Archive of the former Yahoo!Groups mailing list: MOTM

∗∗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.