CGS48 VCO Problems

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srfnsmy
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CGS48 VCO Problems

Post by srfnsmy » Thu Mar 24, 2016 3:35 pm

Hi Everyone!

Having problems with getting my CGS48 calibrated correctly.

I'm running on 12V power (I did switch the 20k resistor for the 14k)

I've wired this according to Zthee's diagram:

http://www.thehumancomparator.net/tmp/instructions.jpg

Got through the first couple steps on the Setup instructions on Ken's page (Zero Trim, Pro SP).

I got to the "WS Trim” section on the setup instructions and its a blob of a waveform that slumps back into the original waveform once I stop adjusting the WS trimmer.

I realize its not supposed to be a perfect sine, but its unadjustable via the trimmer. (Video Below)



Also, the sawtooth at the out put is rounded at the top, and sharp like its supposed to be at the bottom:

Image

But on pin 6 of the CA3140 it looks more like its supposed to:

Image

Sub isn't working at all yet either.

If anyone has had similar problems, or can give me an idea of what to look for, please chime in! I've replaced a lot of the chips, triple checked the wiring and parts etc, still nothing.. :help:

Image

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Post by andy » Fri Mar 25, 2016 1:44 pm

:bump:
Want to known as well!!

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Post by guest » Fri Mar 25, 2016 5:13 pm

i dont have one of these modules, but from looking at the schematic, id guess the issue with the sawtooth being rounded is due to the 100k at the "output" being shorted to ground or something. on the schematic here:

http://www.cgs.synth.net/modules/cgs48_vco.html

the saw out shows 2 connection points, one on either side of the 47uF capacitor. only one of those should be an ouput (the top for dc coupled, the bottom for ac coupled). if the bottom one is shorted to ground, or has a low resistance to ground, you would see a rounded waveform.

as for the sine not being tunable, am i understanding the problem correctly, that turning the trim pot makes adjustments, but then it very quickly just slumps back to where it was, regardless of trimpot position? if so, that sounds like an AC coupling issue. you could try bypassing the 4.7uF capacitor and see if it helps. i have definitely had issues with AC coupling into full wave rectifiers before (im assuming thats part of what the LM3900 is doing).
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Post by BugBrand » Sat Mar 26, 2016 5:10 am

Could be the 240K offset resistor before saw output - that biases the ramp to centre around 0V rather than going 0V to +5V. You could replace with a trimmer to find the correct offset value, then sub in a corrected fixed value.

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Post by srfnsmy » Sat Mar 26, 2016 12:52 pm

I have to admit I'm not very good with troubleshooting and understanding electronics. I watch videos on youtube whenever i get stuck with technical problems which sometimes helps, sometimes not :waah:
guest wrote:i don't have one of these modules, but from looking at the schematic, id guess the issue with the sawtooth being rounded is due to the 100k at the "output" being shorted to ground or something.

Are you referring to the 100k that goes to ground at the output?
guest wrote:the saw out shows 2 connection points, one on either side of the 47uF capacitor. only one of those should be an ouput (the top for dc coupled, the bottom for ac coupled). if the bottom one is shorted to ground, or has a low resistance to ground, you would see a rounded waveform.
the panel has jacks on the front for both AC and DC Saw outputs so both are being used, but neither seem to be shorted directly to ground.
guest wrote:as for the sine not being tunable, am i understanding the problem correctly, that turning the trim pot makes adjustments, but then it very quickly just slumps back to where it was, regardless of trimpot position? if so, that sounds like an AC coupling issue. you could try bypassing the 4.7uF capacitor and see if it helps. i have definitely had issues with AC coupling into full wave rectifiers before (I'm assuming thats part of what the LM3900 is doing).
Yes that is exactly what is happening. I bypassed one of the 4.7uF caps (the one going to pin 12 on the LM3900) and that stabilized the waveform when I'm trimming but its still nothing close to other sines i've seen from this module.

referring to this thread (in fact that is the same panel i'm building):

viewtopic.php?t=150522&highlight=

:hmm:

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Post by srfnsmy » Sat Mar 26, 2016 1:05 pm

BugBrand wrote:Could be the 240K offset resistor before saw output - that biases the ramp to centre around 0V rather than going 0V to +5V. You could replace with a trimmer to find the correct offset value, then sub in a corrected fixed value.
I'm getting a reading of 1.5mv on the TL072 side of the 240K resistor. Seems about right?

One more thing I'd like to add is the only chip I haven't replaced is the LM394 because its being held down by the Tempco thermistor. Wondering if that was malfunctioning or burnt out if it would have this kind of effect on the module?

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Post by guest » Sat Mar 26, 2016 2:09 pm

set your scope to DC coupling and look at the DC output of the saw. since your running at a different voltage than spec'd, i suspect that bugbrand is on the right track.
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Post by srfnsmy » Sat Mar 26, 2016 2:22 pm

guest wrote:set your scope to DC coupling and look at the DC output of the saw. since your running at a different voltage than spec'd, i suspect that bugbrand is on the right track.
Ok great thanks Guest!

I'm out running around for a bit but I'll check it when I return

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Post by yan6 » Sat Mar 26, 2016 2:34 pm

As requested in the other thread here some pics of my board, hopefully it helps

Image

Image

Image

Image

btw I added a simple tl072 amp to the saw and sine to boost them to normal levels

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Post by srfnsmy » Sun Mar 27, 2016 12:43 pm

yan6 wrote:As requested in the other thread here some pics of my board, hopefully it helps

btw I added a simple tl072 amp to the saw and sine to boost them to normal levels
Thanks yan6!

Are you running on 15V or 12V?

I installed a 440K resistor in place of the 240K and that brought the Saw ramp to center at 0V, but its still rounded at the top.. :hmm:

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Post by yan6 » Sun Mar 27, 2016 12:48 pm

I'm running on +-12V

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Post by guest » Sun Mar 27, 2016 1:25 pm

what opamps are you using? also, double check that the 68pF in the feedback is actually 68pF.
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Post by srfnsmy » Sun Mar 27, 2016 2:33 pm

guest wrote:what opamps are you using? also, double check that the 68pF in the feedback is actually 68pF.
I'm using all OPA2134's for the TL072s - out for the afternoon but I'll double check the 68pF caps when I return..

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Post by guest » Sun Mar 27, 2016 2:53 pm

the problem is definitely in that amplifier. although the OPA2134 should work fine, it might be a dud. try replacing it with a TL072 (not another OPA, as the whole batch might have the issue) and see what happens. it looks like some sort of slew rate limiting. its possible that its also a filtering issue, but i would expect the bottom peak to not be as sharp if that was the case. if the cap checks out ok, and the TL072 gives the same effect, try desoldering the 330ohm at the output. then check the output of the CA3140 again, to make sure that its still doing the right thing. check the output of the TL072 to verify that its still doing the wrong thing. you can look at the inverting pin on a scope, and see if it moves at all (it should be flat at 0V). if it deviates by even a few millivolts, that indicates either slew rate limiting or saturation of some sort at the output stage. i think the sine issues will go away once this is solved as well.
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Post by srfnsmy » Mon Mar 28, 2016 1:17 pm

guest wrote:the problem is definitely in that amplifier. although the OPA2134 should work fine, it might be a dud. try replacing it with a TL072 (not another OPA, as the whole batch might have the issue) and see what happens. it looks like some sort of slew rate limiting. its possible that its also a filtering issue, but i would expect the bottom peak to not be as sharp if that was the case. if the cap checks out ok, and the TL072 gives the same effect, try desoldering the 330ohm at the output. then check the output of the CA3140 again, to make sure that its still doing the right thing. check the output of the TL072 to verify that its still doing the wrong thing. you can look at the inverting pin on a scope, and see if it moves at all (it should be flat at 0V). if it deviates by even a few millivolts, that indicates either slew rate limiting or saturation of some sort at the output stage. i think the sine issues will go away once this is solved as well.
I've switched out all 3 OPA2134's for TL072's. Still no change :bang:

I unsoldered the 330ohms resistor at the output and the CA3140 and the TL072 are reading the same.

Put the scope on DC and checked pin 2 of the TL072 and its steady at 0V, but pin 6 seemed to have a pulse turning on and off the reading..? :despair:

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Post by guest » Mon Mar 28, 2016 1:45 pm

so it doesnt sound like its the opamp. the pin6 thing is a bit wierd, as this is the PWM input, and should just be at a constant voltage. pin5 should be a replica of the output, and pin7 the PWM output.

so its most likely a component or solder blob or shorted/busted trace on the pcb connected to that opamp. the most likely culprit in my mind is the trimmer going to the sine generator. if you look really closely at all the traces, and they seem fine (no shorts or opens), then remove the trimmer and the 68pF cap and see what happens. the measure the resistance across all the resistors (although these are most likely fine, as its generally producing the right amplitude/offset it should be). then measure the resistance from pin3 to ground to make sure its a short.

if you want to post some closeups of the top/bottom of the board in that area, i can have a look as well.
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Post by srfnsmy » Mon Mar 28, 2016 2:20 pm

guest wrote:so it doesnt sound like its the opamp. the pin6 thing is a bit wierd, as this is the PWM input, and should just be at a constant voltage. pin5 should be a replica of the output, and pin7 the PWM output.

so its most likely a component or solder blob or shorted/busted trace on the pcb connected to that opamp. the most likely culprit in my mind is the trimmer going to the sine generator. if you look really closely at all the traces, and they seem fine (no shorts or opens), then remove the trimmer and the 68pF cap and see what happens. the measure the resistance across all the resistors (although these are most likely fine, as its generally producing the right amplitude/offset it should be). then measure the resistance from pin3 to ground to make sure its a short.

if you want to post some closeups of the top/bottom of the board in that area, i can have a look as well.
Wow thanks for all the help Guest! So many variables to these things I would have given up by now if it wasn't for this thread :nana:

Im at work now but when I get home I'll try going through these and see if that makes a difference.. I've had a weird feeling about that trimmer so hope thats the culprit!

Will post my findings later..

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Post by guest » Mon Mar 28, 2016 8:47 pm

remove the 68pF capacitor, its the "problem". i was suspecting a lowpass filter of some sort at first because of the rounded waveform, but then i dismissed this because the top was rounded, but the bottom was not. wouldnt a filter affect both sides equally? i guess not. i just built the circuit up, and the 68pF cap causes the exact same waveform. this 68pF cap creates a 10kHz lowpass filter, which the peaks are well beyond.

so i guess the question is, do you want it to sound like the CGS48, or like a saw wave? if the latter, put in a 10pF cap, or none at all.

unfortunately this means the sinewave issue is probably something else. you could try DC coupling it again, now that its centered.
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Post by srfnsmy » Tue Mar 29, 2016 12:11 pm

guest wrote:remove the 68pF capacitor, its the "problem". i was suspecting a lowpass filter of some sort at first because of the rounded waveform, but then i dismissed this because the top was rounded, but the bottom was not. wouldnt a filter affect both sides equally? i guess not. i just built the circuit up, and the 68pF cap causes the exact same waveform. this 68pF cap creates a 10kHz lowpass filter, which the peaks are well beyond.

so i guess the question is, do you want it to sound like the CGS48, or like a saw wave? if the latter, put in a 10pF cap, or none at all.

unfortunately this means the sine wave issue is probably something else. you could try DC coupling it again, now that its centered.
Ok I took out the 68pF and put in a 47pF in (smallest i had). Its a little less round on the top! Its not that I want a perfect saw, I just assumed this would be the same problem that I'm having with the Sine and a couple other features of the board (Sub & Staircase).

Only thing is according to yan6's photos on his thread he's getting some pretty great looking waveforms coming out of his:

viewtopic.php?p=2186741#2186741

So I wonder if it could be something else? But like i said, a perfect waveform isn't important to me, I just want a fully functioning CGS48 :sb:

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Post by yan6 » Tue Mar 29, 2016 12:46 pm

The only difference I have spotted so far is, I used a lm311, where you have something else in a metal can. Also in the far lower left I can barely make out but I may have jumpered one of the 100k

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Post by guest » Tue Mar 29, 2016 1:18 pm

yan6 wrote:The only difference I have spotted so far is, I used a lm311, where you have something else in a metal can. Also in the far lower left I can barely make out but I may have jumpered one of the 100k
i was curious about this after i did my experiments. is the saw in your oscope shot taken from the TL072, or the CA3140? that 68pF cap definitely rounds the waveform off a lot. i pulled it in and out a couple of times to verify it was the culprit. i even made a little low pass filter of just a 220k and a 68pF elsewhere on the board, and it did the same thing to a pure saw wave. so im trying to figure out how you got a clean saw with a 68pF installed.
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Post by srfnsmy » Tue Mar 29, 2016 1:41 pm

yan6 wrote:The only difference I have spotted so far is, I used a lm311, where you have something else in a metal can. Also in the far lower left I can barely make out but I may have jumpered one of the 100k
The metal can is also a LM311. Just the older style. I was trying to make out the wire jumper in place of the 100k, but it looks like there is something connected to it that is in the shadow of the 4.7uF?

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Post by yan6 » Tue Mar 29, 2016 2:06 pm

All my scope shots are from the banana jacks: ie the outputs

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Post by srfnsmy » Tue Mar 29, 2016 2:17 pm

yan6 wrote:The only difference I have spotted so far is, I used a lm311, where you have something else in a metal can. Also in the far lower left I can barely make out but I may have jumpered one of the 100k
One thing I did just notice is that I have a 220pF Stryo where you have a 220pF Ceramic in place between pin 1 and pin 5 of the LM3900. I looked on the schematic and it notes that it should be ceramic. I know this is a different part of the board from what we are focusing on, but I wonder what kind of difference that makes? :hmm:

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Post by yan6 » Tue Mar 29, 2016 2:20 pm

Im headed home in a few minutes if you need any scope readings from anywhere just let me know

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