I noticed a discrepancy of resistor values on the Large Tetrazzi silkscreen and graphic at the spots where the Upton/Donton and Upmod/Dnmod controls attach. All of these come through a 470K resistor to the base of a transistor, with a pull-up or pull-down resistor. Sixteen locations in total, but with four of them having 10K and the other twelve having 4.7K for the pull-up/down. This bugged me to go back to the SMD board in my hand Tetrazzi to check. Turns out that all of these are 10K on the final product.

While looking at the board and probing around I saw a lot more 10K resistors and discovered that in the Upmod and Dnmod circuits, where transistor pairs have marked 22K resistors going from their bases to Mido and 22K from their collectors to either ground (Donmod) or +12V (Upmod), these were all using 10K. That’s a total of eight resistors per oscillator or 32 total that should be changed to 10K on the schematic. Two resistors in each section that should be 100K instead of 470K were also found.

The effect these changes is to double the maximum depth of the cross-FM modulations. Peter must have made these changes after he designed the paper circuit.

Notice all the 103 (10K) SMD resistors in the photo above. To assist in locating the changed values, I printed another graphic image and colored all the locations that should be 10K orange. (This includes the four that were originally 10K, some that were 4.7K, and some that were 22K.

The two resistors in each section that should be 100K instead of 470K are colored yellow.

The graphics are duplicated diagonally. Above are two details of the upper and lower right, which are identical to their corresponding diagonals. Here’s an updated schematic. The changes are in the top half, on the left and right, in the Donmod and Upmod parts of the circuit. Sorry if it’s a bit fuzzy. I’ve also shown the capacitor values I’ll be using, 56pf instead of 1000pf in the comparator, and 4700pf instead of 1000pf in the integrator.