1. Calibrate Bandpass Filters
Changed R53, R56, and R59 to get the bandpass filters to track each other more closely. I have two MOTM-410s and when all six filters were tuned to generate maximum output from a 1 KHz sine wave, the settings for the FREQ knobs varied between 2.0 and 4.5 (a knob variance of 2.5). After making this mod, the knob variance at 1 KHz is zero and is less than 0.5 over a 10 KHz range.
- Replace R53, R56, and R59 with 500K pots (preset the pots to a value of 180K).
- Set SWEEP, DEPTH, and all FREQ knobs to zero.
- Power up module and let it warm up for at least 10 minutes before continuing.
- Apply a 1 KHz sine wave to IN jack.
- Put one channel of scope on IN jack, trigger the scope off of this channel.
- Put second channel of scope on OUT 1 jack.
- Set FREQ 1 to 4.5.
- Adjust R53 until OUT 1 is at maximum (output will be 180 degrees out of phase with input).
- Put second channel of scope on OUT 2 jack.
- Set FREQ 1 to zero.
- Set FREQ 2 to 4.5.
- Adjust R56 until OUT 2 is at maximum (output will be 180 degrees out of phase with input).
- Put second channel of scope on OUT 3 jack.
- Set FREQ 2 to zero. Set FREQ 3 to 4.5.
- Adjust R59 until OUT 3 is at maximum (output will be 180 degrees out of phase with input).
- Power down module.
- Disconnect 500K pots and measure their values.
- Replace R53, R56, and R59 with the measured values.
- Make sure that only one of the FREQ pots is set to non zero at any time. Otherwise it will be difficult to get maximum output at resonance (the three filters interact with each other causing a dip in the output when more than one is set to the same frequency).
- Give the circuit plenty of time to warm up before doing this mod (at least 10 minutes), the filters tend to drift quite a bit when power is applied.