IC bag Locate the ferrite beads 2 They are axial parts gray colored with no markings These are non polar and are soldered into L1 and L2 by JP1 Locate the MTA 156 power connector Solder into JP1 Note that the connector has a locking tab on one side This side is the inside facing relative to the pc board Note the silkscreen symbol for JP1 has a line on one side indicating this is the side where the locking tab goes Locate the MXL1013 op amp Solder into U1 Note that Pin 1 is the square pad Pin 1 is the pin near the very small dimple in the top of the part All of the ICs point to the bottom on the pc board Locate the TLO72 op amp Solder into U2 Locate the JRC4558 op amp Solder into U3 Locate the JRC4556 op
13. bracket 4ea 1 4 inch aluminum spacers 6 KEPS nuts 2 for attaching bracket to front panel 4 for board small tie wraps small heat shrink tubing Organic Solder No clean Solder PC Board MOTM 420 Tn addition you will need a very small amount of silicone heat sink compound Available from Allied Newark Mouser Maplin and many others SYNTHESIS TECHNOLOGY PAGE 3 MOTM 420 ASSEMBLY 6 25 99 WWW SYNTHTECH COM GENERAL INFORMATION Thank you for purchasing the MOTM 420 Voltage Controlled Filter VCF If you have any issues concerning the building or use of the kit please contact us at 817 281 7776 or by email syntht yahoo com This kit should take the average builder between 3 to 4 hours The VCF kit contains many different resistors and special parts However please remember this is NOT a speed contest it is an accuracy contest There is no rule that you have to complete the entire kit in one day as long as you wash the flux off Successful kit building relies on having the proper tools Here is a list of what you will need to build your 420 Soldering iron 5 0W max power A heat shrink gun optional but highly recommended Needle nose or chain nose pliers Diagonal cutters Allen key set for securing the knobs 1 16 or 1 6mm Magnifying glass to read the capacitor codes and to inspect solder joints Lead bending tool optional but makes the job go much faster DVM Digi
14. e board DO NOT SOLDER this part until told so You have been warned OUuUUdUOdU Uo U Locate the 4ea 100K 1 resistors and solder into R1 R2 R3 and R10 all near U1 Locate the 44K2 1 resistor and solder into R5 by TP1 Locate the 475K 1 resistor and solder into R6 above U2 and C3 Locate the 221K 1 resistor and solder into R9 Locate the 51K1 1 resistors and solder into R11 R12 and R19 all by U1 STOP Save the big black tempco resistor for later DON T LOSE IT PART 3 BOARD WASH 1 o 0 o 0 Verify all the resistors are in the correct position Verify all the resistors are flat on the board Correct if needed Check solder joints Wash the board in warm water gently scrubbing both sides Shake the board a couple of times blot dry with an old towel the leads will frazzle a good towel Let dry at least 15 minutes Take a little break You are about 1 3rd of the way finished PART 4 CAPACITORS EXCEPT C10 amp C11 polystyrene Board Wash 2 g Locate the CAPACITOR bag Set the polystyrene 2200pf caps the big silvery ones with the long thin leads aside THESE CAPS ARE SOLDERED IN AFTER THE ICs g Locate the 22pf cap and solder into C13 between R35 amp R36 g Locate the 1000pf yellow box cap and solder into C2 next to U2 g Locate the 0 1M caps 7 Solder into C1 C3 C4 C6 C7 C8 and C9 SYNTHESIS TECHNOLOGY PAGE 7 MOTM 420 ASSEMBLY 6 25 99 WWW SYNTHTECH COM Loc
15. efore the FM1 input can be set to control the filter from 0 75V oct to 0 75V oct This control range can be adjusted by changing R12 AND R19 they both need to be 1 resistors of the same value EXPONENTIAL CONVERTER The voltage output of the summing amp U1B is divided by the ratio of R4 R4 R5 TP1 The reason is that the base emitter junction of Q1A wants to see a 18 02mv change for each 1 00V change at the summer output Also the collector current of Q1 has a temperature dependence This temperature coefficient is cancelled by R4 That is why R4 and Q4 must be in thermal contact they need to be as close as possible to the same temperature So the desired attenuation ratio is 1V in 01802V out or 55 5 1 That is why R5 TP1 value is close to 54K if TP1 is in the middle Clever yes SYNTHESIS TECHNOLOGY PAGE 14 MOTM 420 ASSEMBLY 6 25 99 WWW SYNTHTECH COM Now we need a reference current for the VCF which is what the current source supplies with 0 0V out of the summer This current is set by R6 and is 15V 475K or 31 6uA this same topology is used in the MOTM 300 Q1B is a current mirror that actually drives the 2 OTA stages R7 amp R8 are protection resistors and C2 overdamps U2A since it is operating open loop The idle cutoff frequency is 250Hz the exact value is left as an exercise for the reader FILTER SECTION The filter is based on the Korg MS 20 circuitry The MS 20 used a cust
16. ers with the beveled corner in the upper right corner into the 7 holes Place the flat washer on the jack then the hex nut Hold the jack with one hand on the backside keeping it square Tighten the hex nut with a nut driver NOTE when tight not much of the exposed threads of the jack are exposed You are now ready to attach the pc board to the bracket and then wire up to the panel PART 9 ATTACH PC BOARD TO BRACKET PANEL In the HARDWARE bag locate 4 6 32 3 8 screws 6 6 KEPS nuts and 4 spacers Locate the mounting bracket The pc board attaches to the bracket with the 4 screws threading from the top of the board through the spacers through the bracket and then out the bottom of the bracket The 6 KEPS nut attaches on the bottom of the bracket Note the bracket has 2 long mounting flanges with a hole in each These attach to the 2 threaded studs sticking out of the rear of the panel The 4 pots each stick in its panel hole when the bracket is screwed down on the 2 threaded posts Attach the pc board to the bracket The flanges will point upwards when the pc board is sitting on the bracket Note that the bracket holes for the pc board are actually oblong This is to allow adjustment for the pc board to firmly press up against the back of the panel As a start set the 4 screws ALL THE WAY TO THE LEFT of the oblong holes Loosely tighten the 4 KEPS nuts on the bottom THIS IS A VERY IMPORTANT STEP SO PAY A
17. from the summing amp exponential converter The OTA s output current is the product of the audio signal the Vdiff part and the current into the ABI pins In order to peak the filter at the cutoff frequency called Resonance we take a portion of the filter output and feed it back into the input This is the function of U4A Panel pot VR7 SYNTHESIS TECHNOLOGY PAGE 15 MOTM 420 ASSEMBLY 6 25 99 WWW SYNTHTECH COM is a simple voltage divider U4A is a non inverting amp whose gain is set by the combination of R18 and trim pot TP2 the MAX Q trim R32 is a trick to get a log like response from linear pot VR7 The 6 diodes across the opamp is a voltage limiter Each diode has a 0 6V drop so 3 diodes in series is a 1 8V limiter The 3 audio inputs are summed by The individual gains for each stage 15 75K 100K or 0 75 Since we have 3 inputs and each can be connected to say a VCO output at 5V pk pk the input summer will start to clip at some point This is just a compromise between pot settings on the front panel versus all 3 inputs going full blast You can adjust R22 up or down in value to suit The filter output is amplified by U4B It s gain is 39 12 or about 3 25 Therefore the end to end gain on a single signal is 75 3 25 2 4 which duplicates the MS 20 characteristic Note that the MOTM 420 can be used with ANY input levels not just the 5V pk pk standard MOTM levels This makes the MOTM
18. llowing reference directions assume that you are looking at the pc board with the panel to the right and the power connector to the left g All of the ICs are pointing the same way and all notches are down 1 Al ofthe diode s bands are to the left g The braided wire on the coax goes to the beveled side of the jacks g The parts in the the right places and the panel pots switch is wired correctly 0 No solder shorts or missing joints BE SURE to carefully examine the pins on Q1 for solder shorts SYNTHESIS TECHNOLOGY PAGE 17 MOTM 420 ASSEMBLY 6 25 99 WWW SYNTHTECH COM USE OF THE MOTM 420 VCF The 420 is a stand alone module that can be used for filtering just about any audio source you can t plug a guitar or mic directly into it but use a direct box or preamp first The important thing to remember is the top row of jacks is for control voltages and the bottom row is for audio inputs You can use audio signals to modulate the filter The filter has three modes LP low pass HP high pass and NF Notch filter A low pass filter is like a BASS tone control It passes lower frequency and attenuates higher frequencies The point where this starts is called the cutoff frequency Frequencies below the cutoff pass through while frequencies above cutoff get reduced in volume A hi pass filter is like a TREBLE control again higher frequencies pass through and lower frequencies are a
19. lux applying heat to the pad for about a half second first then applying just enough solder to make a small puddle that looks like a tiny pyramid Enough solder should flow in the hole such that on the top component side a small amount is on the top pad as well The rule of soldering don t use too much you can always add more Cut the leads flush with the top of the solder joint with your diagonal cutters Locate the 100K resistors 3 and solder into R13 near C5 R20 and R21 above R18 Locate the 4K7 resistor and solder into R7 above TP1 Locate the 8K2 resistors 2 and solder into R8 above R7 and R14 by SW1 Locate the 2K7 resistor and solder into R15 Locate the 22K resistors 2 and solder into R16 and R32 top right corner Locate the 10K resistors 6 and solder into R17 bottom right corner R25 R26 R28 R29 and R30 all are near U5 Locate the 2K2 resistor and solder into R18 below TP2 Locate the 75K resistor and solder into R22 by VR5 Locate the 4 220 ohm resistors and solder into R23 R24 R27 and R31 all by 05 Locate the 82K resistor and solder into R33 top right corner Locate the 12K resistor and solder into R34 below R18 Locate the 39K resistor and solder into R35 Locate the 510 ohm resistor and solder into R36 SYNTHESIS TECHNOLOGY PAGE 6 MOTM 420 ASSEMBLY 6 25 99 WWW SYNTHTECH COM PART 2 Precision Resistors EXCEPT R4 IMPORTANT NOTE R4 is the last part soldered on th
20. older the IN 3 coax in J6 to the IN 8 jack Solder the IN 2 coax in J5 to the IN 2 jack Solder the IN 1 coax in J4 to the IN 1 jack Solder the orange white wire in J2 to the FM2 jack The white wire solders to the LEFT lug and the orange wire to the BEVELED lug Solder the orange white wire in J3 to the FM 1 jack White wire to LEFT orange to BEVELED Solder the 1V OCT coax in J1 to the 1V OCT jack Use a ty wrap to bundle the coax together Trim the excess Rotate all of the front panel pots fully counter clockwise Locate the KNOBS Notice each knob has a white line on it Place the knob on the pot shaft align the white line to the 0 tick mark and tighten the hex screw The silver part of the knob has a protective clear plastic overlay that can be removed if desired Gently rub with your fingernail across it and it will peel off kkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkk CONGRATULATIONS YOU HAVE FINISHED BUILDING THE MOTM 420 kkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkk SYNTHESIS TECHNOLOGY PAGE 13 MOTM 420 ASSEMBLY 6 25 99 WWW SYNTHTECH COM All that s left to do is test it But before we do please read the following Theory of Operation THEORY OF OPERATION The MOTM 420 has 3 basic sections a voltage summing amplifier for the control voltages an exponential voltage to current converter and 2 current controlled integra
21. om OTA gain stage and the MOTM 420 uses a LM13600AN OTA stage Vintage op amps are used to duplicate the sonic characteristics of the MS 20 filter OTA stands for Operational Transconductance Amplifier The gain of the OTA is Iout 19 2 Icontrol Where Vdiff is the differential voltage between the and input pins and Icontrol is a control current fed into the ABI pin So an OTA has a Current Out Voltage In transfer hence the term transconductance note a standard op amp is Voltage Out Voltage In As every good EE knows the simplest low pass filter is a series resistor and a cap to ground The corner frequency is the familiar F 1 2 p RC Since a variable C is hard to make most filters use a variable R esistance to make a variable filter Now the clever reader will also recall Ohm s Law I V R Which can be re written R V I Hmmmm if an OTA has a I V function and since the for our simple low pass is in the denominator ie 1 R factor AND since 1 R is I V HEY the OTA is a variable resistor whose value is set by the control current Icontrol We got ourselves a VCF Note the following discussion skips tons and tons of college level calculus and complex arithmetic Sorry The MOTM 420 uses 2 filter sections U5A C10 and U5B C11 These sections form the variable RC sections of the filter As stated earlier the R is really set by the control current
22. on parts The precision parts use a slightly different color coding scheme If you are unsure of a resistor s value use your trusty DVM to measure it Inserting the wrong resistor in the VCF kit will cause interesting behavior And it s very hard to find the error Lastly the large black tempco resistor is the last part soldered on the board DO NOT solder it until told to do so You will start by soldering in ALL of the 5 resistors 7 Findthe RESISTOR bag g Find the MOTM 420 blank pc board There is a copy larger than actual size of the silkscreen which shows where the parts go at the end of this document It will be useful if you locate the part on the print first put the part in the board then check off the silkscreen All parts are inserted from the side of the board with the white silkscreen the top side SYNTHESIS TECHNOLOGY PAGE 5 MOTM 420 ASSEMBLY 6 25 99 WWW SYNTHTECH COM OUUU UU Q Q We will stuff the resistors by value to make things easier The resistors and other long leaded parts are inserted on a 0 4 inch spacing The important thing is to be sure that the part is sitting all the way down on the board Push the leads in the holes push the part on the board and then bend the leads on the bottom outwards to a 45 degree angle roughly This is called cinching the leads and keeps the part from falling out From the bottom of the board solder using the organic f
23. shrink gun is highly recommended Solder the wires in VR1 to a pot the Red wire goes to the LEFT 8 lug the Black wire goes to the MIDDLE 2 lug and the White wire goes to the RIGHT 1 lug yes I realize the pcb numbers don t match the pot numbers Oh well Don t forget the heat shrink goes on first When you are finished soldering and the tubing is SYNTHESIS TECHNOLOGY PAGE 12 MOTM 420 ASSEMBLY 6 25 99 WWW SYNTHTECH COM OU UU l shrunk on insert the pot into the top panel hole FREQ Remove the outside hex nut and flat washer Place the pot in the hole with the 3 wires pointing down towards the jacks Put the flat washer and hex nut back on the pot and tighten with a nut driver In the exact same manner attach the VR3 wires to a panel pot This time be SURE the pot goes into the second hole marked FM1 Secure to panel In yet again the same exact manner attach the VR2 wires to the remaining panel pot This pot goes in the third hole marked FM2 The wires will cross over the FM1 wires Secure to panel Using a ty wrap bundle the 3 pot wires together Now you will solder to the 7 jacks Each jack has 3 lugs from the rear we will refer to them as LEFT TOP and BEVELED In all cases there is nothing soldered to the TOP lug Solder the AUDIO OUT coax in J7 to the OUT jack For all coax connections to the jacks the braid goes to the BEVELED lug and the inner conductor goes to the LEFT lug S
24. synthesis technology MOTM 420 Voltage Controlled Filter Assembly Instructions amp Owner s Manual Synthesis Technology 6625 Quail Ridge Dr Fort Worth TX 76180 817 281 7776 www synthtech com July 26 1999 MOTM 420 PARTS LIST Please carefully check that all parts are in your kit If you have a suspected shortage please call or email If you get free extra stuff keep it for next time g Capacitor bag containing the following 15 parts 2ea 10mfd 25V Electrolytic C14 C15 lea 3 3mfd 50V non polar Electrolytic C5 lea 33mfd 35V low leakage Electrolytic C12 lea 22pf marked 220 ceramic axial C13 2ea 2200pf axial polystyrene C10 C11 lea 1000pf yellow box marked E102 or 1N C2 0 1mfd marked 104 ceramic axial C1 C3 C4 C6 C7 C8 C9 g Resistor bag containing the following 36 parts 6ea 10K brown black orange R17 25 26 28 29 R30 4 220 ohm red red brown R23 24 27 R31 4ea 100K 1 brown black black orange R1 R2 R3 R10 3ea 100K brown black yellow R13 R20 R21 3ea 51K1 1 green brown brown red R11 12 19 2ea 8K2 gray red red R8 R14 2ea 22K red red orange R16 R32 lea 1 3W tempco large black R4 this mounts over Q1 lea 44K2 1 yellow yellow red red R5 lea 475K 1 yellow violet green orange R6 lea 4K7 yellow violet red R7 lea 221K 1 red red brown orange R9 lea 2K7 red violet red R15 lea 2K2 red red red R18 lea 7
25. ta ozz ozz 62 LZA SZA 2 oun azt pte z gt gt AST 9SStUWON vea sca ava 8 an p a ewe ast T pon MOOT 5 aor A sua 9 AL ots SEU NI NI Fs Ore gt see ot z e i adoozz zr a ast sew D sr ray OTS NI fe oeu sta pra zza Tai n 9 009 asa w oosetwt wr o wa BAL L ADYCANOHA 1 22 AL Ast Le tats T TATS J zta 2 2 2 55 T T T TATS TTA zwa 22 55 Hoot 3T AT 5 zan zr va AST 8 xoz 25 1 sT CADE A eTOTIXW a 5 T MOOT ast Z J 5 95 U 12 oxy 2 ime moot anto ae NSLP Le ER AST AST D AST ST MOOT T MOOT ST Oma ast ta za ou WILINI ast SYNTHESIS TECHNOLOGY MOTM 429 FI
26. tal Volt Meter or oscilloscope to check the output 1 Philips screwdriver Small flat screwdriver for adjusting the trimmers Fingernail brush for washing off the organic flux Old towel for blotting dry pe board Heat sink compound this is REQUIRED for proper VCF operation Available from Allied Newark Mouser Radio Shack Farnell and most all electronic supply companies Postal regulations force us not to include with the kit FF HF HF HF KH For more information of tools used and suggestions see the MOTM FAQ and Tutorial pages at http www synthtech com You also may find it useful to purchase additional heat shrink tubing 1 8 diameter 2 1 shrink ratio and solder Just be sure it is NOT rosin based flux HOW TO FOLLOW THE DIRECTIONS Please read the entire instruction before proceeding There may be valuable information at the end of the instruction Each instruction has a check box next to it After you complete the instruction check the box This way you can keep track of where you are in the process VERIFY THE PARTS LIST g Verify that all of the parts are in the kit as shown on the parts list SYNTHESIS TECHNOLOGY PAGE 4 MOTM 420 ASSEMBLY 6 25 99 WWW SYNTHTECH COM A WORD ON SOLDERING There are 2 very different types of solder used in the kit Most of the soldering uses Organic Flux solder This is strictly for use on the pc board and is NOT to be used on the front panel wiring
27. the right hole Secure and trim off any excess g Find the orange white gray twisted wire This is soldered into SW1 Solder the Orange wire into the top 1 hole the White wire into the middle 2 hole and the gray wire into the bottom 3 hole g Find the 2 long orange white twisted pairs They go into J2 and J3 Solder the Orange wire into the RIGHT hole 2 and the White wire into the LEFT 1 hole g Find the 3 red black white twisted wires They go into VR1 VR2 VR3 In all cases solder the White wire into the LEFT 1 hole the Black wire into the MIDDLE 2 hole and the Red wire into the RIGHT 3 hole Be sure you solder the shorter conductor end into the pc board YOU ARE NOW FINISHED WITH THE PC BOARD WORK BREAK TIME SYNTHESIS TECHNOLOGY PAGE 10 MOTM 420 ASSEMBLY 6 25 99 WWW SYNTHTECH COM PART 8 FRONT PANEL PREPARATION You will now attach components to the front panel It is HIGHLY recommended that you use a set of hollow shaft nut drivers NOT PLIERS to tighten the nuts This prevents scratching NOTE all references to part orientation is from the REAR of the panel Locate the 7 Switchcraft jacks Notice that from the rear there is beveled corner This corner is ALWAYS CONNECTED TO GROUND USUALLY WITH THE BRAIDED CONDUCTOR Each jack has flat washer a lockwasher and hex nut Remove the nuts and washers from each jack Place aside Keep the lockwasher on the jacks Insert the 7 jacks lockwash
28. tors that form the actual filter INPUT VOLTAGE SUMMER Refer to the schematic page marked M420 SCH for the following discussion The precision DC op amp U1 sums the various sources of control voltage to feed the exponential converter Op amp section U1A is an inverting summer The 1V OCT input is summed at unity gain set by the ratio R2 R10 The filter s initial cutoff frequency is set by panel pot VR1 and gain resistor 9 if you want to reduce the spread of the filter s range increase R9 to like 390K 470K The filter has a variable FM2 input that due to the 51K1 gain set resistor can achieve approximately 0 5V OCT A second inverting stage U1B is used because the sense of the PNP exponential converter is such that positive control voltage inputs will increase the cutoff frequency We have added a special type of input circuit for FM1 it is called a reversing attenuator Resistors R12 and R19 are in between the panel pot VR3 Note carefully R19 goes to the input of the following inverter stage while R12 goes back to the first stage The reason is explained below If the pot is rotated all the way to the left CCW the wiper connected to the input jack J3 is all the way over to the R19 side Therefore the voltage applied to J3 is split into to paths one path has a gain of 2 the R1 R19 path and the other has a gain of 0 66 the R2 VR3 R12 path The net effect is of course 66 2 1 33 Ther
29. ts VCF initial cutoff frequency resonance gain at filter cutoff level set for FM1 control voltage level set for FM2 control voltage level set for audio input IN 1 2 3 selects filter mode HP LP NF 15VDC 18 ma 15VDC 18 ma 2U x 5U 3 47 x 8 72 88 1mm x 221 5mm 4 375 inches 111mm PAGE 20 251 qesus eser sume v 5 ama zequnn 5 BOVLIOA WLOW Starr WLOW 8Ts 888 XL HLYOM LAOA r WINL TIwnd 3299 N ALIVAA ast SISIHINAS vr vr zT asz SPIPNI SP TPNT wot d 4 sto j 9ST WLW J dL ea gt 3 zaz asz wot gt eta 1 ast TT azz ast esstucn cea aen g azz L g aze ast ast e
30. ttenuated A notch filter has a dip at the cutoff frequency if the signal is above OR below cutoff it passes through At the cutoff frequency it is attenuated Note that as a side effect the phase of the output signal gets shifted relative to the input This can also add new sounds FILTER SLOPES AND RESONANCE The amount the frequencies get attenuated relative to the cutoff is called the filter s slope The MOTM 420 in LP HP modes have a 12dB oct slope 12dB is about 1 44 For example if the LP filter has a 500Hz cutoff a 1Khz sine wave is attenuated 12dB at 1Khz 24dB at 2Khz etc The Notch filter due to the circuitry used has only a 6dB oct slope If you are in NF mode and the filter is set to 500Hz then a 500Hz sine wave is attenuated 6dB at 500Hz about half Note these are ideal figures and the actual filter will have slightly less attenuation You can get sharper slopes by daisy chaining MOTM 420s If you have 2 then take the OUT of the first one into the IN 1 of the second one If both filters are set to the same FREQ RES and MODE settings the slopes will be twice normal 24dB oct for LP HP and 12dB oct for Notch A popular filter effect is to add resonance to the cutoff frequency Resonance is just adding additional gain right at the cutoff frequency The effect is that the filter sounds brighter squeakier If you add too much resonance the filter will self oscillate
31. y the same SYNTHESIS TECHNOLOGY PAGE 16 MOTM 420 ASSEMBLY 6 25 99 WWW SYNTHTECH COM SETTING THE 1V OCT TRIMMER First apply power to the VCF and wait for 10 minutes This allows the Q1 R4 temperature to stabilize to operating point Although the purpose of the trimmer is to set the filter s cutoff frequency to double for each volt applied ie 1V 200Hz 2V 400Hz 800Hz etc the easiest way is to set this is use the VCF as a VCO From the above MAX Q procedure the VCF will oscillate and act as a VCO over a limited range of control voltage This is because the exponential converter is not as precise as those in VCOs although the MOTM 420 s exponential converter is better than any VCF ever made The best way is to beat the output of the VCF with a VCO or synth that is in tune Or if you have good pitch you can just set it by ear plug a source of accurate 1V OCT into the jack ie a Kenton Pro 2000 b set the MODE switch on LP c set the FREQ to about 6 d set the RES to 9 e Set the 1V OCT ADJUST pot to the center position You should now be able to play the VCF as a VCO Now adjust the trimmer for the best tracking The tracking is not that great but over a 3 4 octave range you should get fairly close Remember the exact cutoff frequency is not that important as it is for a VCO TROUBLESHOOTING If your 420 does not work please verify ALL of the following before contacting us The fo
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