Posts Tagged ‘SEM’

SEM Modifications – LFO Section

September 30, 2011 Leave a comment

Location of LFO Signals

As part of some design decision ages ago, Oberheim opted to only provide a sine wave output for the LFO section even though the triangle and square waves are available in the circuitry. Bringing both of these out is easy and well worth doing. The square wave is a little more work as it is present as a timing element in the circuit and its voltage is well beyond usable ranges as it swings pretty much completely between the +/18.5v power. For this, we need to construct a simple divider. Take a look at the picture of the component side of the SEM where I’ve marked the locations you need to bring out. In both cases, they are on pin 6 of the 741 ICs.

Attached Triangle and Square Wave Output Mods

The triangle is very straight forward, all you need to do is attach a 1k resistor to pin 6. The square requires you to make a small voltage divider between pin 6 and ground to bring it down to standard modular voltage levels. If you are unfamiliar with a voltage divider, you should make sure conversant now. You see these all over schematics and you also will regularly need these in your mods. Wikipedia has a good entry on the topic and I use this handy calculator for determining the correct resistor values. There are many such pages out there. In this case, you need a 1k8 resistor attached to pin 6 of the IC and a 1k resistor attached to ground. You then need to attach a wire to both of these resistors. I started by twisting the resistor leads together with the wire and then soldered and applied heat shrink to the join. I then used wire to connect the two resistors and used heat shrink to make sure they would be no shorts. This is the same method I’ve used throughout on all the breakout mods. It should be clear from the image.

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SEM – The Machined Panel Arrives!

September 29, 2011 Leave a comment

SEM Panel Ready for Wiring

If my work space is any indication, the hardest part of any project is the panel and enclosure. I have all kinds of fully stuffed PCBs sitting around that just need a proper panel or housing and they would be finished. I’m not even talking about one or two projects, a quick look around reveals 4 such PCBs and I know for a fact there are at least as many in various storage boxes hiding from my direct glare. I used Frontpanel Express in the past but while the product was good, I found using their software left something to be desired. in particular, if you wanted a dial gauge of some sort, getting it to line up properly was a frustrating exercise in 1/128″ increments. Even then it seldom was perfect. Equally frustrating was that the cost was just high enough to be on the edge of tolerable. Fortunately, I have found a better way and projects are now slowly ending up behind panels again much more quickly.

The key is to use HPGL graphic files for you text, gauges, etc. and import these into the Frontpanel Express software. The cost is considerably less and you have much greater control over graphic elements. This is probably no great revelation to any of you who have used Frontpanel Express. I had heard about this from the first time a made a panel. Unfortunately, HPGL is not the friendliest or widely supported format out there. I experimented with multiple methods on the web: using old HP plotter drivers, printing to file, and reimporting; saving to SVG or other vector files and uses translation programs, etc. None of them worked even close to satisfactorily. Typically one of two things occurred.  Either the output vectors were choppy because there were not enough points stored for the curve or the scale was off.  The later was particularly frustrating because a couple of the methods to get HPGL eventually yielded smooth outputs, but I could never get the scale right.  It always seemed off by just a slight amount so the gauges were always ever so slightly off.

The absolute best solution I’ve found is Corel Draw.  It supports HPGL natively, outputs perfectly, and is really easy to use and in many ways optimized for this kind of technical layout.  I also have Adobe Illustrator I still always use Corel for my panels (but Illustrator for pretty much anything else).  I’ve also heard that Inkscape supports HPGL but I’ve not tried it so I do not know.  In Corel, the trick is to scale everything up so that you get sufficient output points for all the curves to be smooth.  Just to keep the math easy, I scale everything up 10x larger than my desired final output.  Then I import the HPGL into Frontpanel Express at 10%.  Unlike my previous attempts, the scale is dead on perfect. I still use Frontpanel Express’ software to specify the milling of holes etc. but Corel does all the labeling, graphic, and gauge type stuff.  One really great thing about Frontpanel Express is that it is trivial to have them mill “d-holes” on your panel.  If you are using banana jacks, spend the extra dime for these.  You jacks will never turn on the panel again.  I even added Chinese labels to my panel as graphical elements and they came out great!

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SEM Modifications – VCF/Filter Section

September 29, 2011 3 comments

The filter section needs less modification for my project than both the oscillators (because there is only one VCF) and the LFO section.  I love the SEM filter as it is and as such, I really have no desire to upgrade any of the components or make any modifications that might alter the sound in any way.  Like the oscillator section, I do want more modulation sources than the SEM provides with the stock config on the rear Molex connectors.  The two CV inputs to the VCF are found on pins H1 and H2 while the EXT modulation source is on H3.  Just like the oscillators, I want the inputs on one of the pots of the front panel to be switchable from going straight to the VCF modulation or routed to the EXT modulation input.  See the post on the oscillators for more about how to make the leads, soldering, et.c

The only other wrinkle is that I have also added a waveshaping circuit that takes the outputs from the oscillators and outputs both triangle and sine waves.  I’ll have more information on this circuit in a future post.  I want these to have their own dedicated volume control and feed directly into the filter just like the pulse and saw waveforms.  I love the alpha pots with the push pull switches like the ones I used for the oscillator section to route the cv to either the modulation or EXT modulation inputs.  I use these all the time and they will be used on the panel to control the volume of the triangle/sine waveforms and also to select which is being feed to the filter for each oscillator.  Unfortunately, these pots only come in a linear configuration so I’ll have a later post showing how to make them behave with an audio taper by adding a resistor to change the curve of th pot response.  Looking over the schematics and probing around a bit on the SEM shows that there is a group of 100k resistors on the upper left part of the board where we will need to add a 100k resistor in parallel with the ones already on the board to add the additional CV input to the VCF.  Towards the middle-left part of the board, there is a 220k resistor where we will need to add 2 additional resistors for the sub-oscillator inputs.

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SEM Modifications – VCO Section

August 3, 2011 Leave a comment
SEM VCO Section – Top of PCB

A quick look at my proposed panel should make you realize that there are more connections than the SEM accomidates on the molex connectors and features available than are not  on the SEM.  There are a total of 3 frequency CV inputs on the panel and only two on the SEM (not counting the EXT input) and there is no pulse width modulation input on the SEM.  The addition of sine and triangle waveform outputs will be handled in a later post, this post concentrates on how and where to add the additional cv inputs – the frequency and pw mod.

The SEM has provision for two frequency mod inputs and an input that routes to the EXT of both VCO1 and VCO2 on its molex connectors.  The frequency mod inputs for VCO1 are B1 and B2 with the VCO1 EXT source on A1.  For VCO2, the frequency inputs are D1 and D3 with EXT on E1.  A quick look at the SEM VCO schematic(taken form Kevin Lightner’s Synthfool site) clearly shows where the external modulation points enter the VCO.  If you look at the inputs B1,B2 on VCO1 and D1,D3 on VCO2, you see that they just run through a 100k resistor summing node before getting mixed together the 741 opamps A2 and A7.  All we need to do to add additional frequency inputs is attach additional 100k resistors in parallel to those already there.  You can add as many as you need – I needed only one additional.  I could have used the EXT input without having to add any more inputs, but I find that I generally want to have the mods active without having to engage the EXT.  In order to maximize the usability of the SEM though, the second CV input on the panel is routed through a pot which has an integrated push pull switch.  When the pot is pushed in the mod runs through my additional input.  When the pot is pulled out, it routes to the EXT mod location on the SEM front panel.  This gives me the best of both worlds.  Make sure you are attaching your additional resistors on the correct part of the PCB so that they are before the 741 input and in parallel with the other 100k input resistors.  Oberheim hand matched the 100k resistors on the SEM but I am not going to bother as I do not need my extra modulation input to track precisely as I intend to use the existing 100k matched set for v/oct inputs and my additional input with an attenuator for LFO modulation.  the original SEM needed to be able to precisely track incoming pitch CV as in a 2 voice there was a sequencer for one of the inputs and in the 4 and 8 voice there was a programmer that needed to have predictable inputs.  If you need this kind of precision on your additional inputs, you will need to hand match your 100k resistors to the ones on the board or just replace all of them with hand matched ones.

Mods all attached and completed for the VCOs

Resistors mid-wire with heat shrink

The pulse width modulation is a similar add-on.  If you follow the schematic to where the PW mod internally to the SEM comes in, you will see another summing opamp (A5 on VCO1,  A10 for VCO2).  This time, use a 47k resistor attached as part of the summing resistor nodes.  You can where all the mods need to attach in the picture.  I’ve shown the top of the board to give you some landmarks.  You can attach to the components on top of the board but I tend to instead make all my modifications to the bottom surface of the PCB for several reasons.  First, it is really easy to melt a nearby wire or worse a near-by component.  The underside of the board simply gives you more room to work.  Secondly, these types of mods are easily reversible, putting your SEM back to its stock factory state.  Having these on the bottom not only makes reversal easy, but also repairs and undoing them in the future if you should choose to do so for some reason.

There are a couple of ways to go about attaching the resistors.  First, locate where you are going to attach the resistors on the bottom of the board.  This is best done with a multimeter using the continuity function (where it beeps when you have a connection).  Put one probe on the top of the board in the locations shown in the picture and find a corresponding solder pad on the bottom.  Once you do this you will need to solder the new resistors to these locations.  I used to make a small loop in each end of the resistor and trim them close.  Then I would solder to the appropraite pad, then attach the wire.  I have found over the years that any movement in the wires tends to fatigue the resistor lead rapidly and you will end up with it breaking off.  Now, i still form the loops but instead I attach a short length of wire to one and and then solder it to the board.  This simple extra step provides some flexibility and will not break off the way soldering a resistor directly to the board will.  Remember, solder is a physical connection, not a mechanical connection and these sorts of mods are relatively fragile.  After you have made your wires with the appropriate resistors, make sure you use some shrink wrap to cover the resistor.  You don’t want these shorting out on you once you are finished.  I use clear shrink wrap so I can see the values of the components.  I’ve included a close-up picture of a resistor attached to the SEM with the wires attached so you can see.  I’ve never had one of these fail.

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SEM +/-15v Secondary Power

July 23, 2011 Leave a comment
+/-15v Aux Power Board

+/-15v Aux Power Board

My plans for the modifications to my SEMs require the addition of two pcbs.  One is the dual waveshaper board that will add sine and triangle output waves for the two SEM oscillators and the second is a CV processing voltage board that adds 8 inverting/attenuating controls as well as a pair of inverters for the envelopes to add negative going envelopes to the SEM.  I already have a 5u modular system and so all my DIY stuff is designed to run on+/-15v and tapping off the SEM after the power regulation to use its regulated +/-15v seemed more than a little inelegant even by my standards.  I really didn’t want to redesign the boards (although I had to call in massive help on the waveshapers – more on that later) so I decided to add a +/-15v regulator board to provide me the necessary power for the auxillary boards.

One thing about me that is fairly constant is that I’m willing to spend extra cash on a project if it is going to either save me time or likely be of higher quality.   This is true because if you are getting into DIY, it isn’t going to save you anything over purchasing a unit where economies of scale come into play.  A quick look through the Mouser, Digikey, Newark catalog should make it painfully obvious how these companies do not exist to service the likes of you and me where standard pricebreaks beigin in the 1,000s of peices.  Additionally, I despise  working or any kind of strip/proto board.  For my power needs, I noticed that Ken Stone has a power supply board , the CGS66, designed to use an AC wallwart at 18v AC and deliver regulated +/- 15v DC.  I ordered four of the boards and built them minus the AC filter stage so they really are just 15v regulators with standard amp type connectors for the remaining modules.  The SEM in my casework leaves enough room that these boards are mounted to the back panel.  You can see the parts I left of for the AC filtering in the photo on the right.  Works great and now I have power for the other boards.

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SEM Power Section

July 22, 2011 Leave a comment
SEM Power Section

Always, always take the time to replace the electrolytic capacitors in your vintage synths if you find yourself opening them up for a repair anyhow.  It is relatively cheap and your synth will generally sound and behave better after you have given it a fresh set.  Electrolytics have a limited lifespan and therefore *all* your vintage babies probably are due.  Go ahead and splurge on some nice high temp and high quality caps to ensure you don’t have to do this for another twenty years.  The SEM is particularly easy as there are only two electrolytics on the whole synth.  To open an SEM, there are three srews on the PCB to remove and then you have to pull the two boards apart.  I hate this part.  They are connected with a series of pins around the edges and it takes a good bit of force to get them seperated.  This is another bad design decision as there is no way to seperate the boards without flexing them at least a bit.  I have had solder connection go bad from this process so try and minimize the number of times you remove the PCB.  If you are following along at home, try and open it, make *all* the modifications I’m documenting and then close it up rather than pulling it apart for each one.  There are not many mods to do (at least here) and chasing down intermittent solder joints is not fun.

There is not much to the power section on the SEM since the main work was handled off the SEM and in my case in the external supply as you can see in the schematic.  I’ve pointed out the interesting bits in the pic.  I’ve had two occasions over the years where a malfunctioning SEM was due to one part in this section.  Note the white Molex power connector – this is one I had to repair as it had broken off.  If your SEM is acting at all flakey (or if it is completely dead), start by checking the voltages coming out the power section first.  If they are not correct, your problem is likely to be here.  Once when I was probing around with a 5v control voltage trying to find where to insert the pulse-width modulation mod into the VCO1 circuit, the probe slipped and shorted across something on the board.  My head was turned at the time looking at the scope so I didn’t see it.  The result was a non-functioning SEM.  The gate LED would flash constantly and there were some random gates going on but none of the voice sections were outputing anything correctly.  The second time was more horrific.  It was late and I was trying to troubleshoot a problem that had derailed my day and I accidentally reversed the power supply voltages.  This of course released the magic smoke in the circuit and the SEM was completely dead.  In both cases, the voltages coming out of the power section, particularly the negative side, were way off.  As all the parts are readily available from Mouser and cost a total of something like $5 I decided to just replace the whole power section’s ICs and caps.  In both cases it turned out to be the 2N3638A transistor and therefore an easy fix.  I am not sure if I just got lucky or if the design of the SEM is such that the transistor sacrifices itself in the case of shorts or other power problems. I would not replace anything besides the electrolytics as a matter of course, but if you are having issues, check the voltages in the supply section.  You might get lucky and have an easy fix!

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SEM Molex Connectors Repair

July 16, 2011 Leave a comment

The bummer of most restoration or modification type projects is that often the piece you are working on has some other defects and they generally tend to not be minor either – at least that has been my experience. It typically takes some fairly major repair to even get me thinking “while I am in there anyhow fixing X, I might as well go ahead and do that whole Y thing I have been thinking about forever.” The most common problems in SEMs are broken Molex connectors on the back or a whole host of diffuse symptoms related to the power section. In this post, we will address the Molex connectors.  The picture shows you every Molex connector and pin type you will need to make any connections or repairs to your SEM.

First off, as much as I adore the SEM, the Molex connectors are complete crap. For as well as everything else seems to be designed, the use of the Molex connections on the rear of the unit and the interboard pin headers always kind of baffled me. True, the SEM is an older piece from the late 1970s but even then more standard 0.156” and 0.100” headers were available and common. If you have any broken connections on the rear of your SEM, you might be thinking what I was thinking: “I’ll just replace these awful Molex connectors with 0.156” headers.” It is a good idea except for the fact it will not work. The pins are spaced close enough that the 0.156” headers would fit but the pins themselves are too large to fit through the PCB. You would need to find a header with 0.156” pin spacing but with the smaller pins of a 0.100” header. I searched connectors high and low and never found a solution.  So, we are pretty much stuck with the Molex connectors as they are still made and available.  Until recently, they still made regular appearances inside computer cases. 

 I say “pretty much stuck” because you could opt for two alternate routes. First, you could just hardwire everything to the board and remove the Molex connectors entirely. This works great except that it makes removing the SEM from whatever housing you are using a major pain as you would have to unsolder all those connections.   It is however a common response and I’ve seen plenty SEMs go this route.  Alternately, you could use some sort of mid-cable interconnect but I did not find any that I particularly liked and they made the wiring messy to boot. Consequently, as much as I hate them, I had to order some of the old Molex bits.

Making Housings for Cables

A quick search of the web will turn up that what you need for wire housing are the 3 and 4 position Molex housings – Molex part number 03-06-232 and 03-06-2042 respectively. Mouser and Digikey both carry these.  These are just the ones you need to plug into the back of the SEM. You need to order the pins separately and these are male crimp pins, Molex part number 02-06-2103. You really should use a crimper for these and not solder them. Just go and buy a cheap crimper online. There are tons of them around $40 that work great and beat the hell out of soldering all those damn pins. Now that I’ve railed against soldering, I’ll let you know that if you are using 24AWG or 26AWG wire, you probably will need to solder and crimp these. The pins themselves are rated for 18 – 24 gauge wire but I’ve pulled the 24 out fairly easily so I end up crimping and soldering them. Here is the easiest way I’ve found to make the connections.  First, strip just enough off your wire so that the wire sticks slightly past the crimp section of the pin. Tin the wire with solder. Do it. Really. Always. I know you hate it but just do it – your life will be easier. Next slightly tin the inside of the pin. Just a little in the crimp section and slightly below. You are not trying to fill up the pin. If you put too much on, you will not be able to crimp it well. Next crimp the tinned pin onto your tinned wire. Now apply heat to the crimped assembly until you see the solder melt. This creates a great connection and is easier than soldering and them crimping.

Replacing Broken Terminals on the SEM

Now, the more likely problem and the one that for some reason never comes up on the Google searches, is what to do about those broken off terminals on the SEM itself. The above instructions will let you make plugs to interface the patch points but that only works if there is a terminal there to begin with. Pretty much every SEM I have seen either has missing terminals that have broken off (because they are crap) or some that are so weak that they are going to break off any second. My old tech (who did original warrantee work on these) once told me they pretty much expected to replace 2 of the housings any time a SEM same in because invariable one would break off just trying to get the SEM out of the housing. For this, you need the 3 and 4 position housings – Molex part number 03-06-1032 and 03-06-1042 respectively. Here again, these are sold without pins. Once you order the replacement pins, you will see why these are so fragile. The part is a female PCB pin with solder tail – Molex 02-06-7103. First, clean out any remaining pins from the broken one, use wick or a pump to clean the PCB holes. Next, insert the pins as far as they will go into the housing and then use a pliers to pull them back down. You do not want there to be any play in the connector when you solder it down. If the connector can rise up from the PCB, it is going to break off. Next, insert the connector back into the PCB observing the correct orientation of pin1 and resolder.

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