
David Hillel Wilson of the
New England Synthesizer Museum has been sending tips and info to the AH list for some time now. These tips usually just come in as an informational email about every month or so. I noticed they are informative and not really meant to engage discussion although of course sometimes they do. They are written more like informational pieces. I asked David if he would be interested in having me put them up on Matrixsynth for others as well. He said yes. The following is the first of hopefully many more on Matrixsynth.
"To see if your ARP 2600 needs the filter mod, turn the frequency and resonance both all the way up. If the sound is still low enough in pitch for you to hear, then you need the mod. If your dog starts barking, you don't need the mod.
In order to modify the filter without unsoldering it, I solder 4 5K6 (Green-Blue-Red) resistors in parallel with the existing 4K7s, and on the foil side of the board. This gives a resistance near the value 2K7, which is what ARP used in the Solus, in which they had fixed the filter themselves, so it's still authentic ARP. Unfortunately, I'm not quite sure how to describe which foil traces to solder to (I've done so many of these I now just do it from memory). Try this: Look for the big chip in the middle (two rows of 7 pins each, spaced 1/10 inch apart, with the two rows 3/10 inch apart). Near the 4 corners of this chip (an LM3900/CA3401E), there are four ovals, each made out of two 3 pin transistors that are glued together. Thus, each oval will have 6 points around the edge. Two of these points (the Emitters) are connected together with a diagonal line that cris-crosses the oval. The only other place each foil trace goes is to a single connection - This is one end of the resistor for that oval. four ovals equals four resistors. The other end of each resistor is tied to a foil trace that encircles the whole circuit, but only makes 4 connections (one at each oval) plus one more in the expo converter. Solder a 5K6 resistor from each pair of emitters to the long outside circular trace, and you're done. (Or if you're near New Hampshire U.S.A. or are willing to send the board, I can do it for you).
David Hillel Wilson
Curator
New England Synthesizer Museum."
Image via
this post.