Vintage Synth Scans via Tim Stinchcombe

ARP 2500 pictured. You can find the full size scan for this and more at Tim Stinchcombe's site. You'll find Moog and EMS as well.

Update via alt-mode on AH who owned and sold a 2500: "The modules are labeled so you know which column corresponds to the input/output. Yes, there is a small gap between the module and the matrix but it usually isn't a problem, provide you don't need to get out your bifocals (more on that later). There also tends to be similarity between modules and the inputs used. Most of the oscillators have VC inputs on columns 1, 2, & 3 in the upper matrix. There is also a color-coding scheme for the slider caps that, if you have the right caps on the sliders for your modules, helps make things clearer but you need to move the caps around anytime you rearrange modules and some of the colors are impossible to find these days.

The upper matrix tends to be for keyboard CV and Gate signals and you are correct that the jacks on either side allow access to a particular row of the matrix so you can patch between the upper and lower matrices as well as external gear.

Each row is like a giant multiple. You can send one output to many inputs from the same row. You also need to dedicate a row or two of the lower matrix to audio outputs.

To patch between modules, you select the column of your output on the module, slide the matrix lever for that column down to an unused row and slide the matrix lever for the input of the destination module to the same row. So, in some sense you are limited to 20 "patch cords" in the lower matrix.

The middle few rows of each matrix are not connected to anything so it is the place where you leave disconnected inputs and outputs until you need them.

The fun of a 2500 is that you can have lots of "happy accidents" as you move the sliders since you can end up influencing other modules or finding unintended modulation sources. The downside is that you can get some crosstalk between the rows of the matrix but I never had too much of a problem with that.

My frustration with the 2500 was that I had to squint a lot (or find my glasses) to line things up on the matrix. It depends on how you have the synth set up but you really want those matrix switches nearly level with your eyes so you can see exactly which row you are using."