[sdiy] Hammond VCF patent vs. 102100 schematic comparison

[Aaron Lanterman]

The schematic and description in the Hammond 102100 service manual Bob has on his website was spread over a couple of scanned pages. I tried a bit of photoshopping to splice them together to form single pictures. 

Here's the VCF from the 102100 schematic:

users.ece.gatech.edu/~lanterma/hammond_vcf_splice.png

and here's the circuit description in the text:

users.ece.gatech.edu/~lanterma/hammond_vcf_description.png

There's a some differences in component values; on the scan I can't make out most of the component values so in most cases I'm not sure. There's also a few minor differences on the periphery.

The most interesting differences between the patent and the 102100 schematic that I can see:

1) "R146" and "R417" on the patent are actually each made up of two parallel resistors on the 102100 schematic; in each pair, one of them is notated as 1% and it says "factory selected." 

2) Looking at the patent, consider the output of OA1 and the + input of OA2. On the patent, there is no direct connection between them, but on the 102100 schematic, there is a 220K resistor connecting them. 

3) On the patent, C34, R148, R149, and R150 form a strange positive feedback network around OA2 - Don Tillman suggested that it "looks like complete trouble." In any case, it appears that this network is *entirely absent* on the 102100 schematic.

- Aaron