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Modifying the XR2206 kits

A project log for LissaJukebox (aka dedicated Lissajous machine)

This is my attempt to build a device that will trace Lissajous curves on a CRT with input from the user.

Emily VelascoEmily Velasco 01/29/2019 at 21:382 Comments

Let's talk about the XR2206 function generator. First, some brief background, then how I'm using them in this build, and lastly, the modifications I'm making. 

Background

The XR2206 is, according to its manufacturer, Exar,  a "monolithic function generator integrated circuit capable of  producing high quality sine, square, triangle, ramp, and pulse waveforms." From what I understand, the chip is considered obsolete, and is no longer made by Exar. However, knock-off versions are still manufactured in abundance by factories in China, and its these chips that serve as the heart of the XR2206 function generator kits that can be found on eBay, AliExpress, Banggood and elsewhere for a mere pittance ($6-$9 US dollars). 

The kits are a quick, fun build, and make a useful, if not high quality, function generator that's good for hobbyists. They usually come bundled with a laser-cut acrylic case and, IMO, look pretty attractive when assembled. 


How I'm using them

Since these kits are cheap, easy to build, and small, they seemed like the perfect way for me to supply variable waveforms for my project. Assembly went quick, maybe a half hour for each one, and everything went smoothly except for one screwup on my part that required me to bodge in a bridge wire after I overheated a via on the board. 

As nice as these kits are, they do have some features that limit their usefulness as the "brains" of a Lissajous console. These kits are going to serve as the main point of interaction between the console and the user. Each board has three potentiometers and two sets of headers that can be manipulated to created the desired waveform output. Here's an overview: 

Because these kits are so inexpensive, the components used in them are really cheap. The potentiometers mount to the board through solder connections, and two tabs, and the headers use tiny plastic jumpers smaller than your pinky fingernail. These components work fine in kit form, but not as part of a console. The potentiometers do not provide a way to be panel mounted, and the jumpers are fiddly to manipulate and are apt to get immediately misplaced.

The modifications

The first modification I made was replacing the stock potentiometers with more robust versions that can be panel mounted. While I was at it, I swapped in a 10K pot for fine adjustment because I found that the stock 50K pot wasn't well suited for making fine adjustments. I gave all the pots long leads so I would be able to place them where I want them in a front panel. It was at this step that I ended up needing that bodge wire. I wanted to make sure the kit worked before putting aftermarket pots in, so I soldered the stock pots in first. The kit worked, so I removed the stock pots. Unfortunately, one of the pots took a via with it when it came out. If I were going to do this again, I would solder in the long leads, and then solder the stock pots to them, instead of soldering them right to the board. 

The next step was to do something about eliminating the header/jumper range selector combo. As you can see from the schematic, the frequency range is set by selecting one of five capacitors of various values. 

I contemplated using a patch cable and banana jacks (as found in synthesizers) but ultimately decided that a rotary switch would be easier for the user, more aesthetically pleasing, and importantly, cheaper for me to purchase. Here's how I wired it into the circuit: 

And here it is attached to the board. As you can see, I left the original header on, and attached the switch to its contacts. I didn't actually connect all five capacitors to the switch, and that's for two reasons: 1.) The deflection coils of the CRT yoke don't respond much to the frequencies at the top or the bottom of the XR2206's range, so those range selections were kind of useless in this application. and 2.) Rotary switches are quite expensive, and my local electronics store only had available one with three positions and one with four positions. 

 That's all for now! 

Discussions

Emily Velasco wrote 01/29/2019 at 22:54 point

You're welcome! Let me know what you think of them when you get them. I'm curious if all of the kits have the same flaws (clipping on the triangle waves at higher amplitudes, for example) even if they come from different manufacturers

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Dean Segovis wrote 01/29/2019 at 22:49 point

This is moving along quickly. Of course, now I want one, or two, at my work bench to experiment with. Hello eBay!

Thanks for the post and the info :)

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