XR-2206 squirrely at 15 Volts (XR-VCO)

warrenmaximus

I got my XR-VCO to work yesterday finally, after a long battle with RF and general squirrelyness at lower frequencies. Even at the higher frequencies where it behaved better, I could see that there was RF happening. Yes, I built mine the hard way (if there is another, I plan to buy the pcb). The entire saga is documented here:

http://www.experimentalistsanonymous.com/ve3wwg/doku.php?id=synth_vco

To make a long story short here, I had enormous difficulty taming the XR-2206 until I dropped the supply voltage to it (and the peripheral circuits feeding it). I used a 220 ohm dropping resistor and a 100uF decoupling capacitor. I've identified the changes I made on the schematic towards the end of the page listed above.

I am unable to find it now, but another fellow had reported this problem with a breadboarded Exar test circuit. He found it worked ok at 10 volts but when squirrely at or near 15 volts. This was information I kept in my back pocket as a last resort.

I am assuming from a general lack of posts on this subject that this is rare, and perhaps all of this is related to layout.

My question really is one of curiosity- has anyone else here encountered this problem? My 2nd clue was that it would behave as soon as the power was switched off, and the voltage of the supply was decaying. I have five XR-2206CP chips and they all behaved similarly.

I also want to add my thanks to Thomas Henry for his book "An Analog Synthesizer for the 21st Century". I plan to also buy his VCO Cookbook (I think it was), which I saw references to this morning.

XR-VCO-Installed.png

Description:

This is my home brewed synth in progress. The XR-VCO module is the one on the right. The entire project's documentation can be found here:
http://www.experimentalistsanonymous.com/ve3wwg/doku.php?id=analog_synth

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XR-VCO-Debugging-but-working.png

Description:

This is a shot of the module being debugged with the oscilloscope.

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Tim Servo

Hi Warren,

I'm unsure of exactly what the problem could be, but perhaps you're getting some HF oscillation around one (or more) of the op amps? I might try some small value caps - say 20 to 30pF - from the output back to the input. I sell a PCB for the XR VCO, and I haven't run into this problem with those boards. Hope this helps. Otherwise, drop me a line if you're interested in one of my PCBs.

Tim (has also been described as squirrely) Servo

warrenmaximus · Posted: Sun Mar 11, 2012 10:14 am Post subject:

Hi Tim:

I did check for that, but oddly everything on the opamp side was well behaved. I have the XR-2206 tamed now with the reduced voltage, so to go back and recreate the problem now would be difficult. I do plan eventually to build a bench top function generator, so I'll probably revisit it then.

I was just curious if anyone else had a similar experience.

I have an email into Magic Smoke to get a proper NTC resistor, since I've got too much drift in the current unit. I had used two 1K NTC of unknown sensitivity out of desperation. Hopefully the drift will be much reduced with the correct part.

With the correct temp resistor, do you find that you need to recalibrate at all? I was considering if I need to bring R1 out to the front panel.

I've also ordered a 10 turn 500 ohm trim pot for R1 (replacing R1 and R2). I used a normal trimpot in the original build. Somehow I missed that little detail of "multi-turn". Smile

Thanks, Warren

NoizeToyz · Posted: Tue Mar 26, 2013 5:32 am Post subject:

Hey,
I see this is an old topic, but I wanted to confirm that I'm getting exactly the same problems with some XR2206 chips.

I think there's a dodgy batch (I got some off eBay which is never good!) It seems that even though they should work up to 26v, anything above 12v makes them just output crunchy noise.

For the record the chips that I had this problem on were stamped with F0919 (I presume this is a batch number).

Stay away from the brown acid...

warrenmaximus · Posted: Wed Mar 27, 2013 5:29 am Post subject:

Interesting that. I did get my chips off of ebay and perhaps they have the same issue. They do however have a batch number of F0730 for what its worth. Oddly enough, each of them acted somewhat differently in the original circuit.

I eventually got around the problem as articulated under the heading "Debugged":

http://www.experimentalistsanonymous.com/ve3wwg/doku.php?id=synth_vco#debugged

I essentially dropped the voltage to the chip and applied a low pass filter.

Warren.

marcmarc