Synth Service Manuals

This post consists of links to synth service manuals. I'm putting this up primarily for my own reference but putting it here it can benefit others as well. I will continually update this post.

http://manuals.fdiskc.com/tree/

http://www.synthfool.com/docs/

http://www.synfo.nl

http://www.fantasyjackpalance.com/fjp/sound/synth/index2.html -includes great quality Minimoog schematics

http://www.pro-one-synth.org/manual.php?section=5

http://www.hinzen.de/midi/juno-106/

http://www.rhodeschroma.com/?id=servicemanual

http://www.korganalogue.net/korgms/service/ms20/m20s.html

http://redchapterjubilee.tripod.com/moogsource.html

Obligatory "sorry for not posting" post

Sorry I haven't posted in the past couple of months. My excuse is as follows: Lay offs at work that I survived, super busy at work from taking on all the extra work, looking for my first house because of my new found job security, got robbed of my laptop and camera, setting up new laptop, mailing in rebate form for laptop (are they seriously still doing this shit?), being sick for the past three weeks.

I'm going to commit to a new post with sound files of the Polymoog. Looks like there won't be any pictures for a while though.

Up $#!+ Creek Without a Pedal. Polypedal Alternatives for the Polymoog (part 2)

Having gotten the expression pedal working, I then need a momentary footswitch or two. There are four back panel controls that can be connected to a footswitch:

• EXT SYN enables or disables external synth connected to S-TRIG and KYBD OUT
• TRIG MODE switches between single and multi triggering of the low-pass filter
• SUSTAIN is not a true sustain, it really switches release on and off, and release must be the same as decay
• GLIDE switches the glide (portamento) on and off. Internally to the Polymoog this would only affect the VCF when keyboard tracking is on.

I decided SUSTAIN and TRIG MODE were worth having. Unfortunately, in the days of old, Moog decided sustain pedals should use 0.206" TRS plugs rather than 1/4" TS. Their thinking was this would prevent the wrong plug going into an output. To find out more I recommend going to Fantasy Jack Palance and looking at the minimoog and Micromoog manuals (By the way, I should mention I used a minimoog scan from that site in a previous post without giving proper credit. The scans on that site are excellent). Anyway, the pedal contacts connect through the tip and ring of the TRS. The sleeve was meant for the shield. I knew I needed to get 0.206" plug so I looked at mouser. The only available option was the switchcraft plug here. Incredibly, they cost $9.10 each. Ouch. I had to buy the Cinch Jones connector too so i bit the bullet.

For the pedal I considered the Roland FS-6 because it is a dual pedal with each pedal being selectable between momentary and latching. Latching would be handy for the TRIG MODE because there is otherwise no where to select the trigger mode on the Polymoog front panel. However this pedal needs a battery, which I felt was silly. I also would prefer a piano-type pedal. I ended up choosing a dual Kurzweil pedal, KFP-2M.

My original plan was to cut off the 1/4" plugs, solder on the 0.206" plugs and be done. Once it arrived I was impressed with the 1/4" plugs and couldn't bring to cut them off.

I decided to grab some cable and make double-headed plugs instead. The switchcraft plugs actually use screw terminals instead of solder lugs, which I liked. Soldering onto the existing 1/4" plug proved to be a little difficult, but I got both working on the first attempt. Here is the final result plugged into the back of the Polymoog:

Up $#!+ Creek Without a Pedal. Polypedal Alternatives for the Polymoog (part 1)

When I got the Polymoog it didn't have the Polypedal to go with it. This is unfortunate because it can offer a lot of useful control, especially of the filter cutoff. On the other hand I really don't need a giant, dusty, probably broken, 30+ year old box that can only be used with one synth. I started doing some digging to see what my options were. First a run-down of the controls on back panel:

There's S-TRIG OUT and KB OUT. The Polymoog has a built in monophonic keyboard circuit that is used to trigger the filter, provide filter keyboard tracking, and control external synths. The Polypedal doesn't use either of these. I also didn't need either for my purposes, although the S-TRIG IN plus a screw driver to short it is very handy for troubleshooting the filter envelope.

The SWELL, FILTER, MOD AMT, and PITCH are all 1/4" TS 0-5V controls that are intended to be provided by the two expression pedals of the Polypedal. The EXT SYN, SUSTAIN, TRIG MODE, and GLIDE are all 0.206" 3-conductor jacks that are triggered by a contact closure between the tip and ring of a TRS plug. On the far right are the two accessory power outputs. In part 1 I'll be talking about the SWELL, FILTER, MOD AMT, and PITCH connections. In part 2 I'll talk about EXT SYN, SUSTAIN, TRIG MODE, and GLIDE.

The first thing I decided was to use a modern expression pedal, specifically the Moog EP-2. While some of the reviews I read weren't great (along the lines of "not nearly as bad as the EP-1"), I liked that it had an attenuator knob. Also it's from the same company, so at least it matches the Polymoog logo-wise. The EP-2 manual can be found here. It uses a 1/4" TRS plug with Sleeve = ground, Ring = 5V, and tip = output voltage. So my problem was how to get 5V to the EP-2. My options were the following:

• Add external power to the EP-2, e.g. add a 9V battery.
• Replace the Polymoog CV jacks with 3-conductor jacks and rewire to include 5V.
• Use the 5V from the accessory output like the Polypedal.

I decided on option 3. It was the least work and it would keep the Polymoog and EP-2 unmodified. I figured I needed three things-a 1/4" male TS plug, a 1/4 female TRS jack, and a 6-pin male Cinch Jones connector for the power. Then I had a revelation. Rather than get each of those and try to wire them together, I could just get a Y connector with the 1/4" female TRS jack and two 1/4" TS plug. I could cut of the appropriate TS plug and replace it with the Cinch Jones connector. The first thing I found was a product from Hosa, the YPP-136. It was inexpensive and even available at Guitar Center. However, once I saw it I realized a problem, it wouldn't be long enough to reach the accessories jack. I could extend the cable but that defeated the purpose of finding a pre-made connector. After a little more research (amazon similar items) I found a Monster Cable adaptor that could reach. It unfortunately cost $20 but I got it anyway. Please do not take this as an endorsement of Monster Cable. Monster Cable rants are welcome in the comments. After I got the cable, Cinch Jones connector, and EP-2 pedal I got to work.

The Cinch Jones connector needed to replace the 1/4 plug on the left. Verify with a multimeter. Monster doesn't even label the ends L and R.

Cinch Jones connector

Monster Cable stripped. The red wire needs to connect to 5V. Ground and shield are left unconnected since they will both be connected to ground at the Polymoog control input jack. Make sure ground and shield are cut off and aren't contacting any pins on the Cinch Jones. Heat-shrink would have been a good addition in hindsight.

Accessory connector view from inside the Polymoog. 5V is yellow. Note that the connector is "polarized" and the red and green pins are a little bit farther away then the others. Make sure you solder to the right pin on the Cinch Jones connector.

Final result with everything connected. It's definitely a stretch.

Here's what the result sounds like. In the first part I'm lightly controlling the filter cutoff on a patch based on "String." Second I'm attempting to be funky on a patch based on "Clav." Third I'm using the pedal hooked up to SWELL on a patch based on "String." Notice the high level of noise between examples. That's because I don't have the shield board under the filter/output board...and because the Polymoog is a noisy MF to begin with.

Using the Moog EP-2 pedal with the Polymoog by Bross

OTA Voltage Controls in the Polymoog - Pictures

Front panel

This is an inside view showing the front panel on top and the right control board on the bottom.

This is an up close look at the right hand control board. The 4007 chips are all socketed. Behind them are the preset resistors, also in sockets. In the center you see the resistors for the mod amt control (57k and 33k from left to right). Only two of the presets have LFO filter modulation. That's a pretty old school way to handle presets.

Back view of the sliders. All are hand wired. The green wire is ground and the red wire is V₊, which are daisy chained across several controls.

This is the top right board, which handles the VCF, keyboard CV, and other functions. The three 3080s in a row are from left to right mod amt, S+H, and contour. The coresponding transistors Q6, Q5, and Q10 are below each 3080.

OTA Voltage Controls in the Polymoog

Here's a use for OTAs (courtesy of the Polymoog) besides the VCA we already looked at. Technically this could be called a VCA as well. It's taking a voltage from a front panel slider to control the amplitude of filter modulation signals. We will look at the Modulation Amt. control. It affects the amount of LFO going to the filter cutoff. The top half of the schematic shows the front panel slider and other components from the right hand control panel board. The slider itself is a 10k audio taper pot with its wiper terminal in series with a 27k resistor. The 4007 is a CMOS switch. The ZZ control line determines if the output should be in PRE (preset resistors, upper right) or VAR (slider) mode. In VAR mode pin 12 is connected to pin 9 via an n-channel FET. The output of the 4007 is named PVMAN.

The lower half of the schematic is from the top right filter board of the Polymoog. Notice the PVMAN output connects to transistor Q6 which is in turn connected to the control current pin of OTA A9. This is essentially the same pnp transistor configuration we saw with the MG-1 VCA. The input of the OTA is a voltage divider, not a differential input like before. Note that the LFO is ±2.5V. The voltage divider reduces this to 100/(22000+100)*±2.5 = ±11mV. This is within the ±25mV input limit of the 3080. What about the control current I_ABC? With the slider all the way down we can see no current will flow and the OTA will be off. With the slider all the way up to V₊ = 15V the current will be roughly (15 - 0.7)/27k = 0.53 mA. This is also a reasonable result considering I_ABC for the 3080 can be 0.1µA to 1mA.

The output of the OTA goes to pnp emitter follower Q11 which adds several VCF control signals, S+H, LFO, contour, and keyboard amt, which all use 3080s and cutoff (PVCON), which is directly connected from the 4007 without a 3080. The emitter follower in turn drives an npn transistor (not shown) that sinks current dirrectly from the ladder filter. Next time I'm going to take some pictures while my Polymoog is still openned up so I can show what this stuff actually looks like.

Theory of the MG-1 VCA

I will be referencing the schematic in the previous post.

Q: Why is Q28 needed?
A: We want an envelope to be based at 0V, go to some positive voltage, then back to 0V. This is the control signal we expect for a filter, oscillator, and VCA as well. The problem is the 3080's control current pin (5) is internally connected to a diode which is connected to the negative supply voltage. If we wanted to connect a control voltage (with series resistor) directly to this pin, our CV would have to based at the negative supply instead of ground. This single PNP transistor circuit is the simplest solution. Here's how it works. When the envelope is 0V, the transistor doesn't conduct and no current is passed to the OTA. Note the -15V on Q28's collector at the start and finish of the envelope. During the peak of the envelope the transistor conducts, so the base-emitter voltage will be roughly 0.6V. The voltage across R104 is (4.5-0.6), giving a current of (4.5-0.6)/15k = 0.26mA. The collector voltage is then roughly 0.26m*10k+(V_ABC). V_ABC is a diode drop above the negative supply, so the collector voltage is 2.6+(-15+0.6)=-11.8. This corresponds to the -12 shown at the envelope peak on the MG-1 schematic. Actually, if we neglected the diode drop voltages of the transistor and OTA instead of using 0.6V, we get -12V exactly.

Q: What are R96 and R97 for?
A: According to an article on OTAs from Nuts and Volts magazine posted here (follow the link at the bottom to UsingOTAs2.pdf), the input resistors "help equalize the source impedances of the two signals and thus maintain the DC balance of the OTA."

Q: Why are they 100 ohm?
A: If they were larger, they would do a better job of equalizing the source impedance. However, there are two concerns. The OTA has a finite input resistance. The data sheet gives a typical value of 27k. If the resistors were larger, the differential input voltage would be reduced. The output of the ladder filter is already quite low. The second concern is noise. The larger the resistor, the noisier. These resistors are at the input of an amplifier, so any noise will be amplified as well.

Q: What is the gain of the OTA?
A: We found I_ABC is 0.26 mA at the peak of the envelope. We also know the transconductance (g_m) of the OTA is 19.2*I_ABC. This gives a g_m of 5mS. We can see our input voltages are two reverse polarity 2mV_pp signals, so a total differential input of 4mV_pp. The output current swing is then 4mV*5mS=0.02mA.

Q: What is the output voltage?
A: The OTA output is fed to the master volume pot in series with a 1k resistor. This is a clever arrangement that allows a standard pot to be used while the resistance, dominated by the smaller resistor, can be designed. The max output voltage peak-to-peak is 0.02mA*1k//10k = .02*10//11=18mV_pp. This corresponds to the 17mV_pp shown on the schematic.