MOTM-820 Assembly

I'm resuming a project that I started last spring, and then put in the shelf due to house-related projects.  This is a circuit board for a Synthesis Technology MOTM-820 voltage controlled lag processor:

At this stage, I have just installed the white power connector at the lower right, and the ferrite beads immediately above it.  All resistors and caps are installed, but none of the semiconductor devices are yet.

In this photo, the ICs are installed and I'm installing transistors.  There are two stuffed here, one in the center of the board, and one at right center; they are not soldered yet.  When I solder discrete devices, I generally like to do at least two at a time, and preferably 3-5 at a time, so that I can alternate between devices, doing a pin on the first one, a pin on the second one, etc.  That reduces the heat buildup in the devices.  

Here is a closeup of the solder side of the board.  The bending of the leads is called "cinching" the leads; it keeps the device in place while it is being soldered.  I bend them in different directions to help prevent shorts.  After it is soldered, the excess lead lengths are cut off.

With all of the transistors in, this phase of assembly is complete and the board is ready to wash.  In its kits, Synth Tech divides board stuffing and assembly into two phases: all of the parts that can tolerate water immersion are installed first, using an organic acid-based solder.  This solder does a good job of removing oxidation from pins and board traces, but it must be washed after soldering to remove the acid flux.  After the final wash, the remaining components are installed using a "no-clean" solder that doesn't have to be washed, but requires more care in soldering.  The board, ready for wash:

This weekend, I'll do more work on it and post more photos.  Once the potentiometers are installed on the board (they go along the top edge in the photo above), it'll start to look more like the final product.  

Saving Private Fizmo

As I posted back in September, I acquired an Ensoniq Fizmo that needed the factory voltage regulator replaced.  I finally found time to do the replacement a couple of weeks ago and the Fiz is up and running how.  Here's how it went:  

Fizmo on the workbench:  

To get to the CPU board, which contains the voltage regulator, you have to flip the synth over and take the sheet-metal bottom off.  (Rest the edges of the top on a couple of lengths of 2x4 wood, so that the weight of the synth isn't bearing on the knobs.)  Upon doing so, you are greeted with this:

There are six screws on top holding the CPU board in.  (In the above photo, all but one have been removed.)  There are also three that go through the back panel.  All of the rear panel jacks are mounted to the CPU board, so once it is loose, you have to slide it forward, tilt it out, and then very carefully slide it out from under the two grey ribbon cables.  There is one connector, just visible at the upper right, that needs to be disconnected.  Like so:

With the CPU board removed, you can see the panel board underneath.  The ribbon cables are going to a third board, underneath the metal shield.  I didn't remove the shield, so I don't know what that looks like.

A closer view of the CPU board.  The voltage regulator is at lower right:

Just for the heck of it, while I had it open, I took this shot of the underside of the pitch/mod wheels:

Here, I have already drilled out the pop rivet holding the regulator down, and removed the black heat sink out from under it.  The regulator's leads are still soldered in:

Here is the board with the regulator completely removed.  The silkscreened lines show where the regulator and the heat sink go:

A close-up of the culprit:

Here is the new one, soldered in, but without the heat sink installed yet.  The synth came from the buyer with the replacement regulator and decoupling caps in baggies; apparently he had gotten them from Ensoniq, but never got them installed since he didn't know how and there isn't a tech in this area.  The new regulator is an RoHS part and getting solder to stick to the leads was an absolute pain.

You have to mechanically secure the regulator to the heat sink to ensure good contact.  Without the heat sink, the regulator will burn up.  I found some machine screws that I had left over from a cabinet hardware installation, and a matching nut.  The screw was too long, so here I am cutting it to length with the screw cutting mechanism of a 3-in-1 tool:

The heat sink, ready to install.  The white stuff is heat sink compound, which helps with heat transfer from the regulator to the heat sink.  There was also a plastic electrical insulator between the old regulator and the heat sink (not shown here) which I re-used.  

The complete installation.  The board is pictured here mounted in a Panavise circuit board holder.

And from the back.  There's a small metal washer under the nut.  I only finger-torqued it, to avoid cracking the board.  I put Permatex thread locker on the nut to keep it from backing off.

Soldering on the decoupling caps.  These may not have been strictly necessary, but one theory has it that lack of decoupling encourages the factory regulators to fail.  In any event, it can't hurt.  Only problem is, there's no place on the board for them, so you have to improvise.  Most people use tantalum caps for this, but the ones that came with the repair kit that I got with this Fizmo were polystyrenes.  That's OK with me; polystyrenes are non-polar, so you don't have to worry about getting them backwards.  However, they had huge thick leads which were pure heck to get soldered to the regulator's solder-averse leads.  Here is the result; the white and red stuff is heat shrink tubing, ready to be shrunk.

You can't just leave those things dangling out in space, so the only alternative is to tape them down to the board.  I throughly mummified them in tape, just to preclude any possibility of a short.  

When I bough the synth, the little red lens that covers the alphanumeric display had come off; the owner had it in a baggie.  I needed to find something that would stick it back down without damaging the display; there is very little contact area for it to adhere to.  The perfect thing was electrically insulative, clear RTV silicone.  I put a bit all around the edge with a toothpick, put the lens on, and left it to set up overnight.  Here it is before I put the lens back on:

And finally, it's playing again!  The Fizmo in action:

The wall wart power supply gets pretty warm.  It's rated 1A so, with the new regulator (which probably dissipates more current than the old one), it's running right on the edge.  Wall warts rated higher than 1A are kind of hard to find.  I might replace it with a 1.5A open-frame linear.  

Miscellaneous update

The JD-990 has developed a nasty buzzing noise in the right main output.  I'm not sure what's up with that yet.  I had noticed a couple of weeks ago that the outputs have become kind of noisy.  They had a weird low-level rustling noise, kind of like some of the bad digitally generated white noise generators from the '80s.  

I'm working on an idea for a Statescape.  I've created a bunch of patches with "reverse" envelopes; that is, they have very slow attacks and abrupt releases, so that when you play a note, there's kind of a reverse-tape effect.  I'm going to record a track of these, run it through a long-period delay, and then reverse the result.  The reversed track should have "normal" sounding notes, but reversed decay of the echo -- it builds up instead of fading out.  I might also try adding the delay after the track is reversed; I've done a test track that way, and the results were encouraging.  There are other possible variations, such as forward or reverse reverb.  

There is an electronic music wiki that I have contributed a considerable number of articles to recently.  Most of my stuff has been technical articles; another contributor has added a few articles on artists, but there is very little else on artists, companies, or works yet.  I welcome other contributions.  You have to set up a user ID with Wikia, but it's free.

Minimoog mystery solved, maybe

Back in March, I put up this post on the mystery of who owns the first production Minimoog. At the time, there appeared to be two contenders: The Eboard Museum in Austria has a unit that they advertise as being S/N #1001. The Audities Foundation owns one that several third parties who have seen or used it claim is #1001 (the Foundation itself made no such claim, which should have been the first clue, but there was a confounding factor which I'll get to in a moment).

The Moog Archives seems to have resolved the dilemma. It now appears that the Eboard folks do in fact own production #1001. Audities also owns a unit #1001 -- but it's not a production unit; it's a prototype Model C! (All of the vintage production Minis are Model D, except for the handful known as the "Welsh Minis".) The Audities photo that I reproduced in the March post is not this synth. Here is that synth:



Note a few things about it. The big thing is the pitch and mod wheels -- they are completely different from anything that ever appeared on a production Mini. In fact, I'm not sure that they are wheels at all; they may be sliders. Second, note the A-440 oscillator switch; it's a plain toggle switch rather than the typical rocker, and it's a bit to the right of where the production Mini has it. Third, the pilot light: It's higher up on the panel than the production article.

(The thing sitting on the panel that covers the keyboard keys' hinge mechanisms appears to be a ribbon controller of some sort. My guess is that it was added later.)

I had speculated in the March post that Moog may have assigned serial numbers to prototypes and then later re-used those numbers for production units. And sure enough, it appears that that's what happened here. Another clue to the Eboard one is the white-background logo plate, whose legitimacy I had questioned back in March. The Moog Archives now says it's legit; it's the earliest version of the R. A. Moog logo. Unfortunately, we don't have the Audities one to compare to; if it ever had a logo plate, it appears that it was removed when the ribbon controller was installed.

I think we can conclude that the Eboard Museum legitimately has the first production Minimoog. The Audities Foundation has a prototype.

The Hammond is working!

Started it tonight. At first, I was getting no sound except for a slight hum from the reverb channel. Checking, I found that two tubes in the preamp were cold -- the 12BH7 and the 12AU7. And I thought I had them packed very carefully. However, it got me thinking: I've had problems before with tube sockets not making good contact. So I pulled the offending tubes and pushed them back in, very firmly.

Started it again, and it works! Some of the drawbars are a bit crackly, but I think that will work out with some playing. As usual with this unit, it takes two or three tries with the start switch to get the start motor to engage, but that's no big deal. Everything works and the rotating mechanisms are all quiet.

To celebrate, I hereby present some Hammond-guts porn. The preamp:

The power amp:

The reverb amp:

The run motor, scanner, and flywheel, illuminated by the pilot light:

Why does the Juno-60 sound different from the Juno-106?

As you already know if you've been following this blog, I have a Juno-106 that I bought new in 1984. I've always been a fan of its sound, and I keep mine going despite the reliability issues with the 80017A IC's. I've never owned its prececessor, the Juno-60. But many people who have owned both say that they don't sound quite the same -- the words usually used is that the 60 is possibly a bit more mellow (that dreaded means-anything-anyone-wants-it-to-mean word, "fatter"), while the 106 is possibly brighter, "cold" (another one of those non-words), and some people even say it sounds "digital".

So let's start by getting a couple of things on the board out front. The VCO, VCF, and VCA circuits are nearly identical. "How can you say that", you might ask, "when the 106 uses those 80017A's that keep failing, and the 60 doesn't?" Consider: The 60 uses a VCF filter circuit based on the IR3109 quad OTA. It uses a BA662 VCA to control the resonance. A second BA662 serves as the voice's VCA proper. As for the 106? The notorious 80017A is really just an encapsulation of three ICs and some resistors. The IC's? An IR3109 and a pair of BA662s! It's the same circuit, just in a smaller package. A lot of people don't realize that when you look at an ordinary IC, most of what you see is packaging; the actual integrated circuit is a tiny bit of silica embedded in the plastic. Roland bought a bunch of unpackaged 3109s and 662s and had someone encapsulate them, and voila, the 80017A was born. Similarly, the Juno-60's DCO circuit: the counter IC that times the DCO, the reset transistor, and the wave shaping circuitry are encapsulated into the much-less-infamous (because it seldom fails) MC5534 in the Juno-106. The voice circuits are, for all practical purposes, the same.

So assuming that there is a difference in sound (and I've heard enough reliable witnesses say there is), where could it be coming from? Let's take a look at the rest of the audio processing: the portion that follows the summing amp, which combines the six individual voice signals into a single mono signal. A lot of Juno players don't realize that there is not a highpass filter per voice; Roland cleverly placed the HPF switch on the panel to suggest that the HPF precedes the VCF, but it isn't so. There is only one HPF circuit for the whole synth, and it works on the summed mono output of the voice circuits. Following the HPF is a seventh VCA, which is tied to the VCA level control on the panel (that bit looks like a kluge; maybe I'll write about it later), and then the chorus circuit, which takes the mono input and produces a stereo output. There's a bit more stuff for the master volume control and the various outputs, but all of that is bog-standard IC amp and buffer circuits.

The chorus circuit on the two synths looks the same; there may have been minor improvements that have eluded my quick scan of the circuits, but they both use the same bucket brigade ICs, the same control circuits, and the same gain make-up circuits (there is no noise reduction, which anyone who has heard either synth on headphones has already realized). However, I found some differences in the HPF circuits. First of all, on both synths, the HPF is not a voltage-controlled filter. It's basically a set of four passive RC filters. The panel or recalled setting controls an 1-to-4 analog demux which routes the signal through one of the four.


The one on the Juno-60 is pretty straightforward. Here's the portion of the schematic:

The four outputs of the analog demux are on the right (the two inputs from the control CPU are on the bottom, and the mono signal enters at the top left at pin 3). The pins are labeled as to the corresponding position of the HPF switch on the panel. As you can see, going up from output 1 to output 3, the signal gets routed through progressively smaller-valued capacitors; the smaller the cap, the higher the cutoff frequency. The output for position 0 has no cap; it's just a straight wire, so position 0 of the HPF is actually no filter at all. It's straight through.

Now here's the corresponding circuit from the Juno-106:

As you can see, it's more complex. First things: I am pretty sure that the CPU is sending the two input signals, A and B, inverted. Therefore, the pin labeled "Y0" corresponds to position 3 of the HPF switch, "Y1" is position 2, etc. The opposite interpretation doesn't make sense when you look at the circuit.

Now note the first difference: The straight-wire output corresopnds not to position 0, but to position 1. The circuits for positions 2 and 3 look pretty similar to the ones on the Juno-60. But what's all that business connected to the position 0 output, around IC4b? Well, it sort of looks like a Sallen-Key filter, as used on the Yamaha GX1. What's it doing? Note C8, the 0.01 uF cap shunted to ground. That's a lowpass filter! This part of the circuit is acting like a bass boost. (C6, I think, is just there to keep IC4b from self-oscillating.)

Second difference: Note IC4a. In both the 60 and the 106, the "seventh" VCA that I mentioned earlier immediately follows this HPF circuit. Although the VCA is an oddball part ("uPC1252"; the only data sheet I've found is in Japanese, but some Googling reveals that it was manufacturered specifically for dbx), it doubtless is based on an OTA circuit, and like all OTA circuits, it loads the input some, particularly as the gain is decreased. The Juno106 uses IC4A to buffer the input to that VCA. The Juno-60 doesn't have that buffer; it couples almost directly, only separated by a DC-blocking capacitor. That means that the 1252 VCA is loading down the outputs of the passive filters on the 60, which introduces high-frequency rolloff. So the HPF actually acts a bit more like a fairly wide bandpass filter, particularly as the VCA level control on the panel is turned down and the input impedence of the 1252 decreases. That doesn't happen on the 106 because the buffer amp provides a constant high input impedence for the HPF output.

Third difference: The analog mux used is a different part. The Juno-60 uses a 14051; the 106 uses a 4052. They work basically the same way, but possibly the properties of the analog portions of the two circuits are different. I need to look into that some more.

To me, the biggest difference is in the configuration of the filters, with the 106 providing one "high pass" position which is actually low pass. And, the loading of the filter circuit on the 60 is probably significant; circuits with some rolloff above 8KHz or so are often perceived as "warmer" by listeners. Maybe I'll have to get a 60 so I can compare them myself.

Bringing in the Hammond

Last weekend, my stepson came over and helped me move the Hammond in from the garage to the new studio space. Before we moved into the new house, it was stored in (climate-controlled) storage, and then it had to wait in the garage for a while. All in all, it hasn't been played in about 2-1/2 years. So there are a few things that need to be done first.

About the organ: It's an A100, one of the spinet styles that Hammond produced mainly for the home market. Despite that, it's a full-up tonewheel organ, with exactly the same layout, sound generation, and controls as the venerable B3. In fact, if you are looking for that B3 sound but find the price tag daunting, you can pick up an A100 and get that exact same sound for $500-1000 less. I don't know why it is that the A100 should sell for that much less than the B3 when they both use the same components. The only difference is that the A100 contains a built-in power amp (two, actually) and speakers, so you don't have to have an external tone cabinet to play it. (Despite that, it does have a socket for connecting a Hammond tone cabinet, or with the proper adaptor, a Leslie.)

The A100 weighs about 350 lbs., and this one has a magnetic attraction for my toes. I actually dropped it on my toes once! So, of course, as we were moving it in, my shoe got stuck in a gap in the floor between the hallway and the room, and it nearly ended up on my toes again. But I eventually got myself unstuck, and now here it resides in all its glory:



Once it was in place, the first step, after a good vacuuming, was to unlock the generator's locking bolts. The generator, and all of the rest of the rotating mechanism, is suspended by a set of springs for mechanical isolation while in use. For transport, it has to be locked down to avoid damage. Here is one of the locking bolts, protrouding from the underside of the generator shelf:



Next step: oiling the generator and motor/scanner assembly. The generator has two funnels on top of it. To oil, fill each one of these funnels with Hammond oil once, and then let it drain. The oil runs into a resevoir and from there to a bunch of little cotton threads which convey it, via capillary action, to the many bearings inside the generator. I don't have a good shot of the funnels (there will be a video of this part up next week), but here is the front of the generator:



The motor/scanner assembly has a little pot on top of the motor which contains a cotton pad. To oil, squirt oil on the pad just to the point of saturation. More will not do any good; it will just wind up all over the place and possibly crud up the contacts of the scanner. Here's a shot; the motor is the square box, with the oil pot on top of it. The scanner is to the left:



Note that this is the run motor. The start motor is at the other end of the generator; it doesn't require regular oiling. However, while I was in there, I put a bit of oil on the mechanism that couples the start motor to the generator shaft. This particular organ has always had a bit of trouble with the start motor not engaging the shaft, and some oil seems to help.

Now, since the organ hasn't been started for a while, I'm going to wait a few days for the oil to propagate through the mechanism. As it happens, I had to go out of town for a few days on business anyway.

The next step was to reinstall the tubes, which I had removed and packed away before the organ was moved from the old house. This organ has a bunch of tubes carrying the Hammond brand. Now, Hammond didn't actually make its own tubes. I'm not sure who made them. They are all noted "Made in Holland". Here's a 6Y5 full-wave rectifier tube, from the reverb power amp:



And a 12AX7 from the preamp:



So when I get back, it will be ready to attempt to start. Before I do that, since it hasn't been on for a while and the filter capacitors are likely completely discharged, I need to come up with a way to limit the power inrush the first time it's turned on. I think I know where I can borrow a variac, but if I can't find one, the backup plan is to plug together all of the long extension cords I bought while were were building the house, about 200' worth, and plug the organ into that. That much wire should do a fairly decent job of limiting the inrush. I'll turn on the run switch for a few seconds, without trying to start it (I'll bet it won't start with that much voltage drop). Then I'll get rid of all the extension cord and attempt to start it.

A few glam shots: Some of the drawbars.



The start and run switches. Older Hammond tonewheel organs have a run motor (the one in the photo above) which is an old-style synchronous motor. It does a fine job of regulating speed once it's started, but it does not have enough torque to start by itself. So there is a second, compound motor which does the starting. It's sort of like starting a car: You switch on and hold the start switch (it's spring loaded) for about six seconds, while the start motor cranks it up. Turn on the run motor, wait a second or two for things to stabilize, then let the start switch go.



Something that a lot of Hammond players don't know about: This is the patch panel that alters the fixed presets (the reverse keys that don't correspond to a set of drawbars). You change them by moving the wires from one terminal to another. Hammond put a paper sticker on the back of the generator compartment cover that explained how. I was surprised to find this still intact when I first took the back off of mine, and I preserved it. I'll summarize it in a post next week. Note the little white cloth sack hanging from the far edge; it contains spare terminal screws. I've got some special plans for this panel, which I will explain in a future post.