The REAL Moog 905 Reverb Schematic Revealed!!!

The Moog 905 reverb module is one of those parts of the modular system that typically never got as much attention as some of its flashier counterparts like the filters or oscillators. For those in the know however, it went on to play an essential role in helping to create the patches that have become a part of the popular canon of sounds and effects that the instrument has become known for.

Back in the 60's, reverb springs were a pretty new and exciting way to spice up the sound of an electronic instrument. Guitar amplifiers, home organs and even PA systems utilized them to add spaciousness to an otherwise dry sort of electronic sound.

This was a relatively cheap and easily portable solution to mimic the sort of reverberation effects available in a professional recording studio, which usually used a large hard-surfaced reverberant room or specially treated acoustic chamber with a speaker and microphones inside to create their sound.

Reverb springs work by using one or more actual metal springs suspended between two electrical transducers. The input transducer is hooked to circuitry that actually shakes or twists the springs in response to an electrical signal...the bigger the signal, the greater the movement of the the spring(s). The signal travels down the springs, and is picked up by another transducer that converts this motion into a small proportional electrical signal that is then amplified and mixed with the original signal. The delay in this signal caused by the time it takes to travel down the springs, plus the fact that the signal "bounces" up and down the springs several times as it decays gives it its reverberant quality. Shake or smack the springs, and you get a great explosion of sound as they clang around!

 

 

Bob Moog realized that this technology would be valuable in creating space and realism (or even unrealism) for electronic music, and so the 905 Reverberation Unit was born.

 

 

Using it was simplicity itself...it had an input and output jack, and a single knob. When turned completely to the left, only the dry sound of the instrument was heard, along with some coloring from the electronics involved (more on this later). As the knob was rotated clockwise, the dry sound was slowly lowered, and the reverberation effect from the springs was slowly added to the signal. At the full clockwise rotation of the knob, only the output from the reverb springs was heard. This allowed for a wide range of effects that added the reverberated sounds to the original dry signal in various amounts.

 

There has always been some confusion about the actual circuitry involved in this module.

This early 905 schematic, dated from mid-1966, details most of the circuitry fairly well, but the value of the resistor connected to the collector of transistor Q3 is missing from the document. The mix knob is either drawn incorrectly, or missing the necessary info to show that the dual-ganged potentiometer elements must be connected to operate opposite from each other for proper blending of the two signals to occur.

 

This module's schematic was also included in the Norlin Modular Service Manual, but it was reproduced with a number of nasty mistakes (as were some other schematics In that collection as well) that have continued to circulate around the web. Some resistors were shown improperly connected, some resistor values were not representative of what was actually being used in later production models, and the dual element blend pot was again drawn incorrectly connected so that no blending of the two different sounds would occur. The resistor connected to Q3 is still shown here with no listed value. The input to Q2 is shorted!

So, not much help.

Trying to build a working unit from this schematic would be frustrating, to say the least...

 

Over time, the schematic has had some much-needed corrections made (shown here in red) and was reposted by various individuals, but there are still errors present in these modern day versions, including a resistor on these newer models that is not shown in any of these schematics.

 

After a lot of sleuthing on the web, examining various circuit board photos of these modules, and more lately seeing some excellent photos at the Moog Forum posted by an individual called Rezin, I've been able to piece together what I believe is the most accurate version of this schematic to date.

 

For those like myself who are planning to recreate one of these modules for their own system, this may be very helpful in capturing the magic that this module brings to the party!!!

 

 

After examining the various photos available, I created this stuffing guide using a picture of the copper side of the board to enable me to see just how the various components were really connected together "under the hood" in an actual production model unit.

At this point, I began to notice some differences.

In all of the schematics up to this point, the circuitry connected to the output of the springs was shown as using the +12 volt buss as the positive supply, and the spring chassis and the return of the associated components was connected to the -6V buss. All other electronics in the module used only the +12 volt buss and ground for their connections.

Looking at the 2.2k resistor and 75uf capacitor connected to the emitter of Q3, you can see that they are actually connected to the ground trace on the PC board, not to the -6 volt supply rail.

The mounting plate of the reverb assembly and the chassis of the springs are connected to the uppermost trace on the board... you can get a better look at how the plate is mounted in these pictures of the unit. Notice especially the soldering lug that connects one of the plate mounting screws to that uppermost trace...

 

So, at this point I was thinking, "Perhaps this trace is the -6 volt rail shown in the schematics?!??"

 

Looking at some better pictures of the various modules available, it looks like that trace is connected to ground as well, through the shield of the cables attached to the reverb springs.

 

OK, so just where is the -6 volt supply rail on this unit???

 

In fact, as you can see here, pin 3 of the power connector (-6 Volts, 3rd tab from the top) is not connected to the circuitry at all in these modern examples of the 905 module. You can however see a red wire connected to pin 1 (+12 Volts, top tab) and a green wire connected to pin 2 (ground, 2nd Tab from top) if you look closely.

 

 

This makes sense to me because if -6 volts was actually connected to the main chassis and springs, it would be fairly easy to short this supply to ground, as the springs tend to poke out quite a bit inside of the module.

 

Yikes!

 

This might be one of the reasons this was changed in ongoing versions of the module.

Having the extra headroom that connecting this circuitry to both supplies provides doesn't make a lot of sense here anyhow, as the signals that are generated by the spring transducers are fairly small, even if you shake the springs vigorously. It would probably make more sense to do this at the front of the circuit, as the single transistor is driving a fairly low impedance load and could use the extra "oomph" from a larger supply, and the input to many spring assemblies is often isolated from chassis ground and left floating for just such a purpose.

 

I've read accounts from various individuals that this module adds a subtle but pleasant amount of coloring to the oscillator signals run through it, even if no reverberation is applied, and it is often used in this way as a "thickener" to the overall sound. This is probably due to the nature of the electronics being powered from a single ended supply, and the characteristics and biasing of the transistors used in the input and output stages. This is, in some ways, much like the acclaimed discrete transistor mixers used in in the earlier modular systems, which added their own pleasing element of distortion to the sound, delighting modern listeners. (Cha cha cha)

The electronics in the 905 module also invert the phase of the dry signal passing through it, so if it is mixed with the original signal source this will cause various cancellations as well, depending on the settings.

 

So, after all this investigation, intimation, instigation and implication, this is the schematic that I believe now represents the connections and component values of most production models of the 905 reverb.

 

 

Using the pictures of various production models seen on the web as a guide, there have been some component value changes from that early 1966 schematic, and one addition.

The input resistor R1 was changed from 33k to 15k.

The resistor R8 was changed from 100k to 220k.

The resistor R9 was changed from 220k to 330k.

The resistor R15 was changed a number of times during production. On some units I've seen it was a 330k, or a 470k. I show it as a 150k in these schematics because that seemed to be the most common, and that's what was used in the unit photographed recently by Rezin. Any other changes might have been dictated by the transistors used in that particular example, which are usually not readable in most of the photos I've seen.

A 100k resistor R17 (not shown in any previous schematics) was added from the module output to ground.

In the unit photographed and examined by Rezin, all transistors are NPN types 2N3391A.

The reverb springs available back when this was designed typically had an 8 ohm input impedance, and a 2-3k ohm output impedance. There's lots more choices nowadays, including multi-spring units with varying decay times, so some experimentation with different units might help you create your own perfectly reverberated customized listening experience as you roam this nation's highways and byways....

 

 

 

So, for those intrepid individuals who would like to build their own unit (or repair a janky one) I hope that this will help you in your efforts. It helps lessen any hum pickup if you keep the reverb springs as far away from the power supply and transformers as possible, and use good quality shielded cables to make connections to and from the unit.

Many, many thanks to Rezin and Tom from the Moog forum for their most excellent photos and insights into this mysterious unit (as the dry ice fog slowly clears).

 

So, join us again next time as we go ...

 

FORWARD INTO THE PAST!!!!!!

 

 

 

 

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A Quick Peek Under The Hood...The Moog 930 Powerful Supply!

Here's a look at the Moog 930 power supply, the supply that powers the larger 35 and 55 modular systems.

This module has been on my radar for a while as a prime candidate to be refurbished. It's the lifeblood (the beating heart, if you will) of the whole system, and aside from cleaning an intermittent pot decades ago, it's never had (or needed) any other service.

At 40+ years old, it's certainly more than ready to have the various electrolytic capacitors replaced. These type of capacitors don't last forever, and even though the ones used in this supply were very high quality devices, the time had come for them to be swapped out with nice new ones, and have the entire unit completely cleaned and refurbished. I've read that you can typically expect about 10-20 years of reliable service from most electrolytic caps, so these have done very well indeed!

The supply creates 4 different voltages...+12V, -6V, +15V, and -15V. Each voltage has its own separate card complete with adjustments to set output voltage, current limit, and overvoltage trip points.

Interestingly enough, the two 15 volt cards play almost no part in the Moog 55 operation. The only thing powered by the +15 volt supply is the trigger buffer card, which just draws a few milliamps of current, and the -15 volt card is not used anywhere in the system. These are both fairly robust cards too, with each one capable of supplying 1.5 amps of current. The only use for them I could find was at the accessory connector in the back, where you would connect a Moog ribbon controller, sample and hold controller, or drum controller, and none of these draw anywhere near that much current.

In the schematics for the CP3-A mixers they are shown as being connected to the 15 volt supplies, but in the cabinet wiring diagrams and in reality they are only connected to the +12 volt and -6 volt supplies. These supplies are even brought up to the top cabinets through the cabinet interconnect cables, but unused.

 

As you might imagine, I have big plans for these supplies in a future project...

...Yes, big plans indeed...Excellent.

 

 

Although Moog put their name on this, the individual supply cards were actually made by Powertec (apparently no longer in business) and are marked on the boards as model 1607. A nice bit of kit back then, and still pretty nice even today, especially after some much-needed maintenance.

 

This is what the cards looked like before being refurbished. Although the caps looked to be in pretty good shape physically (no cracks or bulges in the rubber end seals), measuring them showed that they were at less than 50% of their rated value. Not really surprising, given their age.

 

 

I replaced them using a combination of Sprague and Nichicon high reliability capacitors. This should carry the system at least several decades into the future. While I had the cards out for servicing, I replaced the large power transistor mica insulators with new thermally conductive pads to help them transfer heat to the large black anodized heatsinks more efficiently, and run cooler.

 

Each card had the old dried up heat sink grease underneath the card mounts removed and replaced with fresh thermally conductive paste to help transfer heat from each card's heatsink to the metal chassis for better cooling.

 

Here's what a card looks like after being cleaned and refurbished with new caps.

 

 

All connector points are cleaned by being lightly burnished to remove any corrosion with a small square of 3M Scotchbrite green scrub pad sprayed with a little Deoxit D-5, then wiped clean with a soft cloth. The same treatment is applied to the screws used for the connectors as well.

 

Here's the newly refurbished cards being re-installed back into the chassis, which has been previously cleaned inside and out with Windex to remove any old heat sink grease left behind.

Now it's starting to look powerful!

 

 

I created a duplicate AC input plug to power up the supply outside the Moog 55.

This will allow me to completely test and adjust the individual cards before the assembly is returned to service inside the modular system. All of the pins of the two Molex connectors (one is AC power in, the other is DC voltages out) are scrubbed with a small stiff nylon brush and D5, then cleaned with a soft clean cloth. This will remove any built up corrosion and make sure we get a solid electrical contact for all of the supplies.

 

Here's the supply back in place, and ready to go for another few decades ( at least).

The eagle-eyed among you will note the yellow factory " birthday sticker" showing the date of the system cabinet assembly, August 20th, 1974.

Happy Birthday to an old friend.

This year (2015) it will be 41 years old, and still going strong.

The work continues...

...And the beat goes on...

...Yeah, the beat goes on...

 

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The Secret World (And Surprising Hidden History) Of The Moog 960 Sequencer!

This particular blog entry started simply enough.

As you probably already know, Moog is making a limited run of their Modular systems, including the 15, 35, and 55, as well as the Sequencer Complement B.

An incredible feat, and I could scarcely belive my eyes when I saw this announcement (and the entire Analog Resurgence from Sequential Circuits and others) at the NAMM show earlier this year. For a few moments, I worried that I was actually having a stroke, and this was what my sputtering and sparking brain was coming up with to try and entertain me in the interim...("Hey, it's actually 1974, and everything's going to be just fine, so don't freak out, OK??")

 

I saw a thread on a forum recently where someone was asking why the sequencer was so expensive ($8500). A lot of hoopla erupted after that, with people alternately trying to explain why something like this might cost what it does, and others declaring that this was nothing short of highway robbery, and they'd much rather have their brand (insert name here) sequencer which is just as good, or maybe better, and just who does Moog think they are, anyway?!??!!?

As I continued to read through this thread, I thought that maybe I should profile the 960 sequencer next, and provide some pictures showing the high level of craftsmanship and quality that actually went into creating this excellent and iconic piece of technology, so I removed it from my System 55 and started taking some pictures of it, and that's when things began to get really interesting (more on this in just a bit)...

Now, to put things into perspective, the Sequencer Complement B is actually a collection of two 960 Sequencers, mated with two 962 Sequential Switch modules, a 961 Trigger Interface module, and a 994 Dual Multiples module all contained in a beautiful solid walnut extension cabinet. A serious amount of handbuilt technology for a serious amount of money?

 

Well, back in 1974, this same setup went for $3,295, and rightfully so. An inflation calculator will show that in today's dollars, this would now cost $15,894!!!

Suddenly, the $8500 price tag begins to look a bit better...

 

It's certainly not difficult to see why these would cost what they do, given the cost of components these days that would be used, the amount of effort required to locate and procure them, and the sheer amount of hand labor that goes into creating something like the 960 Sequencer. I haven't seen one yet, but Moog claims (and I have absolutely no reason to doubt them) that they will be making these modular systems and assemblies using the same components and manufacturing techniques that they used in 1974. This is almost unheard of in this day and age, and shows their commitment to getting this reissue right...for those that can afford it, of course.

Take a look at the beautiful wiring looms that are required to connect all of the components together. This would be difficult to make if you were already experienced at making these, and did this every day, for years. Now imagine that you have to recreate this starting from scratch, and the task begins to take on some weight...there's a lot of stuff to get right.

Boards will be created from the original artwork and lovingly hand-stuffed just like in the old days, using period correct components, photo etched front panels, etc, etc.

The switches are very difficult to come by these days, and expensive, and the pots...don't even get me started on the pots!!!

Allen Bradley hasn't made these type J series pots in years, but back in the day they were considered the absolute top shelf component, Military Grade, and they were expensive even back then.

It's a testament to their quality and ruggedness that every one of them in my 55 modular is still working 40+ years later. Switches too!

The Canadian company PEC makes what I would consider to be the closest currently available replacement, and even these run around $15-$20 each.

The 960 sequencer uses 25 (!) of these.

Almost all of the circuitry that powers the brain of this system has been out of production for years. The chances of finding replacement chips for these are low, which means that they will cost many times their 1960s-1970s prices (cheap enough back then, not so much now). These are Motorola RTL chips, an early type of digital logic chip.

As you can see, the 960 Sequencer uses a fair number of them, as does the 961 Trigger Interface and the 962 Sequential Switch modules. Major Kudos to the good folks at Moog for marshalling the resources to make these incredible instruments available again. How the heck did they do it?

 

 

And now, on to the Hidden History part of today's Feature Presentation, which is already in progress...

Hey! I've owned my System 55 for 30+ years, and gleefully modded various parts of it as well as performed all my own maintenance of it, and I only now realize that I've taken certain parts of it for granted.

Even in this sophisticated system, there were still sections of it that were hand built on perfboard (!) for certain functions. My system 55 is a pretty early one, and it seems that these hand-built boards were created like this before finished printed circuit boards were available.

The CP3A mixers were built this way...

Boards that do Trigger buffering...

 

...As well as hybrids like the attenuverter for the 904 filter...a circuit board grafted to the board that power is connected to.

 

I've seen pictures on the web of other modules that were more or less hand made, and after spending time modifying and refurbishing sections of the modular many years ago, I sort of ignored these oddities.

In less than a year, Moog was building the 55 modulars with actual printed circuit boards in these locations. Here you can see these boards in a later model. Notice that the individual hand-matched 100k ohm summing resistors have been replaced with a resistor network package for both the CV summers and the CP3A mixers (those light blue rectangular packages).

 

As I was taking some close up pictures of the sequencer, this (finally!) caught my eye...

What The What??!!!?!?

I'd always assumed that this was just another factory hand built module, but as I was paying more attention to it while photographing it, I suddenly realized that this board really didn't look like any other Moog handmade boards, and looking carefully at the opamps revealed a datecode from 1976,

after the module was built at Moog, and purchased by the college.

So now, a new facet of the history of my system had been revealed...it had been modified by someone while it was still at the college!

 

The board appears to duplicate the circuitry for each of the three analog voltage outputs of the sequencer using 308 opamps (a very good opamp at that time) to replace the discrete component opamps that were being used at that time on the sequencer. This would make the output voltages less sensitive to drift with temperature changes, and make it more stable, especially for live performances where temperature changes are likely more pronounced.

 

This is what the discrete component opamps would have looked like before all the circuitry was removed and replaced with the current board assembly...

 

The quality of the work is pretty good, which is probably why it never attracted my attention before. The area where the original circuitry was has been completely depopulated, and the board was nicely cleaned to remove any flux and solder left behind...a very pro job.

 

It makes me wonder if the work was done locally by a teacher or student, or if it was shipped off somewhere to be professionally modified. There were places in the 70's that specialized in performing modifications to synthesizers, although I can't recall reading about anyone performing this particular mod. I'd love to hear from anyone who might recognize this or have more information about it!

The only things I would have done differently would be to use low drift cermet trimmers like the Bourns 3362 in place of the carbon Piher ones that were used, which would make it even more stable.

Yeah, I know...picky, picky.

Trimmers like these were certainly much more expensive and less available back in the mid 70's, and even Moog used cheaper carbon trimmers in their original discrete component version.

 

After finding this, I began to wonder if there were other mods that I had glossed over as well, so I began to look at the other modules in my system with a fresh perspective.

 

I didn't have to look far.

 

The same discrete opamp circuit was used in the 962 sequential switch, and sure enough, there was the same replacement board installed in its place, a true chip off the ol' block.

This one used a precision multiturn trimmer (a nice touch) and the same board material and datecoded opamp. These were likely modified together, and a very nice job they did of it, too!

 

Now my curiosity was up. I had another 962 sequential switch that I has gotten quite a while ago from a friend, same circuitry but a different front panel layout. This panel was the same size as the one for the power switch and pilot light in the lower right corner of the main cabinet, so I had moved the power panel to the rear of the unit, and installed the much more useful sequential switch in its place.

Looking at the board for this unit, I found this...

 

Same idea, but an even more elegant implementation...a handmade etched and drilled printed circuit board, also complete with the 308 opamp. Whoah! It's apparently a thing, I guess.

Am I the only one who doesn't know about this?

 

 

Again, some nice low drift cermet trimmers certainly wouldn't be out of place here, but there's not much else to improve this groovy little mod, which apparently, has occurred to more than one set of crafty hands...

 

 

 

 

 

 

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