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MFOS Synth 6: PCB Complete

Pete Brown - 04 January 2012

Over Christmas vacation, I completed all the component mounting on my MFOS Ultimate board.

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:

The board now has installed all the capacitors, resistors, diodes, ICs, trimpots, and transistors specified in the parts list.

Here's the overall shot of the board. For the most part, I tried to use the best components I could reasonably get. One place where I decided to spring for a little bit better was with capacitors 49 and 50 in the filter. I don't have a DMM sensitive enough to measure pF capacitance, so I couldn't measure and match those. There are circuits you can build which will let you measure capacitance, but I'm not particularly interested in building one. Instead, I sprung for 1% tolerance capacitors figuring that will get them pretty darn close.


A thing of beauty!

Here's a shot of one of the three oscillators. You can see the 2k ohm tempco (temperature compensating) resistor mounted over the SSM2210. I don't have any appropriate thermal or potting epoxy around, so it's just touching it right now. I'll bed it once I test everything and I pick up the epoxy.



After I took these photos, I cleaned the bottom of the board using Flux Off Rosin Flux remover by Chemtronics. What a mess! It took the whole 10oz can to clean the board (at $17 a can!), and then it still (due to the "X" holes) redistributed some of the sticky flux to the component side. Once I finished up the can, I managed to get almost all of the flux off the top, and all of it off the bottom. Still, what a mess. You have to spray enough of the remover on to liquefy the flux and run it off the board. Any dissolved flux which stays on the board simply spreads out. That stickiness, if not removed, will attract dust and dirt, making more of a mess of the board.

I'll give it this: the bottom *did* get clean. The burnt flux from removing a couple mistakes (one transistor soldered backwards, for example) was all washed off, as were the sticky hard globs of resin.


Of course, this is before I solder on all the front panel wires, so there will be flux from that too.

Flux can be corrosive over time, not to mention ugly, so it's a good idea to clean it off. Other approaches to consider include using 99% alcohol if you can get it. I just ordered order some of this, because it is likely the same type of stuff for less than half the price of the Flux Off.

Rosin flux is more benign than most, and actually cures hard to entrap the contaminants. Don't get water washable flux unless you intend to do a really thorough job cleaning the board. Typical water washable flux is highly corrosive if only partially cleaned.

I've also heard that the particular solder I used for this project (60/40 from Radio Shack) tends to have quite a mess of flux. Folks have recommended Kester 44 Rosin Core 63/37 as a better replacement with both more stable soldering (less time in the intermediate state) and cleaner flux. I have some on order for the next few boards :)

Here are a few more glam shots of the board



For the next step, I'll either work on testing and calibrating the oscillators here (which will require building the 1V/Octave test board), or I'll start wiring some of the panel. Whatever I end up doing, I'll be sure to write about it here :)

posted by Pete Brown on Wednesday, January 4, 2012
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1 comment for “MFOS Synth 6: PCB Complete”

  1. CarlDsays:

    Getting all the flux off is a pain. Back in the 80's and 90's I worked at a place where we made very sensitive analog circuits (think picoamp currents). For cleaning, the boards were actually loaded into a regular dishwasher, washed with special soap, and then baked in a low temperature oven to dry. For my own hobby boards, I use Flux Off and a toothbrush. I do all SMT, which is a bit easier to clean - and smaller, so there's less flux in the first place.

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