On 9 Feb 2018, at 19:11, Richard Brewster pugix@... [cgs_synth] <cgs_synth@yahoogroups.com> wrote:It is probably due to power issues. The first place to start
troubleshooting is power distribution and the supply itself. Can you
give more information? It could be as simple as that you are overtaxing
the power supply.
Richard Brewster
http://pugix.com
On 2/9/18 10:23 AM, Mike Burnham mikejburnham@... [cgs_synth] wrote:
> All my oscillators in my Modular system to some extent bleed into each other and you can hear modulation when listening to pure waveforms.
> How can I avoid this or clean them up?
> I have 3 cgs VCOS and 3 EFM vcos
> I presume i need to use shielded cable to the outputs and have the shield grounded only at the pcb end?
>
> I suppose I should check to see if vcos are modulating through the PSU..
>
>
>
> Sent from my iPhone
>
> ------------------------------------
> Posted by: Mike Burnham <mikejburnham@...>
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> The CGS Modular Synth home page: http://www.cgs.synth.net/
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Message
Re: Bleeding oscillators
2018-02-09 by Dave Kendall
Agree with Richard Brewster that power should be looked at. . .
If built as stock, they have only 100nF bypass caps on the power inputs. It’s possible in most cases to replace them with 10uF bypass caps in the same locations, and add 2.5mm - 5mm pitch 100nF ceramic bypass caps on the bottom in parallel.
Be careful when removing the original 100nF caps - the PCB traces are quite thin and easily damaged.
In many cases, you have to pre-bend the 10uF caps’ leads slightly near the base of the cap, so they fit outside the 0.156” 4-pin power connector. It’s a bit fiddly, but they won’t quite fit unless you do it - the bypass cap solder pads are a bit too close to the 0.156” connector for most electrolytic caps. It’s worth it, and relatively easy with a bit of care.
FWIW, I also added ferrite beads (the small hollow ones) around the + and - input wires right on the power cables right near the 0.156” 4-pin power connector housing. Heat-shrink them on to keep them in place. There's no room on the boards for extra ferrite beads.
Some people add 100nF bypass caps to ICs on the board too. Wire them across +V and 0V, and -V and 0V. EFM boards don’t do this on any of the ones I have seen. Comparators and logic ICs are good candidates.
The VCO4 D has bypass caps quite a long way from the input (IIRC, at the far end of the PCB! ), so it’s a bit harder to mod, but could use a bit of improvement. . . .
VCO4E and VCO3D are easiest to work on. Don’t know about the others - early ones and 3500 and 4600 series etc.
None of this is a magic fix, but it will improve noise immunity to some degree on the EFM modules.
Some modules radiate noise more than others on to the power rails. It may be worth temporarily removing some suspect modules to see if the problem lessens or changes.
Modules with lots of LEDs can be noisy, particularly those with older regular LEDs, not more modern super- and ultra-bright ones.
Ken recommends powering some modules from their own “dirty” supply for this very reason.
Post some more info on your power system, and good luck !
D.