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MFOS Synth 4: Resistors all soldered

Pete Brown - 16 December 2011

Phew! The resistors are all soldered. The board has a few hundred of them, so there was quite a bit of soldering.

Over the next several months I'll be building what is definitely my most complex electronics project to date: the MFOS (Music From Outer Space) Sound Lab Ultimate, Ultimate Expander and (if Santa brings one) Sound Lab Mini-Synth Mark II, likely all in the same home-made wooden case, side by side. The Ultimate and Expander are together a 3 oscillator monophonic true analog synthesizer with filters, envelopment generator, ring modulator, sample and hold and more. You patch between the different logical modules using banana cables, so it's a bit of a self-contained modular synthesizer. The Mark II is smaller, newer, and has a few fewer features, but a sound of its own. You also patch that with banana cables, and can integrate the two. This blog post is another in the series. Previous posts include:

Here's an image of the board with all the resistors in place. In the oscillator section to the left, one resistor in each of the three is not used. I marked it with a silver sharpie. In addition, a second resistor is left empty in each oscillator if you use the temperature compensating resistors. Since I'm going that route, that slot is empty and marked with a brown sharpie.


Overall the resistor soldering was enjoyable but monotonous. The only problem I ran into was a mistake I made when ordering. When I ordered the 3M resistors, I accidentally ordered 1/8W instead of 1/4W. Looking at inventory at Mouser, I realize why: they have no small quantities available for 1/4w 3M resistors. It's possible the board only needs 1/8W (I haven't calculated it), but rather than take any chances, I put two 1/8W 3M resistors in parallel in the two spots that required 1/4W 3M resistors.

NOTE: Don't do this. I was only thinking about wattage, but didn't realize that resistance drops when you do this. The two 3M resistors end up producing only 1.5M actual resistance. Thanks to Michael in the comments for saving me some annoying debugging later :)

Other than that, I forgot to swap out two replacement values for R184 and R189 as recommended in the MFOS page. I did that after I took the photo.


I could have just ordered some replacements, but I really didn't feel like waiting, or wasting the money. Here's another shot of the board with resistors in place.


Once everything is assembled, I'll set up some better lighting and take a few wallpaper-quality shots of the board. Here are some opinions on soldering all these resistors:


I actually found it relaxing to solder all those (well, once I got a decent fume removing fan). Resistors are pretty immune to all types of crappy techniques: other than making sure you have the right values, you don't have to worry a whole lot about them. Soldering transistors, on the other hand…

Next up will likely be capacitors or the trimmer pots.

posted by Pete Brown on Friday, December 16, 2011
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7 comments for “MFOS Synth 4: Resistors all soldered”

  1. Michaelsays:
    Just a word or warning - if you actually did what you wrote you are in trouble... 2 x 3M in parallel are not 3M anymore... (more like 1.5M)
    see: http://en.wikipedia.org/wiki/Series_and_parallel_circuits

  2. CarlDsays:
    Pete - Recall that Power (W) = V^2 / R, so you should be fine with the 1/8W resistors that you have. It would take over 612 volts to overload 1/8W 3 meg resistor.

    Since the highest voltage available in this circuit is 24V, any resistor with a value of over 4,608 ohms can be safely populated with a 1/8W resistor.

  3. CarlDsays:
    Just to clarify - you should be fine using a single 1/8W resistor instead of a single 1/4W resistor - you still don't want them paralleled, since as Michael already pointed out, that'll result in 1.5M resistance.
  4. Kelpssays:
    Since you're not experienced in electronics and I've always wanted to learn it, do you have any tips on books and sites for a begginer?

    I wish we had something similar to Radio Shack here in Brazil. :(
  5. Petesays:

    What got me started was the Netduino and some experimentation. As you can see from the mistake in this post, I still have a lot to learn. :)

    Lately, I've been reading "Getting Started in Electronics" by Forrest Mims

    It's REALLY good. Very quick to read and the explanations are perfect. No fluff.

  6. Kelpssays:
    Thanks a lot Pete. I'll take a look at that.

    Also, I found a few good component sites either in Brazil or that sell to Brazil. Time to start blogging this stuff in order to keep a record and help others with the same interest ;)


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