Primitive drum machines are interesting to study. I spent some time with an oscilloscope checking out how the signal flow works inside this Seeburg Select-a-Rhythm (made in 1968). There are three basic functional blocks in its pattern generator; (1) clock dividers, (2) interval logic, and (3) preset selector switches. The clock dividers are divide-by-2 flip flops, structured in a chain so they produce steady streams of pulses at the basic rhythmic intervals; sixteenths, eigths, quarter note, half note, etc… this machine has six dividers in it. They are interesting modules, which I believe should be referred to as thick-film networks, which is sort of a precursor to surface mount technology. They are the six orange rectangular parts arranged in a horizontal line in the photo below. Other drum machines in this era such as the Rhythm Ace have divider chains built from discrete transistors.
Many thanks to Ramiro Jeancarlo for sending a hard copy of the Seeburg SAR-1 service manual. It has almost complete detail on the workings of the drum machine, and even has a tutorial section on logic gates. Ramiro has developed some amazing mods for keyboards and drum machines that allow them to interface with custom mechanical gate sequencers. Some of his music can be found here - Staccato du Mal. And thanks to Lace D. for scanning the hard copy into a PDF document.
So, each of the dividers has a Q and /Q output (square waves of opposite polarity). These outputs feed the interval logic, which is a network of diodes and capacitors that work in combination to generate a master palette of trigger pulse patterns; there are about 38 pulse patterns available in this machine; they are sent out via the thick bundle of wire attached to the bottom of the board. Note that in this machine, the wires are connected to the board via easy-to-remove miniature pin jacks. It’s almost like a patchbay! I didn’t experiment with reconfiguring these connections but I’m sure you could have a blast rewiring it to alter the rhythm presets. Anyhow, the trigger pulses are negative polarity, which is interesting because this machine runs on a positive power supply only. The trigger pulses dip below ground!
Finally, the bundle of wire carrying all of the available trigger pulse patterns is connected to the preset switches. These switches combine the pulse patterns into sets defining the rhythmic style for each preset. For instance, a steady 8th-note pattern might be assigned to hihat for TEEN BEAT. Dig? Thats where the beats come from. The actual drum oscillators are on a separate board.
Ok, now for THE MODS…
I realized that the chain of clock dividers can be rearranged into many different topologies, altering the master palette of pulse patterns in rhythmically interesting ways. I designed an add-on board using CD4053 analog switches to accomplish this. It’s seen on the left side of the photo above. In designing this mod, I organized the Seeburg’s dividers into swappable pairs, and put toggle switches on the front panel that activate the swaps.
The dividers are normally configured like this: (signal flowing to the right) clock input–>A-B-C-D-E-F. I experimented with different swap-pairs until I found three that were the most rhythmically interesting; A-B, C-F, and D-E. So, with these three swap pairs there are eight combinations available. (note: apparently divider A is a divide-by-3, creating swing/triplet patterns)
- A-B-C-D-E-F (000) … original
- B-A-C-D-E-F (100)
- A-B-F-D-E-C (010)
- B-A-F-D-E-C (110)
- A-B-C-E-D-F (001)
- B-A-C-E-D-F (101)
- A-B-F-E-D-C (011)
- B-A-F-E-D-C (111)
Each of these combinations alters the presets in an abstract way. There are seven variations on top of each original preset, for a total of 136 available beats… funky and interesting things happen. Here’s a few examples of what happens when we rearrange the RHUMBA beat.
Here’s a recording of the MAMBO pattern and its seven variations. The original MAMBO is heard first and last:
Also, I added CLOCK IN and CLOCK OUT features to this machine. Here’s a recording of the Seeburg syncing with a Frostwave FatController and Korg MS20.
And finally, feel free to download a collection of 47 beats from the modified Seeburg, split into two archives:
PS. … some favorites