Restoring an ARP 2600 Synthesizer

The ARP 2600 in my workshop

I recently received a truly resplendent gift from my friend Kate: an ARP 2600 synthesizer, owned by their dad, who died recently. It’s in OK shape, but parts of it don’t work, and I’m doing some research to figure out when it was built and what has happened to it since!

And I’m going to get it back in working condition so I can make R2-D2 say whatever I want.

Yes, my workshop is churning chaos. But I also really, really love it there.

First, I had to answer some fundamental mysteries. I knew that it came from Kate’s dad, who I’d had a great time talking about science fiction and sound with. But I didn’t know just when it was built or even what condition it was in.

Initial with-screwing showed that the sliders were more “off-road adventures” than “sliders”. Their response was very sketchy and I couldn’t get the filter to do anything at all. All I could do was bypass it to get sound out.

So it was time to open it up!

If you know the ARP 2600, you’ll see that it has grommeted screws down the sides of the control panel. These had all been removed, so obviously someone had been in here already. Initial searches on Teh Interwebs told me that the panel is held in place by the screws in the handle, so

Let’s goooo!

So, I took the faceplate down. Let’s take a look at it!

The back of the control panel. Just LOOK at those gorgeous traces. Arnold Perlmutter wasn’t a hip guy. He was an electrical engineer who realized he could build a good synthesizer for musicians and music teachers. So those traces’ amazing shapes serve a function and aren’t there just to be beautiful. But they also came about during the height of psychedelic rock and I think he was influenced by the aesthetics around him, too.

Let’s look at each of those modules, potted in black epoxy to ironically maintain trade secrets. And you’ll note that one of them lacks a black block.

The 4017 voltage-controlled oscillator, the first model oscillator they used, incorporating Perlmutter’s design that allowed it to stay in tune. The company replaced this design after a while with a more reliable one, but since these are still working, it looks like we’ve got a keeper!
Sample & Hold is also working fine! I love S&H just faintly desynched from a regular oscillator, like an LFO used to control multiple modules. That way, it puts out multiple evolving sequences that keep a relationship to each other over time, even if they never quite repeat.
The Ring Modulator. This is where R2-D2’s voice comes from. I’ve been really enjoying this effect.

And finally, the filter. Before 1976 or so, it’s a filter that was close enough to Bob Moog’s legendary transistor ladder design that Moog wrote them a letter and told them to cut it out. After that, it’s their own design which I’m told is inferior, losing high frequency harmonics.

Ah. Definitely not the answer I wanted.

OK, so it’s the later ARP filter design. And yet, we can see by the Tonus logo, the model of oscillator, and rectangular grill pattern on the front, that the synthesizer is one of the earliest models from 1971!

OK, so, let’s talk about that floppy disk.

As I found the floppy

This disk was just rattling around in there.

400k Mac floppy.

StuffIt came into existence in 1987, toward the end of the 400k floppy era. So the last time someone was in here, they had 400k floppies in good condition and were expecting to use them for the foreseeable future. In fact, this would have been readable on a stock Mac for at least a decade, so good call. Kate confirms that it’s her dad’s handwriting.

MacPaint was published from 1984 until 1988, but we don’t know what version it is.

It’s the “etc” that intrigues me, particularly given that this disk seems to have been hidden in here. Even with StuffIt’s incredible compression algorithm (it felt like cheating at the time), getting MacPaint onto a floppy with enough room for “etc” means that the cetera can’t be THAT much bitmapped porn. But the disk label might be a lie, and it’s Kate’s dad’s history as a 1980s hacker blowing up the crimes of the Reagan administration.

Hey, if you like weird science fiction art, you should definitely support my Patreon because I make a lot of it.

Or it could have fallen off the desk while he was putting it back together.

Apparently, he opened it enough that putting the screws back on the face plate was a pain. So it could have just fallen in there. Until we find a working Mac with a floppy drive running MacOS 5 or so, we won’t know.

The power supply with replacement power capacitors.

More to the point, I couldn’t get a picture of the bottom of the caps, but I found the date code on the replacement power supply capacitors: 1988. So we know that the repair happened no earlier than 1988.

The repair in question was not only the power supply (used only on the very first “mid-1971” models, with examples from November, 1971 using one that’s easier to repair), but also to the filter.

This model, since we know it’s from 1971, originally shipped with the Moog Violator filter. The later one may have been installed in the original broke in some way, or perhaps it was a misdiagnosis, or perhaps Kate’s dad wanted to “update” it.

Here’s what he did:

The modification to the board

There are traces carefully cut, then bridged, with a resistor soldered in. Thanks to some fellow vintage synthesizer nerds I tracked down across online synth scenes, this is a mod for making the replacement filter work.

You’ll also note that there’s flux all over the place. The original boards had no apparent flux. This is definitely an amateur repair, though definitely has some craft to it.

However, someone — almost certainly Kate’s dad in 1988 — apparently gave up troubleshooting and desoldered the resistor, severing the filter from the rest of the circuit. At first, I thought it was a cold joint that had broken by accident, but the rest of the soldering seems quite competent and I don’t see the rough edge on the solder that you see when it breaks.

This looks like evidence of 34-year-old frustration.

I resoldered the resitor when I thought it had just broken loose…

…but the filter still doesn’t do anything.

At this point, I decided to redesolder the resistor and, since I’d found a dude in Wales who sells kits for the original one anyway, I decided to order one. I’m told that it’s arrived at my mailbox, but I’m in the wrong state right now to check.

That means I get to cut out the old one and look for damage!

The bottom side of the filter module

…and I sure don’t see any, which is concerning. If one of the two op-amps was popped, I’d have guessed that something had been hooked up backward, or perhaps it was defective. But this all looks fine.

So that’s where I leave this first exploration: the machine was built between “mid-1971” and November, 1971. The power supply is like the Grey Meanie short-run prototype that preceded it, but unlike all later models, the earliest of which I’ve found is from November. Sometime not long after 1988, someone — probably Kate’s dad — either tried to modify or repair the filter by replacing the module with the newer, non-patent-threatening one, then got frustrated that it didn’t work, a desoldered the resistor that connected it to the rest of the synth.

This project is an incredible opportunity and some deep fun.

I bet Fifth Worlders have this kind of religious experience, assuming the abundance of mushrooms where they migrate.

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