Sunday, 1 December 2013

The Soundlab Diaries - Part 3 - Build disaster.

Problem 1

Two months into the build I noticed that occasionally VCO1 would "warble" when I touched the coarse tuning pot(entiometer). At first I thought it was a just a dodgy component that had been damaged as I worked, but closer inspection showed that several of them did it.
I put a post on the Electro-Music forum to see if others had the same problem, and they did, it seems that the little green Omeg pots I used are well known for going wrong after a few months.
These pots are sealed so there is no way they can be cleaned or fixed, and as it seemed they were all going to fail eventually I didn't have much choice, if I wanted a reliable instrument they would all have to be changed.

Problem 2

There are no other pots on the market the same size as the ones I have used, so I can't just buy a set of replacements and wire them in.
The panel is already pretty tight for space so anything larger would not fit.
The only affordable, smaller,  ones are clones of the tiny American "Alpha" pot. People on the forum reported good results with them but they require a 7mm fixing hole, much smaller than the 11mm drilled in the panel for the green Omeg ones, so they won't fit in easily either.

Possible Solutions

If time was not an issue I would simply make a new panel with smaller holes, but that would mean a complete strip down and re-wire, which would set me back months, so I have broken one of my personal rules and  ordered a new set of pots without knowing exactly how I am going to fit them.This normally ends up being completely the wrong thing to do and I regret it, but on this occasion I can't see any other way through.
There is nothing on the market to make these fit so I am going to have to make something. I have had similar problems before and in the past have tried these ideas.
  1. Reduce the size of the holes by filling them in with plastic metal and re-drilling, but in this case that would mean a complete strip down and would take ages. 
  2. Stick on a thin skin of aluminum, or plastic laminate, on the front panel and drill new smaller holes in that, but again that would mean a complete strip down.
  3. Rings made from rubber tubing slipped over post shafts to make them a snug fit, but I don't have any the right diameter.
I have just found some nitril o-rings that are 7mm internal diameter and 2mm thick which would give it an external diameter of 11mm... I think this might work and have just ordered some.
Finger crossed.

Update.. arghh.. I have just found some plastic pipe that fits perfectly...

The Soundlab Diaries - Part 2 - First stage build.

Front panel, half way through construction.
The final design was based arund a Music From Outer Space SoundLab PCB. This is designed to be a very simple switched synthesizer for making sound effects,  but it always screamed out to me as being capable of much more.
It is built on 72h (14.4") 3U (Eurorack) format panel, and the lettering was printed on a laser printer, laminated, and glued to the metal before trimming. It is the first time I have tried this method, and it produces a neat easily useable result, but nobody is going to be fooled that it is a professional job.
The sellotape to protect the corners and the Stig sticker are not staying :-)

Modifications from original

Behind the front panel, with main PCB and extension board
Like I said this little beast is capable of more than its original design and I could not resist building in the following modifications.
  • Routing switches changed for 4mm sockets,
  • LFO - Rate in fast mode reduced to 30-0.05 Hz, by increasing C14 to 22nf
  • LFO - LED on triangle output
  • LFO - Inverted triangle output added
  • AR - Threshold level on external input
  • AR - LED
  • AR - Inverted AR output added
  • AR - Attack/On phase output added to provide additional control gate.
  • VCO - VCO1 upgraded to be exactly the same as VCO2
  • VCO - Trigger input via a capacitor so they trigger on any square wave
  • VCO - Triangle output added (Not tested yet)
  • VCF - Control front end modified for logarithmic response (Not tested yet)
  • VCA - Control front end modified for logarithmic response (Not tested yet)
  • Control inputs on VCOs, audio and control on VCF and VCA have one direct socket (input 1) with no attenuator and at least one socket with an attenuator
  • All outputs via 1K resistor to eliminate problems with shorts, and multiple outputs can be directly connected together.
The circuitry for these additions and modifications are on a couple of fiber glass matrix boards, one at each end or the main panel, which are held in place by PCB brackets mounted behind potentiometers


Today, 1st December 2013,  the LFO, AR are completely working, the VCOs are working but not of the modifications have been implemented yet. The VCF and VCA have not been started.
Well that was what I thought...