# 3340 stripboard - pulse width detuning **URL:** \ **Category:** DIY STUFF\ **Created:** [April 4, 2020, 2:01am UTC](https://lookmumnocomputer.discourse.group/t/3340-stripboard-pulse-width-detuning/520 "2020-04-04T02:01:48Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![Myleswise](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/myleswise/32/2668_2.png) [@Myleswise](https://lookmumnocomputer.discourse.group/u/Myleswise)\ **Post date:** [April 4, 2020, 2:01am UTC](https://lookmumnocomputer.discourse.group/t/3340-stripboard-pulse-width-detuning/520/1 "2020-04-04T02:01:49Z")
Hi Gang, hope everyone is doing alright in these crazy times 🙂 I’ve built Sam’s 3340 VCO strip board 2 separate times, and both times I have problems with the PWM detuning the oscillator. I’ve even breadboard it and I have the same issue. All of the features work fine, I can calibrate it to track 1v/OCT, but if I touch the PWM pot or the PW pot it will slightly detune the frequency of the oscillator. I would consider myself an intermediate to semi-advanced DIY’er, I’ve quadruple checked my work and it all checks out. Anyone else have this issue ? I’m using the AS3340 chip FYI, and I have even swapped out multiple IC’s to make sure I didn’t have a dud chip but the results were all the same. Thanks and much love !!! ---
**Author:** ![fredrik](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/fredrik/32/682_2.png) [@fredrik](https://lookmumnocomputer.discourse.group/u/fredrik)\ **Post date:** [April 4, 2020, 2:36am UTC](https://lookmumnocomputer.discourse.group/t/3340-stripboard-pulse-width-detuning/520/2 "2020-04-04T02:36:53Z")
It’s a known issue with the 3340s, see e.g. this subthread: > [@1222 Tuner VCO Module](https://lookmumnocomputer.discourse.group/t/1222-tuner-vco-module/16/27): > > Oh, as a note, I built my #1112 Oscillator’s with ferrite beads as listed on the diagram, and they seem have enough resistance to cause PWM pitch drift when you change the PWM knob directly or apply a CV to the PWM input. This is a pretty common problem with the CEM3340’s as they are super sensitive to changes in current (on pin 3 if I remember right), but when I built my eurorack VCO Maximus boards, which are also 3340 designs, I used the same ferrites without issue so figured I would be ok. Un… ---
**Author:** ![binarymob](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/binarymob/32/5062_2.png) [@binarymob](https://lookmumnocomputer.discourse.group/u/binarymob)\ **Post date:** [July 14, 2020, 4:14pm UTC](https://lookmumnocomputer.discourse.group/t/3340-stripboard-pulse-width-detuning/520/3 "2020-07-14T16:14:11Z")
I just recently found [this](http://www.alfarzpp.lv/eng/sc/AS3340%20tip%20VCO%20_%20PWM%20.pdf) "To improve stability of VCO during PWM excursion, simple solution can be used: " - Is this related to OPs issue above? ---
**Author:** ![Caustic](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/caustic/32/15588_2.png) [@Caustic](https://lookmumnocomputer.discourse.group/u/Caustic)\ **Post date:** [July 14, 2020, 5:46pm UTC](https://lookmumnocomputer.discourse.group/t/3340-stripboard-pulse-width-detuning/520/4 "2020-07-14T17:46:57Z")
ah this basically uses a diode to reduce voltage swing. ---
**Author:** ![Trumac22](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/trumac22/32/3900_2.png) [@Trumac22](https://lookmumnocomputer.discourse.group/u/Trumac22)\ **Post date:** [July 14, 2020, 6:00pm UTC](https://lookmumnocomputer.discourse.group/t/3340-stripboard-pulse-width-detuning/520/5 "2020-07-14T18:00:06Z")
Interesting! Might give it a go - I had this issue pretty bad on mine. I did a few other tricks to tame it a bit which I posted here: > [@AS3340 VCO /w hard/soft sync, FM modulation](https://lookmumnocomputer.discourse.group/t/as3340-vco-w-hard-soft-sync-fm-modulation/249/30): > > So, I kept playing with it and managed to make some headway - I’ve now limited the pitch drift to about the the last 10% of the duty cycle. Not perfect but certainly much better! I stumbled across this thread: [https://www.muffwiggler.com/forum/viewtopic.php?f=17&t=231130](https://www.muffwiggler.com/forum/viewtopic.php?f=17&t=231130) and saw lots people repeating the problem (especially with the AS3340, like I’m using). I managed to treat the issue by including: 10uf electrolytic caps on the power rails \*100nf ceramic cap from pin 4 to ground \*changed t… ---
**Author:** ![analogoutput](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/analogoutput/32/29663_2.png) [@analogoutput](https://lookmumnocomputer.discourse.group/u/analogoutput)\ **Post date:** [July 18, 2020, 11:53pm UTC](https://lookmumnocomputer.discourse.group/t/3340-stripboard-pulse-width-detuning/520/6 "2020-07-18T23:53:50Z")
See also this > **[TH-VCO-MAXIMUS.pdf](https://www.fonitronik.de/application/files/9615/7191/3736/TH-VCO-MAXIMUS.pdf)** > > 3.50 MB which is the build guide for the Thomas Henry VCO Maximus. It’s a different circuit than Sam’s but might provide some ideas: > As usual Thomas Henry did design this project for 15V, alas, time > has shown that this circuit could not be used with 12V w/o a lot of > changes. Especially the PWM did have an impact on the overall > pitch. I am very thankfull that ‘chrissugar’ from Romania did a lot of > valuable work and tests to tackle the PWM/pitch-issue. I amended > this documentation accordingly. Evidently one of the modifications referred to is to power the 3340 with +11V from a TL431. Also here: [https://modularsynthesis.com/mrmrshoes/maximus/maximus.htm](https://modularsynthesis.com/mrmrshoes/maximus/maximus.htm) > The PWM control did affect the frequency as has been noted in various forums. I changed the 10R power series resistors to ferrite beads and added a -5V regulator which stabilized the frequency when changing the pulse width. (Not clear where that regulator fits in.) ---
**Author:** ![fredrik](https://yyz2.discourse-cdn.com/free1/user_avatar/lookmumnocomputer.discourse.group/fredrik/32/682_2.png) [@fredrik](https://lookmumnocomputer.discourse.group/u/fredrik)\ **Post date:** [July 19, 2020, 12:20am UTC](https://lookmumnocomputer.discourse.group/t/3340-stripboard-pulse-width-detuning/520/7 "2020-07-19T00:20:58Z")
You can use a -5V regulator for the negative supply, instead of relying on the built-in zener. (not getting the feeling that these mods are very scientific, though 😀)