Double Sequential Circuits Prophet 10 Restoration

While we’re working from home due to the virus and have all of the synths and all our recording gear in the same place, we took the opportunity to make this video about the restoration of two Prophet 10s we’ve restored recently. One of them proved to be one of the most arduous restorations we’ve ever done due to massive damage to its microprocessor system. It was a long road to get it working, but when it was done, we celebrated by MIDI chaining the two of them (which we had also both retrofitted with MIDI) and making some fantastic and massive PROPHET 20 sounds! Check out this video for a recap of the restoration process and some demos of the synths’ powerful sound.

EML 101 and 100 1 V/Octave Mod PCBs and Kits now available

EML-101/100 1 V/Octave Mod kit installed

PCBs and kits are now available for the circuit we use to add 1V/Octave scaled input to the ElectroComp EML-100 and EML-101 synthesizers!

The EMLs use between 1.2 V/Octave and 1.4 V/Octave (varying from one unit to another) with an offset of between 4.2V and 4.6V. Our board allows you to feed in conventional one volt per octave control voltage from another synth, controller keyboard, sequencer, etc. and scales it up so that the EML’s second-voice oscillators (Oscillators 3 and 4) can track it. Using a sequencer especially opens up amazing possibilities for this already very powerful synth!

Our original version of this circuit just performed the summing and amplification and required the installation of a new external switch to turn the mod on and off. Our new version uses a multiplexer to automatically perform this switching task when a plug is inserted in the CV input jack. This means that it can be completely invisible– no hole needs to be drilled, and the synth functions 100% as original when nothing is plugged in.

The mod can be used on both versions of the EML-101. On the later version that has a “Sequencer” jack, the Sequencer jack becomes the 1V/Octave input, and on the earlier version, another jack can be selected to be the new 1V/Octave input (we use CM3).

The PCB will come with schematics, bill of materials and detailed installation and tuning instructions with lots of full-color photos. The kit will come with all of the above plus all required parts and the jack already partially wired (because this is probably the thing people are most likely to mess up!)

The PCB and kit can be ordered here!

 

ARP 2600

ARP 2600 circuit board work
We forgot to take a picture of it when it was done, so here’s a picture of it when it was dirty!

The owner of this ARP 2600 got it in an insane trade in the late 80s… in exchange for a Peavey keyboard amp and a TR-505! A lot of it had never worked in the entire 30 years he had had it.

When we do restoration of an ARP that’s in bad shape, we’ve learned that there’s really only one good way to approach it. We basically strip it down to its bones and do everything we possibly can in one fell swoop before even trying to test different systems. It ends up being so much more efficient that it actually costs less than taking a more step-by-step approach.

Continue reading “ARP 2600”

Rebuilding ARP PPCs

Using FSRs (force sensing resistors) to repair PPC "proportional pitch control" pads for an ARP Odyssey Mark III

Because I am now one of those people who thinks they are very busy, I am just going to share a “quick tip” today.

In one of several Odysseys that we rebuilt recently, the “Proportional Pitch Control” pads (otherwise known as PPC, those three spongy white pads that Mark III Odysseys have) were so bad that no amount of cleaning could revive them. I finally was forced to look for another solution, and tried using some FSRs (force sensing resistors) and the results were great.

Continue reading “Rebuilding ARP PPCs”

Moog Multimoog

Moog Multimoog synthesizer
Moog Multimoog

Should I be embarassed to admit I had never heard of the Multimoog until this one showed up on our doorstep? The Multimoog was a Moog monosynth made between 1978 and 1981 and I’m not sure I understand how it was intended to fit into the Moog product line, or what is “multi” about it.  Continue reading “Moog Multimoog”

Moog Taurus (and a Prodigy)

Moog Taurus I bass pedal synthesizer
Moog Taurus I

The Taurus is a funny little (actually quite awkward and heavy) bass synth produced by Moog between ’75 and ’81 that is designed with a one octave, organ style pedal board meant to be played with your feet. It has a limited number of actual features… just one sawtooth waveform for both of its oscillators, the obvious 24db Moog ladder filter, portamento (glide), and a simple attack and decay envelope for the VCA and filter. It’s basically meant to do one thing well, which is make bass sounds, and it does it as well as any other Moog synth I’ve played. Though, I will admit it is a fun and unique experience to sweep the filter with my foot using a giant foot-sized slider.

Moog Prodigy
Moog Prodigy

We had a Prodigy in the workshop at the same time, which is the Moog Taurus’s immediate Moog monosynth contemporary. Comparing their sounds when set to equivalent settings, even before comparing the schematics it was clear that even at its highest cutoff setting, the Taurus’s filter, compared to that of the Prodigy, was still cutting a good deal of high frequencies. It was really designed to be used as a bass synth only. Continue reading “Moog Taurus (and a Prodigy)”

ARP Odyssey Mark III (and it’s our birthday!)

ARP Odyssey Mark III

I just finished up an inside-out full restoration of this ARP Odyssey (Mark III). I’m replacing the slider caps, which were all missing, with bat style toggle switch caps which I soften with a heat gun until I can slide them over the shaft of the slider, and haven’t gotten the green and blue ones yet, but wanted to take this photo and do a post because today is our birthday! Continue reading “ARP Odyssey Mark III (and it’s our birthday!)”

Moog Model 15 Modular

Moog Model 15 Modular Synthesizer
Moog Model 15 Modular Synthesizer (1974)

A Moog modular restoration is a very unique and specific kind of restoration experience, which I’ll try to show mainly in photos because it’s less boring for you. It will be like a vacation slide show with a fun anecdote to accompany each picture.

IEC C14 connector installed in Moog Modular Model 15

The first thing I did was replace the frayed power cable with its giant circular connector and the giant inlet for it on the back of the synth with a regular IEC socket mounted in an aluminum plate. The big Amphenol connectors are for connecting keyboards and other “Controllers” and for connecting other cabinets of Moog modules. The male end of the connector has a metal sleeve that screws on to this end like a garden hose. I also had to rewire the male end of the 952 keyboard for this one, which was damaged.

Opened up front “Console Panel”

The “Console Panel,” the front bottom panel which holds all of the connectors for accessing the signals brought into those Amphenol connectors on the rear, what’s supposed to be the main output mixer, and some other basic things opens up via a hinge on the bottom. Behind you can see parts of the power supply. Notice the capacitor mounted on stripboard. Sometimes you see a huge can-sized electrolytic capacitor in a power supply with many wires soldered to some very large solder lugs that it has for that purpose. My solution for replacing those is often to mount a normal electrolytic on a piece of stripboard, which allows for both easy connection of several wires to each lead and easy mounting of the whole thing by screws. Of course, I replaced all of the other electrolytics in the power supply as well.

Rear view of modules of Moog Model 15
Rear view of modules of Moog Model 15

Each module has a tag showing when it was built and inspected and some other info. These were all built in 1973 and 1974.

IMG_0978

Each module can easily be slid out to be worked on, but the ones that are double-width have two PCBs inside, mounted back-to-back, that have to be removed which is a bit of a pain. Above, here’s a look inside the 921 Voltage Controlled Oscillator. The density of the components on the board at right, as low as it seems, is actually atypical among these modules. Most of them are simpler and have even more spread-out layouts, like the one below which is one of the two parallel boards of the 904A Voltage Controlled Low Pass Filter.Moog 904A VCF PCB Get a load of those polypropylene capacitors! They’re the biggest ones I’ve ever seen. Also notice at bottom left, the transistors of the famous transistor ladder that makes the Moog VCF special.

All of the modules needed a lot of work and had what I think of as a “high casualty rate.” I started off by replacing all of the electrolytic capacitors in all of them.

All the pots in all of the modules are these big Allen Bradley pots which are unfortunately sealed and therefore cannot be cleaned. I guess it wasn’t unfortunate for the first couple decades, but by now they were all very noisy. Sometimes cranking them back and forth a couple dozen times will clear up the noise, but for most of these it was not the case, so I ended up replacing something like 19 of them.

Before I went back through for the in-depth troubleshooting I needed a reliable source of scaled CV, so I did the keyboard next. This included a full Pratt-Read action rebuild which I did not document (this is generally one of my least favorite tasks, that has to be done in many Moogs and most ARPs), cleaning the key contacts and working on the keyboard PCB.  The keyboard PCB had a fun surprise for me.

Moog factory 952 Keyboard PCB correction
Moog factory 952 Keyboard PCB correction

The keyboard was acting strange and there were all kinds of cut traces and wiring jumps on the board. What I finally figured out was that at some point a transistor had failed in the Trigger 1 circuit and was actually causing the fuse to blow. Someone had then come up with the “solution” of cutting traces to cut the Trigger 1 circuit out entirely and adding jumpers to tie CV 1 to Trigger 2. This meant that you could not use the keyboard unless you were playing two notes to activate the second (and now only) trigger circuit. If you patched something up with CV1 and Trigger 1 and played one note you would get nothing. I can’t believe they figured out that stupid workaround but could not figure out to just replace the transistor. Anyway, I undid all the “edits” but it still wasn’t working. Guess why.

One of the rework corrections (shown above after I put it back) was supposed to be there. I eventually figured it out by painstakingly cross-checking the PCB with the schematic. There is no documentation of the change because apparently it was a PCB layout error originally made by Moog and probably corrected in every unit even before they left the factory. The schematic only matches the board after the above change is made.

The keyboard trigger design, by the way, is fairly ridiculous and convoluted due to their insistence on using a keyboard with only one buss bar. Probably not what a modern designer would consider a solid design. There are situations here where an op amp comparator is precariously balancing in its high or low state by a margin of something like 16 mA. This is risky and not remotely the “best practice” way to set up a comparator and indeed, occasionally when I played two notes a semitone apart the comparator would just fail to flip and the second trigger wouldn’t work. I actually changed two resistors to adjust the references for the trigger comparators to give them a little more leeway.

pots, cermet trimmers, capacitors, op amps, transistor

Troubleshooting everything else was… a whole other thing. Most modules pretty much worked but almost all of them didn’t quite work! For example, here are all the things I ended up having to replace to get everything working right in the 951 VCO. Notice that I replaced 5 cermet timmers. Later, as I calibrated the other modules, I ended up replacing probably over half of the cermet trimmers overall as many of them were so bad they were impossible to set.

The VCF had fallen victim to maybe the same person who messed up the keyboard. Two transistors had been replaced but put in backwards because they didn’t realize the ones they swapped in had a different pinout than the originals.

The last step was calibrating everything which takes long enough that I actually don’t think it can fairly be described as “the last step.” It’s more like somewhere between 50 and 100 steps.

welcome to bulbtown

One thing that is kind of funny is that I failed to order the correct replacement pilot lamp multiple times. At one point I actually texted the Bulbtown.com “Miniature Bulb Identification Hotline.” Can you believe that’s a thing? Well, they didn’t text me back but I eventually figured it out.

Moog Model 15
there it is again

There it is again. I was pretty underwhelmed by the Moog Model 15 as a synth but I guess that’s not the point. It’s history and stuff. But gosh, I really wish there was more than one mixer.

ARP Axxe (#2) – repair tips for all ARPs

ARP Axxe
ARP Axxe

It’s been almost a year since I began working on the first ARP Axxe I restored, and I was amazed by how much more smoothly this second one went, though it’s not really surprising as I have worked on over 70 synths including a half dozen other ARPs in the year since then. Continue reading “ARP Axxe (#2) – repair tips for all ARPs”