MOTM-300 resistor change
2000-08-14 by Paul Schreiber
**NOTE** This is NOT a required change! Please read the following carefully! *************************************************************** All new MOTM-300 kits will be shipped with R50 changed from 3M to 2M2 (also called 2.2M). Assembled MOTM-300s will have a 2M2 or a 1M8 depending on the particular performance of that circuit. This resistor adjusts the "High Frequency Track" of the VCO. Specifically, the tracking accuracy from 500Hz and up. No, I can't make it a pot because that causes more problems than it fixes. Newer batches of the MAT02EH NPN transistor used in the exponential converter show *improved* HFT when R50 is lowered. Now, should you immediated tear apart your MOTM-300 to replace this? NONONONONONONO! *ONLY IF* you think that in tracking multiple VCOs ABOVE 500Hz you have an issue, then you can change this resistor. You WILL HAVE TO RECALIBRATE THE VCO IF YOU DO THIS!! The following is the tracking of 1 VCO with R50 changed. Voltage source is Pro-2000. Errors in the Pro-2200 are included in this (about 700uV over the range). Desired Actual With R50 = 3M 25 24.97 24.91 50 49.98 49.92 100 100.01 100.2 200 200.07 200.18 400 400.06 399.97 800 799.97 796.1 1600 1599.5 1587.2 3200 3197 3180 6400 6386 6369 This is an improved *average* error over 8 octaves of 0.065% Let's see anybody else top this!! :) You can see how the VCO "runs flat" in the upper octaves. Now, can you *hear* this? Probably NOT! But, if you are 'zero beating' VCOs without sync in lower octaves and want to maintain that in the upper ones, the lower value resistor will help. If you want a freebie 2M2 resistor(s), send me email. Paul S.