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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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