CGS Bi-N-Tic Filter

Well, the Super Psycho LFO rebuild / hot rodding is on hold for a couple of weeks. The issue: Each of the six oscillators has a 1M pot in the circuit that controls the osc rate. After thinking of various ways of combining the pot in series or parallel with a Vactrol's light-dependent resistor, I've decided that I'm not happy with any of them. There's just no good way to do it such that there isn't some value of the pot setting that makes the Vactrol ineffective, and vice versa. The right way to do it is to take the pot out of the oscillator circuit and have just the Vactrol in the circuit; then make the pot be part of a voltage divider that will supply an offset voltage, which is added to the external control voltage via an opamp voltage-adder circuit, and the sum of the voltages drives the Vactrol's LED. Problem: Although I can make a 0-5V voltage divider using the existing 1M pots, it's a bit iffy as far as staying well clear of the opamp's specified offset and bias currents. I'd feel more comfortable using a 100K or 50K pot, so it can supply a bit more current. But I don't have any. I'm in on a group buy at Muff's place, but it's going to take a few weeks for that to come together. So until that happens, the Super Psycho is in a wait state again.

In the meantime I have something else to play with:

This is a kit for a Ken Stone / CGS 57 Bi-N-Tic switched-capacitor filter. The kit is from the wonderful folks at Bridechamber and includes all parts. A close-up of the circuit board:

And the panel:

The kit incorporates nearly all of the mods and hot rodding that Ken calls out as options in his notes on this module. (The control that Ken labels "DAMPER" in his notes is called "RESONANCE" on this panel.) Switched-capacitor filters are very unusual in the synth world. I'm not quite sure why that is; they have a reputation for being noisy, but I think that rep might be the result of some early designs of the switched-capacitor concept that used mechanical commutators and crummy caps. I'm curious to build this and see how it sounds. I am going to add one mod: a switch to take the most significant bit of the digital count that goes into the multiplexor which selects one of the eight caps, and move it from the 4-bit to the 8-bit of the counter. That will have the effect of dividing the eight caps into two banks; instead of scanning all eight caps in order, it will scan over a bank of four caps twice, then switch to the other bank. I'm not quite sure what this will actually do; I think it will give the filter two resonance peaks. Anyway, it will be interesting to find out.

The Wrong Vactrols

UPDATE: I've posted a video demonstrating the Vactrol turn-off delay.

So I was all set to prototype the control voltage input circuits for the Super Psycho LFO rebuild. The basic idea is to feed the control voltage to the LED half of a Vactrol, and put the light-dependent resistor (LDR) half of the Vactrol in series with the rate pot of the specific oscillator (so there would be 6 Vactrols total). I have some VTL5C7's that I've had laying around from a group buy I participated in on Synth-DIY several years ago. I've never done anything with them.

So I went to the data sheet to find out how much current would be required to make the LDR vary between about 0 and 100K, which seems like a good operating range. (You don't want to get into the high resistances because the data sheet says the response is not specified above 1M or so.) And I saw these curves:

Note in particular the turn-off curve. If I want to bias it to operate between, say, 1K and 100K, then the turn-off time from full on is about half a second! Obviously, if you feed the input something like a 10 Hz square wave, the output is going to flatline. Never mind modulating at audio frequencies.

I know this is a fundamental characteristic of LDRs. However, Perkin-Elmer does make different types. On the same data sheet are the specs for the VTL5C6. Its turn-on and turn-off curves look like this:

Note the difference in the time scale. This one has a turn-off from 1K to 100K of 2 milliseconds. That's more like it! Trouble is, I don't have any of these. My usual go-to parts source, Mouser, doesn't carry Perkin-Elmer. Google showed me that Allied carries them, and I checked and they have hundreds in stock. So that's on order, and I expect them Wednesday or Thursday. Until then...

Rebuilding a Super Psycho, Part 2

OK, I know it's been a while, but I ran into some employment difficulties. But that's all resolved now. Anyway, I've been working on the Super Psycho rebuild. The first step, as I documented in the previous post, has consisted of moving everything from the circuit board that was falling apart to a new board. Below is a photo of what I've gotten done:

A few things to note. I replaced all of the resistors along the top of the board with new ones from my stock, mainly because desoldering the old ones was going to be such a pain; that was an area of the old board where I'd had to add a lot of jumpers and kluges. Most of the capacitors were salvaged from the old board; I tested them all first. The two green 100 uF caps at the left are new from my stock because the old board was an earlier revision and it didn't have these. There's also a 10 uF cap above and below these; I replaced the one at the bottom because the original was borderline (measured 8.1 uF vs. a specified tolerance of 20%).

I've installed MTA-100 headers everywhere that panel wiring will attach, a la Dotcom. This should help considerably with the soldering mess that the old board had along the top edge, where 42 individual wires were soldered in. Look, it's much cleaner now:

(This photo was taken prior to washing, which is why it appears to have a lot of excess flux; it does. I used organic solder for this part.)

I wound up replacing the transistors and the IC sockets. Really, trying to salvage sockets just isn't worth the trouble; I destroyed one of them during the desoldering, and even though I got the other two out, they had distortions that made them very difficult to insert into the new board. Sockets don't cost much. I also decided to replace all of the transistors after two of them had the base leads break off while I was stuffing them. The replacements aren't the same type; the originals as specified by CGS were BC557, while the replacements are 2N4403. (Why those? Because that's what I had on hand.) All they do in this circuit is drive the LED indicators; almost any PNP type should work for that.

So the basic board is almost ready to go. There are four 470K resistors that go on the board just to the left of the 8-pin socket at bottom center in the first photo. The originals were butchered and I didn't try to salvage them. These resistors form a passive mixer for the four oscillators on the board that don't have the switchable waveform. I want to take those signals off the board at this point, so I need to get new resistors that I can stand up and have a long lead to solder a wire to. This is in pursuit of one of the two mods that I've decided to make: adding a second output bus with mix capability. Below is a block diagram of what I have in mind:

There are six of these, one for each individual oscillator. I'll pick off the four non-waveform-selectable ones from the board as stated above, and I'll intercept the other two at the waveform selection switch common terminal on the panel. The signal will come into this block at left. Each will have a pot to mix it to the B bus. There will also be a switch that allows the signal to be removed from the A bus (the on-board bus). To save some panel space, I decided to use pots with pull-out on-off switch capability to implement the A bus switching. I wasn't going to do that originally because of the cost, but I found some inexpensive ones from All Electronics. Only problem: The switch is SPST and the "on" state is when the knob is pulled out. I want the opposite sense, so to create it, I'm going to invert the switch signal and then use it to switch a bilateral switch that will feed the signal back to the A bus.

The other thing I'm doing, which I haven't drawn yet, is adding control voltage capability. I'll do this by putting a vactrol in series with the resistor that determines the frequency for each oscillator. There will be two CV inputs, an A and a B, and for each oscillator there will be a switch that switches it to the A input, the B input, or neither. I haven't decided yet if I'll make any attempt to compensate for the non-linear response of the vactrol.

I'll build all of the new circuitry on a piece of stripboard. I'll make an auxiliary panel to hold the bus mixing pots, the A/B CV input switches, and the extra jacks. I haven't figured out yet how I'm going to do this. One possibility is getting a blank MOTM-format panel with studs on the back for a board mounting bracket; that would allow me to mount the stripboard to the auxiliary panel without any screws coming through the front. Another possibility is using JB-Weld to glue standoffs to the rear of a panel and mount the stripboard that way, Dotcom-style. The third option is simply to mount the stripboard on the bottom inside surface of the case, behind the panels. However, that would make it harder to move the combined modules later.

I had to order some more 470K resistors from Mouser to replace the ones I messed up. I also had to order additional 100 nF (0.1 uF) polyester box caps; they are used for decoupling, and the rev B board uses more of them than the old board did. I've got these now and I'll be installing them next week.

Rebuilding a Super Psycho

A couple of years ago I bought a partially completed Catgirl Synth Super Psycho LFO from someone. The circuit board itself was complete, but none of the panel connections had been made. The person who built it had bought a panel for it from Cynthia, and all of the panel components, still in the bags, came with it.

I set about completing the build, and ran into a problem: This board has a zillion connections to the panel, and when I did the soldering, a couple of pads lifted off of the board. No biggie, I thought at the time; I put in a few jumpers to bypass the damaged pads and completed the build.


Well, when I started testing, a lot of things were flaky. Some of the oscillators worked some of the time and not at other times. There were lots of random oscillation rate changes. When I investigated, I found several other pads that had lifted when the original builder was soldering in the parts. And I noticed that the original builder had had to put in a few bypass jumpers himself. When I set about unsoldering a few parts so I could put in more jumpers, more pads lifted! Pretty much anyplace where the copper wasn't covered with solder mask, it was coming off.

I finally threw in the towel. I ordered a new board from CGS, and Tuesday night, I set about stripping the parts from the old board. Here is what I started with:




I started by just cutting all of the lines to the panel. It was a big rat's nest anyway. When I put it back together, I'm going to put headers on the board and use MTA-100 connectors to connect the panel wiring, a la Dotcom.




Big pile of indicator LEDs, which I never secured to the panel. They are a mix of white, blue, and violet types. Obtained from lsdiodes.com (RIP).




The board, ready to strip. The original builder was nice enough to socket all of the ICs. Most of the panel connections, now cut, were along the bottom edge.




The solder side of the board. Note the numerous workarounds and hacks, including the cap hanging off the board:




A bunch more of the pads came up as I was desoldering. I don't know what happened to this one; I've never seen a circuit board shed copper so badly. Although I wasn't being quite as careful as I would be when soldering normally, I certainly wasn't trying to be destructive, and I don't think I used excessive heat on anything. Nonetheless, this is what happened:




So the board's obviously a goner. At this point, I've removed everything except the resistors:




Below: the new board, together with the old. The new one is a rev 2 which accounts for the slightly larger size and somewhat different layout.





Why didn't I remove the resistors? Lot of trouble. I just laid in a large stock of resistors, so I'll probably assemble the new board from my stock. I'll only go back to the old board for a resistor if I don't have one of the right value. I got everything else off the board: capacitors, transistors, ferrite beads, IC sockets, and the MTA-156 power connector. I'm not sure if I'll be able to reuse that last item; those MTA headers always get distorted and the pins come loose if you apply too much heat to them, and owing to the size of the pins it's hard to desolder one without using lots of heat. I damaged one of the IC sockets; I may replace them all anyway, just in case. Everthing else that survived will be re-used; I'll test the caps and transistors first.

Here is the panel. (EDIT: It's right side up now.) The panel is obviously from Cynthia, although the circuit board is not. I've done a couple of things to the panel: the stock Cynthia design has hi/low speed switches for oscillators 1, 2, 5, and 6, but the square/triangle switches occupy those positions for oscs 3 and 4. I've added speed switches for those two oscs, which are on the left. I changed the banana jacks for 1/4" jacks, since most of my stuff is MOTM and Dotcom.

Before I put it all back together, I'm contemplating some mods. The first thing I want to do is add control voltage capability for the osc rates. You'll note that the Cynthia panel has a jack for this. However, the circuitry isn't on the stock CGS board. On the version of the Super Psycho they sell, Cynthia adds a mod that uses vactrols to add CV capability to the oscillators. I'm going to do that too, but I may make some changes. I want to do the same basic thing, but I'm not sure if I'm going to use vactrols or a Fairchild optoisolator IC. I've got some Fairchild H11F1's, which I'm going to experiment with; I've read that their response curve is not very linear. Since I bought those, Fairchild has released a bunch of new parts, some of which are specifically labeled as being for analog applications. If I'm not happy with the H11F1's, I may order some of those and give them a try. (Note that Fairchild has since discontinued the H11F1.) Why not just use vactrols? Well, I do have some 5C3/2's on hand. But six vactrols takes up a lot of room on the circuit board, and the design of the oscillator circuits necessitates having a separate optoisolator channel for each oscillator.

Where will that board go? Well, I'm going to build an auxiliary panel. That panel will have a daughter board on standoffs, which will contain all necessary circuitry for my mods. And since I have extra panel space, I'm going to extend things a bit. Since each oscillator has to have its own optoisolator channel to have CV capability, instead of just one CV input, I'm going to have two, and each oscillator will have a switch to select CV input A, B, or neither. The extra switches and jacks will go on the auxiliary panel.

The other idea in my mind is to provide some capability to split out some of the oscillators. At first, I thought about just providing individual outs for each oscillator. I can pick off the individual signals from the main board, but they aren't well buffered. And anyway, I thought of something that will be more fun: the ability to have a second mix output, which individual oscs can be added to or removed from at will. The controls and the output will be on the auxiliary panel. I'm not sure how I want to do it yet: should I just have switches to choose one output or the other for each oscillator? Or should I have pots to mix the oscs into a second mix out? I'll have to think about that bit a little more. I might also add a DC offset capability to the second mix.

It's all VSE's fault

Remember this photo I put in my last blog post?

Something's wrong with this picture. And it was about to cause me a bunch of extra work. We'll revisit this...

I received the MTA-156 connectors from Mouser and made the harnesses to connect the +5V supply to the distro board, like so:

The DC output:

The AC input:

Here's the completed wiring to the distro board. The jumper wire is tying the returns together for the two supplies:

Next, I soldered the AC-side connections. For this module, I used 18-gauge zip cord as the power cord. On past modules, I've use 16-gauge, but that's really overkill; these things don't draw much power. Below you see the fuse holder, which is connected to the AC hot; the neutral is connected directly to the transformer, and there are some jumpers per Condor's installation instructions. There are also smaller jumpers that go over to the +5 supply. In the photo, the connections are complete, and I'm gooping all of the exposed terminals with insulating RTV.

The connections are complete, and I'm about to install the fuse. I'll start with a 0.25A fuse. Note the cable staple retaining the end of the power cord on the base.

Putting a plug on the end of the cord, being careful to get the hot side of the cord on the correct prong. (The hot side is the brass screw; the neutral side is the silver screw.) This is an "old fashioned" one that uses screw terminals; I don't trust the quick-connect plugs. In the second photo, note the built-in grommet for strain relief.

Now, it's time to test. But first, let's go back and look at something again. Remember from the last post, when I said that I always wire the +15 with a red wire, and the -15 with a green wire? Well, note carefully the silkscreen designation next to the red wire below.

Yes, I connected them backwards. And no, I didn't catch it before I powered it on. And yes, the smoke test succeeded -- in producing smoke. When I applied power the first time, I got nothing. Checking, I found that the fuse was open. It didn't appear to be actually blown, so I figured it was a defective fuse. I didn't have another 0.25A one, so I put in the next highest amperage I had, which was a 0.75A. I powered it up and I was initially encouraged to see voltage. But then I was puzzled as to why my voltmeter was indicating a negative voltage when I had it connected to the +15 side. Then the voltage started to drop. Then I heard a fizzing noise and saw smoke coming from the distro board's filter caps...

The result. Note the capacitor in the center, and compare to the one on the right. (The one on the right is for the +5V, which wasn't effected.)

So I had to replace them. 330 uF, which of course I don't have. Another order to Mouser. While I was at it, I ordered 35V, 105C types (the originals were 25V). Here, the two failed ones have been replaced; note that they are a bit taller than the remaining original:

Screw the distro board back down, power it up, and everything works. I didn't even have to adjust the voltages; they were all dead on.

Here's the (nearly) completed Block 4, with an MOTM-820 VC lag processor installed. The slot to the left of it is awaiting my MOTM-650. I still need to put on the top, but that's easy; it's just another piece of wood, held on with Velcro at the corners.

So why did I forget to verify the power supply connections? Probably because, just before I worked on it, I was reading this VSE thread on stupid accidents with gear. They jinxed me.

Continuing on Discombobulator Block 4

The adhesive is dry and the bottom rail is secure on the base. Time to add the side rails and the rear posts. I drive in flat-head screws through the bottom of the base (after drilling pilot holes first).




A couple of brads from the nail gun keep the side rails and rear posts from wanting to rotate.




At this point, all the woodwork is complete, except that I haven't put the top rail on yet:




Now it's time to add the power components. The three items you see here are: at left rear, the tall item is a Condor dual-voltage linear power supply that will provide the ±15V. The small object in front of it is the +5V switching supply. This is a return to the power supply configuration that I used for block 1. For blocks 2 and 3; I used a Condor triple-voltage linear to supply all three voltages, but the tri-voltage supply is large, heavy, and expensive, and I haven't noticed any difference in audio quality using a linear for the +5V vs. using a switcher. (Most modules, if they use +5 at all, will use it for noisy things like logic circuits or LEDs anyway.) At right is the MOTM-990 triple-voltage distribution board:




The Condor has posts for soldering on the wires to connect it to the distribution board. Below, the soldering is done: the red wire is the +15V, the green is the -15V, and the black is the return. Note in the photo above that the distribution board has terminals at both ends. You want to make sure you put the power in at the end of the board where the filter caps are; the terminals at the other end are for expansion. The photo below shows this. The MOTM-990 board is new to me; all of the previous Discombobulator blocks used a MOTM-960 for the ±15V, and a separate terminal strip for the +5V. That's kind of a pain in the rear to connect if you need the +5. One of the modules intended to go into this block is an MOTM-650 MIDI interface, which is one of the new series of MOTM modules that needs +5, and it will come with the six-pin power cable that mates with the 990.




A closeup of the +5V switcher. I didn't know until I unwrapped it that it needs a three-position MTA-156 connector for the AC input, and a two-position MTA-156 to connect the output. I considered just soldering directly to the header pins, but thought better of that. Unfortunately, the only MTA-156 connectors I have on hand at the moment are four-position. Yes, I know I can just let them hang off the ends, but I want this to be a clean installation. So off to Mouser to order some. And until they come in next week, I'm at a stopping point on the electrical work.




Now to put on the top rail. The distance between the bottom and top rails is a critical measurement. It needs to be just barely tall enough to allow the stiffening tabs on a Dotcom-format module to slip in between. If it's more than about 1/4-inch taller than that, the mounting holes on MOTM modules won't line up with the rails. And if it's short, of course, Dotcom modules won't fit. So I test-fit before I put the top rail on:




It turns out my side rails are very slightly short, about 1/16 inch. So for spacing, I'll insert these little split lock washers in between the side rail and the top rail; the mounting screw will hold it in place.




Here's the result; a pilot hole is drilled through the top rail and then into the end of the side rail, and a flat-head screw driven. Later, a piece of Velcro will go over the screw to hold the top on, which is why we use a flat-head screw that can be turned down flush.




And the Dotcom module just fits:




Meanwhile, my supervisor DJ keeps a close eye on things. Or not.

Discombobulator, Block 4

Started construction on the fourth block of the Discombobulator modular. With an MOTM-650 and a couple of other things on the way, plus wanting to build some of my own panels, it's time for more space.

I started with pieces that I pre-cut some time ago and put away, at the time I built block 3. The basic frame consists of five pieces -- a plywood base, a bottom rail, two side rails, and the top rail. This is what it will look like when it's done:



It's hard to see in the photo above, but the bottom rail is shorter than the top rail. The ends of the side rails are on the base, with the sides butted up against the ends of the bottom rail. This way, the bottom rail helps stabilize the side rails a bit. The top rail sits on top of the upper ends of the side rails. When it's done, modules will be fastened to the top and bottom rails, by drilling holes in the faces of the rails wherever needed, and using wood screws. The power supplies and distribution will be fastened to the base, behind the modules.

It begins with gluing the bottom rail to the base. A nice bead of construction adhesive:



Press it on there and clamp it down. It's centered at the front edge; there's a clamp on the base at each end to keep it from wanting to slide sideways while the adhesive is setting up.



Admittedly it's not much to look at so far.