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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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