Above you see an ARP 3604 with 2 mods. The top one is called “sum” and the bottom is “2nd voice”
This is a good example of a project that was a mess because it involved repair work, modification, and modifying modifications in order to repair it properly and deliver the best experience for the owner.
First, all the gentleman wanted was the “sum” mod, which is something they wanted after seeing this added to Mike Dean’s 3620. Basically, this mod allows you to add (sum) an external CV into the CV generated by the keyboard. So, you can transpose sequences with the keyboard for example.
Problem was that despite this mod working as intended, this 3604 was starting to develop a CV droop where it begins to drop in CV (pitch) after a key is released. Hardly useful. Additionally, it had an early version of Tom Oberheim’s 2nd voice modification mod to allow duophonic ability from the 3604 keyboard. All of these issues and mods now had to play nice with each other.
Photo above shows a new S&H IC that replaced the op amp module that was drooping. There info online on how to install these. I’ll write it up eventually too. Problem with these is that the “tune” knob, which is effectively the pitch bend, no longer functions unless a key is held down. Hardly useful if you ask me. I had to modify the pot and switch wiring to feed a CV into it that could be calibrated to a perfect 1V and feed that into the summing circuit that all the CV’s hit anyway before it heads to the 2600 itself. Have I lost you yet?
Here is the ol’ Tom O 2V mod. We had to lightly modify this to have the S&H chips CV hitting this as the original mounting locations and sourcing will no longer line up. Also had to input the tune CV to this.
Here’s a shot of my DIY circuit made on “robs strip board” miniature edition which isn’t publicly available yet. On the left there is a trimmer to calibrate the range of the tune pot. On the right there is a inverting summing node with a few different equal power inputs:
1. Keyboard CV
2. Sum input jack
3. Tune knob
Overall, I’m surprised how well this works. The keyboard holds pitch brilliantly and the tune knob works at all times when the switch is engaged for it. The duophonic mode works nicely as well and plays nicely with everything else. Overall, it’s one of the better equipped ARP 3604 keyboards I’ve had leave my workshop.
Note: I love spinning circuit boards for these types of projects but lately I’ve been loving just using my custom strip board as it just gets it happening quickly. I know it’s not pretty, but it ain’t too shabby either.
Fun fact: this is the exact 2V board I pulled out of my bag when Anthony Marinelli & I interviewed Tom Oberheim.