Monday, December 31, 2012

Add Sample-and-Hold to Korg Mono/Poly

After my success with fixing the dead keys on my Korg Mono/Poly, I decided to try some of mods that are out there on the Internet.  One that seemed within my grasp was to, once again, follow the Old Crow and add a Sample-and-Hold circuit to the Mono/Poly.  What's the sound of a sample-and-hold circuit?  Well, this video shows how to setup the sample-and-hold on my modded Mono/Poly to get a great bubbly filter effect:

If you're interested in adding this to your own synth, I'd start with Old Crow's description here.  For my build, I used the same schematic as Old Crow, except I chose to hardwire it into MG1, instead of making it switchable between MG1 and MG2.  So, my modified schematic is shown below.

In terms of buying the parts, I went to Digikey, which is the standard place where professionals go to buy their electrical parts.  They've got nearly everything that you could ever want, but unless you know what you want with an engineer's precision, you can get easily flooded by all the choices.  If you've never bought from Digikey, I wrote a post to help guide you through the process.

For this build, let's talk about how to buy a few of the parts in the schematic...
  • The LF398 is the sample-and-hold chip itself.  I don't remember which one I bought, but try this one: Digikey LF398N/NOPB-ND.
  • The ferrite beads are a bit unusual to see in a hobbyist design.  I'm not sure how to pick the right ferrites for the filtering/decoupling task here, but Digikey M8697-ND is probably good enough.
  • You'll need a switch for "SW2".  Any single-pole double-throw (SPDT) switch will work fine.  Try Digikey 360-1801-ND
  • Finally, you'll see a bunch of caps in the schematic labeled "104".  This means that they are 10 * 10^4 pF, which is 100,000 pF, which is 0.1 uF.  These are commonly ceramic caps.  A decent choice might be Digikey BC2665CT-ND.
So, after getting all my parts, I wired it up on a piece of protoboard and then wired it into the synth (using a terminal strip to allow me to easily remove it for debugging).  Unsurprisingly, it didn't work.  This is a common occurrence with my tangled rats-nest electronics.  So, I spent some time poking around and finding lots of errors in my build.  Eventually, it worked!

Below are some pics of it in my Mono/Poly:

Elements of the S-H mod in my Mono/Poly.  Circuit board, Terminal Strip, and Switch.

Close-up View of my Assembled Circuit.  Ugly!

The red wire is how I chose to connect to the KLM-353 board for getting MG1.

Here's how I wired in the shielded cable to VR17 to get the noise source.

I'm not showing how I wired it into the mod wheel.  I've got additional modifications in this part of my synth, so the pictures would have been more confusing than helpful.  Sorry.

Once I got the circuit working, I was able to apply the sample-and-hold effect to anything that the mod wheel can affect.  On the Mono/Poly, that means the filter cutoff, the pitch of all the oscillators, or the pitch of just oscillator 1.  The most useful sounds come when applying the S-H to the filter cutoff.  The video at the top of this post shows that effect.  In my opinion, it's a great drone that can act as a fantastic bed for moody improvisation.  Enjoy!

Buying Parts and Using Digikey

Before I dive into my circuit modifications, I'd like to talk about how to buy parts.  As a non-professional electronics person, it can be hard to know what specific items to buy.  I mean, if someone says that you need a 10K resistor, where do you get such a thing?

Well, it's great if you can find what you need at hobbyist places like Sparkfun or Adafruit.  They really limit the choices to just the core essentials.  Having limited choices sounds like a bad thing, but it really is a blessing.  They have done all the work of sifting through the thousands of choices available and have narrowed it down to just the few choices that will likely fit what the hobbyist needs.  If you can find the part that you need at one of these places, it'll probably be the right one for you and you should buy it.

If they don't have what you need, you can go to a place like Jameco.  They've got more choices, which can be daunting, but they show a lot of pictures, so you can often shop by the pictures.

If you still can't find what you want, it's time to put on your big-boy pants and step up to "real" stores like Mouser and Digikey.  They're the places that professionals go to buy their electrical parts.  Their stores are ridiculously deep.  They're scary places to go at first, if you don't know what you want with an engineer's precision.  But, with a little practice, they get more comfortable, and then the whole world of electronics is open to you.

Let's go through an example of shopping at Digikey.  Nearly any time that you work with integrated circuits (aka "ICs" or "chips"), you'll have to use 0.1 uF capacitors to provide filtering ("decoupling") of high frequency transients on the power input line.  On most designs (even hobbyiest designs) 0.1 uF caps are everywhere.  Let's say you need to buy some.  In this case, they're so common that Sparkfun does carry them, so you should just buy them there.

Buying a 0.1 uF Cap at Sparkfun

But shopping at Sparkfun is not the point of this exercise.  The point is to try Digikey.  So go to Digikey and search for "capacitor".  You get 275,000 options.  Umm.  OK.  Now what?

Shopping for Capacitors at Digikey.  Lots of choices!

Well, for caps, you need to know what type (composition) of capacitor you want.  How do you know that?  Well, sometimes the schematic tells you ("electrolytic" or "polypropylene").  But, usually, it won't say.  What you need to know is that, for a given cap size, or for a given application, everyone seems to use the same type of cap.  It's tradition.  If you search around the web enough for people using a cap in a similar way as you, you'll be able to find out what everyone uses.

For synth hacking, most caps will be ceramic caps because they're small and cheap.  The biggest exception is for high capacitance caps (1 uF and bigger).  These caps are almost always electrolytic caps.

Returning to our example case of finding a 0.1uF cap for use around ICs, everyone seems to use "cheap" caps, which definitely means "ceramic" caps.  So, on the Digikey page, click on "Ceramic Capacitors".  This gets you down to 126,000 choices.  Note that the webpage has changed...

Digikey's page for filtering through all the choices for "Ceramic Capacitor"

This new page is giving you all sorts of filtering options.  Now's when we really start cooking:

  1. First, click on the checkbox for "In Stock".  Now we've got only 41,000 choices.
  2. Under "Capacitance", scroll down and select "0.1uF".  2,000 choices.
  3. Under "Voltage - Rated", use Ctrl-click to select "25V", "35V", and "50V".  944 choices.
  4. Under "Mounting Type", choose "Through Hole".  159 choices.

At this point, you've narrowed it down to a few pages worth of choices.  If the part you needed was a little less generic than simply a "0.1 uF capacitor", you'd probably only have a handful of choices now, instead of 159 choices.  So, you'd look at the pictures, maybe look at a datasheet or two, and then just pick one and go.

In the case of these caps, you probably want to down-select a little more.  Look under "Tolerance".  We probably don't want to pay for the best caps (smallest tolerance) and we never want the worst (biggest tolerance), so select "10%" and "20%".  That got us down to 128 choices.

Looking down through the first page of choices, the pictures all look like parts that I could work with.  Any of these would probably be fine.  Because I'm likely to be hand-soldering on a crappy proto-board, I think that I prefer the look of the caps with the long leads.  So, I'd probably buy some of Digikey Part BC2665CT-ND.  They're 37 cents individually.

Hopefully, a decent choice for a 0.1 uF capacitor from Digikey.

For cheap general-purpose components like caps, you should always buy more than you need right now.  I always buy extras.  That way, I might not need to order more when I go to my next project.  Having the parts on hand means that you can dive right in when the inspiration strikes and not have to wait for the shipping.  It's so much more fun when you can dive right in.

For something super general-purpose like 0.1 uF caps (or 10K resistors), I might buy 10 or more.  For these caps, you'll see that the price drops to 25 cents each when you buy 10.  Or, if you're really bold, you could buy 100 for only 10 cents each!  I'm not that bold....I'd probably just buy 10.

So, that's how one buys parts.  It's a very important skill to have when you're going to be hacking synths.  Anyone have any sites that they really like to use for parts?

Saturday, December 29, 2012

Dead Keys on an Old Korg? Try CaiKote 44!

My first synth was a Korg Mono/Poly and I still love it. I've had it for 4-5 years. I got it off eBay and half the keys didn't work. Luckily, since it was cheap, I didn't have too many qualms about diving in to try to "fix" (or possibly make more broken) all its broken parts. I didn't have much to lose!  And, once I did actually fix the broken parts, the success inspired me to keep going into the endless world of synth mods. So fun!

But that's not the point of today's post. Let's start simple. Let's fix the dead keys...

Out on the web, one can find a bunch of approaches to fixing dead keys on old keyboards. The best description of how to work specifically with these old Korgs is from Old Crow...his site is really should check it out...

His text and his pictures give you the confidence to dive right in, even if you (like me) might not have ever done anything like this before. Very inspiring.

Unfortunately, following his directions for simply cleaning the key contacts didn't fix my dead keys. Bummer.  After a bunch more internet searching, I came across a reference to this paint-on electrically conducting stuff...CaiKote 44. Oh man, this is where it's at...

A little of this stuff goes a *long* way, so all you need is the tiny little package shown on the left (not the bigger tub on the right). To use this stuff, simply: (1) follow Old Crow's description to get at the keyboard's rubber contact strips, (2) swab a very thin layer of this CaiKote stuff onto each rubber contact, (3) let it dry, (4) put your keyboard back together. Voila!

After applying this stuff, all the keys on my Mono/Poly worked. Outstanding.  After I got my Korg Polysix off eBay, I found that it had a few dead keys, too. I dug out my several-year-old tube of CaiKote 44, put a little bit on, et Viola encore! This stuff is great.

Oh yeah, for those of you on Muff Wiggler (an excellent discussion board), I discussed this fix in this thread:

And I just found this...another guy who used CaiKote on his PolySix.  He's got great pics:

Update: I just learned that LASynthCo is selling newly-manufactured key contacts for the Polysix and Mono/Poly.  They're not super-cheap, but it's a great option to have.

Update: As some of the keys on my Mono/Poly have stopped worked again (I guess the CaiKote is only good for a few years), I bought some new key contacts from LA Synth Co.  They work great!  You can check it out here.

Smell the Solder

Welcome.  I've been into musical electronics for a while now.  And surprisingly, I've found that hacking this stuff has been almost as fun as playing it.  Because I've relied so much inspiration from hacks and mods gleaned from the web, I wanted to push some of my experiences back out there to maybe motivate others. That's what I'm hoping to do with these posts.  So, here we go...

Sorry, no credits on this photo.  I pulled it deep out of my photo archive.  I pulled this off the web in 1998 (!) for use as potential cover art for one of my projects.  It never got used.  It maybe seemed appropriate here.