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CGS735 Synthacon filter / Elby Designs / Ken Stone
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author CGS735 Synthacon filter / Elby Designs / Ken Stone
ZoeB
Hi!

So I got one of these modules, rev 1.5 of the Eurorack version... My partner was lovely enough to build it for me. We got to the part where we're testing / calibrating it, and it won't make a peep without an input, whereas the guide suggests it should be self oscillating. It also sounds very odd with an input, like it is filtering, and there's no background noise, which sounds like it's pretty good for a first attempt... but the resonance is highly unpredictable, and sounds louder than the input signal.

I'm guessing we might have something in backwards in the main board, or have been given one or two wrong parts. (My partner suggests that according to the guide, we had one too many of one capacitor and one too few of another, so she swapped one out for one from her own supply.)

I was wondering if anyone here had successfully built one of these, and if so, whether they would be kind enough to take a photo of how it should look, or comment on any way in which they had to deviate from the build guide and schematic at all.

Thanks either way!
ollepetersson
I had the same problem. On c211 it says in the schematics "Reversed on PCB". So after reversing that and adding some solder on a few bad soldered parts I got it working.
/Olle
home_listening
I just finished one a few days ago and is working OK, very quirky resonance though. I've not used an original and have been told over and over that its an unstable circuit so I wasn't sure if it was the filter itself.

hmm C211? I soldered that in as depicted on the PCB but I'll change it around now and let you know if it improves anything.

I was also having trouble making it self osc as per the callibration guide btw

::EDIT:: Grr, I think I'm in the wrong mood for the patient task of desoldering at the moment. Don't have any spare electrolytic caps at the moment so I'll do it when I can just snip out the old one and pop in a new one. Easy.
paperCUT
I had no issues with mine, it was mentioned in the docs to skip installing one of the diode pairs to get a more controllable resonance. That's what I did and it turned out very stable.
negativspace
I've never built the Elby/Euro version but I've built a handful of the CGS35 in the last several months. I found the design very sensitive to the 2N2222, and when I used one I had in the bin from Radio Shack I had all kinds of weird resonance issues. I replaced it with one from Mouser's stock and it sung like an angel. I have no idea if the Elby version is exactly the same electrically but if so that's a possible spot to check.

Also on the CGS board, the final level control can be omitted and a fixed resistor soldered in its place. I used a 180k in that position, other values also had an effect on the resonance. Again, not sure if this applies to your module but no harm in mentioning it...

It's worth getting it running. Very nice filter!

Edit: on 12v I also skipped a pair of the diodes in all of my builds as instructed.
home_listening
negativspace wrote:


Edit: on 12v I also skipped a pair of the diodes in all of my builds as instructed.


So how many diodes does this leave you with in the divider chain?

I'd assume that in the elby build (12v PCB) they wouldn't even include them if ken mentions to omit in 12V but could very easily be/am probably wrong. I've got eight in a long row. Which diodes did you leave out in the end? I was just on cgs.synth.net and it looks like the original 15v pcb has 10 or so, so I'm thinking they may already be left out...
negativspace
The current CGS35 revision has 8 diodes, 2 of which are marked on the PCB's silkscreen layer to be replaced with links in a 12v build.

I just went to check the 735 and found this in the build notes:

Quote:
IMPORTANT! Further builds have discovered a problem when running this design on 12 volts. If you wish to run on 12 volts and find the module hard or impossible to set up due to uncontrollable resonance, short out every third diode in the diode divider chain. It will still work on 15 volts with these diodes shorted out.
home_listening
Yeah I also read that, but only glimpsed at the pic of the stuffed pcb.

Awesome, Glad I still haven't cleaned since the build - resistors legs-a-plenty here.
ZoeB
ollepetersson wrote:
I had the same problem. On c211 it says in the schematics "Reversed on PCB". So after reversing that and adding some solder on a few bad soldered parts I got it working.
/Olle


Thank you so much! I told Nina what you said and she did a bit of multimeter probing... apparently C211 is indeed the wrong way around. Incidentally, you're right, the schematics do now say "Reversed on PCB" as of 2012-03-31, although it's worth noting that a previous version of the same schematics from 2010-07-16 (also labelled 1.5!) doesn't. I guess a few people had this particular problem then!

Thanks again!
ollepetersson
thumbs up
ZoeB
Thanks again, it's working now! ^.^

Nina's just filing (or drilling, it looks like) the holes down now, so it'll actually fit in the rack...
ZoeB
So, to clarify, this is incorrect:



...and this is correct:



I've briefly written this up for future reference.
Poldenstein
I just stopped before soldering c211 as I noticed the warning in the schematics and came here to check. Glad I did it.
But I have already soldered all the 8 diodes. So do you think I should bother desoldering them, is it that much better?
Also I found a discrepancy between the BOM and the schematics.
In the BOM c207 is stated to be 10uF 50V, while the schematics says it's 1uF 35V.
So which is correct?
Thank you
Poldenstein
Bump?
roglok
Poldenstein wrote:
I just stopped before soldering c211 as I noticed the warning in the schematics and came here to check. Glad I did it.
But I have already soldered all the 8 diodes. So do you think I should bother desoldering them, is it that much better?


I built this circuit on stripboard and left out 2 diodes. Leave them in and use a couple of alligator clips to bridge them - like this you can compare...

I also tweaked the value of the 2K2 resistor as suggested by Ken. Mine does not oscillate on purpose - I wasn't fond of the crazy resonance, so I tweaked it to stop right before oscillation.

Poldenstein wrote:

Also I found a discrepancy between the BOM and the schematics.
In the BOM c207 is stated to be 10uF 50V, while the schematics says it's 1uF 35V.
So which is correct?


which schematic are you referring to?
L-1
I built this a several years ago on my home-etched PCB. I installed all suggested trimpots and, remember, trimmed them so long.. But at the end I got needed balance and now it sounds like a beast!

Poldenstein
Thank you L-1.
It is rev 1.5, both schematics, BOM and silkscreen.
roglok
Poldenstein wrote:
Thank you L-1.
It is rev 1.5, both schematics, BOM and silkscreen.


how about a link?
Poldenstein
roglok wrote:
Poldenstein wrote:
Thank you L-1.
It is rev 1.5, both schematics, BOM and silkscreen.


how about a link?


http://www.elby-designs.com/contents/en-us/otherurl.html?url=http://ww w.elby-designs.com/webtek/index.htm

That's the best I can link..
Click on the panther logo and look for the CGS735 row.
roglok
Quote:
That's the best I can link..

it's a frame based website. the global url won't help - you need to link to the frame source.

i found them:
http://www.elby-designs.com/webtek/panther/cgs735-synthacon/cgs735-sch ematics.pdf

compare it to ken's original schematics and you'll see that 1µF is correct. it's purpose is to eliminate DC offsets at the filter output. i guess the value is not too critical.

35V will be fine, more doesn't hurt...
Poldenstein
Thank you Roglok, you are very helpful. I was crawling in the same direction, trying to find ken stone's schematics.
This is the first time I try to understand schematics, so please be patient.
I decided to remove d203 and d206 as suggested also in elby schematics (did not notice it before). Now this last capacitor, and there's only hardware left...
It's peanut butter jelly time!
I suspect tonight is going to be a long wiggling night. twisted
roglok
good luck with your build, it's a great filter! thumbs up
Poldenstein
After having spent the night wiggling i can say this filter has personality to sell. Glad I removed d3 and d6, as it's wild enough even like this.
Maybe I did not trim it correctly, but LP do not seem to deliver that much with frequency under 12 o'clock.
I am not sure I did correctly with CV SPAN trim as build instructions say to start by turning it fully CCW, but I did not find a dead end on it, you just go on turning forever. I tried to set it so to have the widest frequency range when self oscillating, but I cannot get lower than 12 o'clock, is it correct?

One thing I hate about Elby kits and wonder how no one mentioned are the ORRIBLE plastic jacks. This kits do not come cheap, I cannot understand how one could find acceptable this things. There are already a couple that are too loose and loose signal when cable is moved. I know I will have to replace them in less than an year, and it will not be easy, due to the quirky column miniboard they are mounted on..
I got also a serge vcs that I bought together with this. I will start building it later today.
But then I think I am done with elby kits. Also I do not like the baroque PCB layout with waste of space and solder points (at least that's what it seems from my newbie's perspective).
roglok
Quote:
Maybe I did not trim it correctly, but LP do not seem to deliver that much with frequency under 12 o'clock.


yes, it's the same with mine - the lowpass range is a bit narrow... i think Ken mentions this in the docs, too.
Poldenstein
Thank you Roglok for confirming this.
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