---------------------------------------- | MAESTRO RING MODULATOR (model RM-1A) | ---------------------------------------- Revision Date: 2004 JAN 15 Many things to say about this project... first of all, this project is UNRECCOMENDED for beginner buiders. About the schematic ------------------- Original schematic: http://www.diystompboxes.com/pedals/maestro/ There's a complete PDF about this ringmod, look for Maestro_RM-1A-1B Alternate source for schem: Justine philpott "effectronics" http://www.j.philpott.com/effectronics/ I used it as an alternate source of info. In the original PDF I've found at diystompboxes there are 2 different version of RM: RM1-A has got 2 features that RM1-B hasn't: - External carrier input - Oscillator output I've chosen RM1-A simply beacuse the differences in the schematic were so slight... I used BOTH this schematic and I did many mods, so in the schematic file of GGG there are some variations. First of all, I corrected a couple of values here and there, then I modded the schem for implementing true by-pass. I re-designed the power supply using 7815 and 7915 regulators. Value for R25 is 3k3 ohms, and (most important thing) pin 7 of MC1495 is connected to "-12" and not "+12"!!! That's very important cause you would fry the chip... I used MC1495 datasheet for these corrections. MC1495 is an obsolete chip and don't know if it's still in production. I've been so lucky, cause I took 15 days for finding one of this ic... and I've paid it A LOT of money. The other ICs are very common: a couple of double op-amp (MC1458) and the single op-amp 741. The part of the circuit built around the 741 is the internal oscillator. It delivers a sine wave into the MC1495. By inserting a jack into the external carrier input, you can drive the ring mod with "any" external audio sorce cause the internal oscillator will be disabled. Switch 1 a-b sets the frequency range of the oscillator (low & high frequencies) and the dual gang pot pt3 sets the frequency. The "oscillator out" output delivers the sine wave to the out world... The 1N746 diodes are 3.3 Volts zener. Mc1495 is the heart of the circuit, it does the hard job of multipling the frequency of a guitar signal by the frequency of the internal oscillator sine wave (or an external wave). I believe (I'm not sure about it) that T1 here is used as a variable resistor, and it shuts down the output when there's no input (it's a gate...). About T1, I didn't used 2N4380 cause I couldn't find it, so I used an equivalent P-FET, the 2N3820. This circuit requires a bipolar power supply to work (+-15v). There's nothing else relevant in the schematic... About the PCB and layout ------------------------ As always, use sockets for all the ICs and for the j-fet. If you dan't want to have much pain in tuning the circuit (there's a section later in this file) use multi-turn trimmers. Tuning perfectly this circuit is almost IMPOSSIBLE without them (by the way, I didn't use it and I took hours to set the 5 trimmers... and I'm not entirely satisfied, so I think I'll re-tune it from start...). There are 6 jumpers, don't forget them. There are bunches of empty pads in the layout, and that's normal cause this project requires a lot of off-board wiring if you want to wire all the jacks for modulation control, pitch control, external carrier and oscillator out. The circuit will work without it, although being less fun... I've left 2 empty pads marked as "A" and "B", these are for the tuning the circuit. In order for the circuit to work, I must link BOTH "+15" pads to positive supply and BOTH "-15" pads to negative supply. Also, "+12" to a positive 12 volts supply and "-12" to... right! Guess it... Respect diodes polarity. I used a couple of 3.3v zener and 3x 1N4148. 1N914 will work well. I suggest to build the circuit without J2, J3, J4 and J5, by putting jumpers between: - J2/1 and J2/2 - J2/3 and J2/4 - J3/1 and J3/2 - J4/1 and J4/2 - J4/3 and J4/4 Then tune the 5 trimmers as described below. Once the circuit is tuned and well working, you can add J2, J3, J4 and J5. Also build the power supply unit and use the small circuit (not included in the main board) to obtain +-12V from +-15v coming from the power supply. Trimmer adjustment (as reported in the service manual) ------------------------------------------------------ 1. Signal null adjust Disable internal oscillator operation by turning the "Oscillator Adjust" trimmer (TR5) fully clockwise, or by inserting an unwired phone plug into the Pitch Pedal Input (J4). Apply a signal to INST. jack (J1), advance the VOLUME potentiometer (PT1) and adjust the "Signal Null Adjust" trimmer (TR3) for minimum signal at test point B 2. Oscillator adjust Remove unwired phone plug if used in step 1. Adjust "Oscillator Adjust" trimmer (TR5) for a signal of 5.5 volts paek-to-peak at test point A. For this test set PITCH potentiometer (PT3) at 10 and PITCH RANGE switch (SW1) at high. 3. Carrier null adjust Set VOLUME potentiometer (PT1) at 0. Adjust "Carrier Null Adjust" trimmer (TR4) for minimum signal at test point B. 4. Linear squelch adjust Using the device in an operational manner, that is with a musical instrument providing a signal, adjust the "Linear Squelch Adjust" trimmer (TR1) for optimum balance between carrier suppression and natural-sounding instrument decay. 5. Offset adjust Apply a signal to the INST jack (J1), and advance the VOLUME potentiometer (PT1). Observe the average DC level at test point B and adjust "Offset Adjust" trimmer (TR2) until the DC level does not change appreciably as the VOLUME potentiometer (PT1) is moved between a setting that produces an observable signal and 0. Trimmer adjustment (by ear, unreccomended. That's what I did, unfortunately) ---------------------------------------------------------------------------- -2. Relax yourself. -1. Be patient. 0. Stay calm. 1. Signal null adjust Disable internal oscillator operation by turning the "Oscillator Adjust" trimmer (TR5) fully clockwise, or by inserting an unwired phone plug into the Picht Pedal Input (J4). Apply a signal to INST. jack (J1), advance the VOLUME potentiometer (PT1) and adjust the "Signal Null Adjust" trimmer (TR3) for minimum signal at test point B 2. Oscillator adjust Remove unwired phone plug if used in step 1. Turn "Oscillator Adjust" trimmer (TR5) fully clockwise, then turn it counterclockwise right before you hear a distortion in the sine wave at test point B. Of course, you have to wire test point B to an amplifier. For this test set PITCH potentiometer (PT3) at 10 and PITCH RANGE switch (SW1) at high. 3. Carrier null adjust Set VOLUME potentiometer (PT1) at 0. Adjust "Carrier Null Adjust" trimmer (TR4) for minimum signal at test point A. 4. Linear squelch adjust Using the device in an operational manner, that is with a musical instrument providing a signal, turn the "Linear Squelch Adjust" trimmer (TR1) fully clockwise (you must not hear anything) then turn the trimmer right before you begin to hear the natural sound of your quiet guitar. Connect your amp at test point A for this test. Then play with your guitar and set again the trimmer for optimum balance between carrier suppression and natural-sounding instrument decay. 5. Offset adjust Apply a signal to the INST jack (J1), and advance the VOLUME potentiometer (PT1). Observe the average DC level at test point B and adjust "Offset Adjust" trimmer (TR2) until the DC level does not change appreciably as the VOLUME potentiometer (PT1) is moved between a setting that produces an observable signal and 0. This all means nothing to me. Set the trimmer half-way. Maybe I'm wrong I have noticed no change at lower volume. Ending ------ I'm sure you're a bit frustrated now... You can contact me by e-mail writing at offramp.unoATtiscali.it remove "AT" and put @. That's only an anti-spam prevention system... I'm also "zero the hero" at the stompbox forum at http://www.diystompboxes.com/sboxforum PM me if you want to know more...