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 Forum index » DIY Hardware and Software » Ken Stone designs - CGS
Troubleshooting a CGS75 - Voltage-Controlled Slope..
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MC1495



Joined: Mar 24, 2009
Posts: 19
Location: baltimore

PostPosted: Sun Dec 25, 2011 11:35 pm    Post subject: Troubleshooting a CGS75 - Voltage-Controlled Slope..
Subject description: Troubleshooting a CGS75 - Voltage-Controlled Slope..
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Currently attempting to build the CGS75 from scratch (no PCB) - and running into a little bit of difficulty. Obviously, this is a minefield - but maybe I can explain the problem sufficiently to merit suggestions.

There seems to be a lot that is hinged on the PNP/NPN differential pairs - specifically in the "Exponential 1v/oct" that is attached to the bases of the bottom NPN and PNP.

With the voltage divider attached to the bases, I get short blips - and if I adjust the Rise/Fall CVs, eventually I get to a point where the Envelope (and LED) is sticks at varying voltages after being triggered. Without the voltage divider, I seem to be able to effectively control the "Rise", but have no control of the "Fall."

I'm assuming this potentially has some relationship to the LM3900 - which is ideally configured to control the positive- or negative-going slope, depending on a threshold?

Any advice would be greatly appreciated!
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Mongo1



Joined: Aug 11, 2011
Posts: 411
Location: Raleigh NC

PostPosted: Tue Dec 27, 2011 7:31 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi MC,

I haven't built the VCS, but I did build the DUSG which is similar. With the complexity of these boards I would really suggest buying the PCB from Ken. There's an awful lot that can go wrong, and isolating is going to be very difficult.

Barring that, I'd suggest posting a photograph of the front and back of your project. That way people can at least see where things stand.

Good Luck -
Gary
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MC1495



Joined: Mar 24, 2009
Posts: 19
Location: baltimore

PostPosted: Tue Dec 27, 2011 11:04 am    Post subject: Reply with quote  Mark this post and the followings unread

Don't have a way to photograph this circuit at the moment.

I will say that I just realized I had the input resistors for the "In" stage arranged incorrectly and that does seem to fix, or at least "change" the problem.
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Mongo1



Joined: Aug 11, 2011
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PostPosted: Tue Dec 27, 2011 11:15 am    Post subject: Reply with quote  Mark this post and the followings unread

That's good news!
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wackelpeter



Joined: May 05, 2013
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Location: germany
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PostPosted: Wed Jan 28, 2015 1:13 pm    Post subject: Reply with quote  Mark this post and the followings unread

hello to all

whilst refreshing this old thread i would be happy to get some help from you because i was stupid enough to try this out on stripboard (like all my builds)

well it does Trigger, it does self oscillate when Trigger button is on, i can control the rise time and rate with the pots or CV but can't control the fall time or rate...

i checked the Outputs on the tl074... when the Switches for exp control of rise and fall rate are off then there are 0v at each Output 1,7,8,14

turned on i have a saw or ramp Signal depending on the rise cv pot it goes from 0V to 2V positive edge or to -2V to 0V negative edge...
curves are somewhat mirrored ... or better to describe an log curve when turned fully to one direction and an antilog curve when turned towards the other end... in middle Position it's near or round 0V...

on the fall Output pin1 it's nearly the same but without changes in frequency which occurs on the rise rate...
the only control which wouldn't work is the fall rate pot ... no changes when turning the pot just a slight Change over the frequency... when i had it at 100Hz it raised up to 105 hz but the fall rate can't be adjusted. Output is almost a ramp...

measured the voltage at the base of the 3906 connected to the fall pot it goes from 0v to 355mV
at the base of the 3904 connected to the rise pot it's 0V to 338mV


already tried replacing the trannies... but they were not the Problem as it seems... checked my board for bad solderpoints or Shorts but couldn't find anything looking a few times with a magnifier over it... also reheated most of the solderpoints to be sure that there isn't any bad Joint in between i haven't spotted...

can someone explain which part apart from the 074 and it's surrounding parts control the rise and fall rate... this 4 trannie converter is a miracle to me...

1V/oct Input works too so i guess they don't have that much to do with the fall rate...

anyone there who could help me a bit to understand this circuit roughly and troubleshoot this?

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elmegil



Joined: Mar 20, 2012
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PostPosted: Wed Jan 28, 2015 10:02 pm    Post subject: Reply with quote  Mark this post and the followings unread

wackelpeter wrote:

well it does Trigger, it does self oscillate when Trigger button is on, i can control the rise time and rate with the pots or CV but can't control the fall time or rate...

i checked the Outputs on the tl074... when the Switches for exp control of rise and fall rate are off then there are 0v at each Output 1,7,8,14


If you put a CV into the Fall CV, do you see the same voltage on pin 8? As you switch the CV amount pot from full Clockwise to full Counter Clockwise, does the output on pin 7 go from a positive voltage through zero, to a negative voltage? Compare to the same points on the Rise inputs, or even put your voltage into the both jack and try it. Both sides should behave the same.

wackelpeter wrote:

on the fall Output pin1 it's nearly the same but without changes in frequency which occurs on the rise rate...


Which sounds like you already did that test, and the TL074 and its associated circuits are probably just fine.


wackelpeter wrote:

the only control which wouldn't work is the fall rate pot ... no changes when turning the pot just a slight Change over the frequency...


Now I'm not clear; does the CV on the fall actually work then? In that case, your problem is likely the fall rate pot or the 82K resistor it connects through into the rest of the circuit. Verify you see V+ on the CW side of the pot and ground on the CCW side. Verify that you see the voltage on the wiper change as you adjust the pot.

wackelpeter wrote:

already tried replacing the trannies... but they were not the Problem as it seems... checked my board for bad solderpoints or Shorts but couldn't find anything looking a few times with a magnifier over it... also reheated most of the solderpoints to be sure that there isn't any bad Joint in between i haven't spotted...


Have you verified the orientation of the transistors? Since you built it on stripboard, you wouldn't have the PCB silkscreen to guide you.....

wackelpeter wrote:

can someone explain which part apart from the 074 and it's surrounding parts control the rise and fall rate... this 4 trannie converter is a miracle to me...


The TL074 has an input buffer followed by an attenuverter -- an inverting attenuator. I haven't examined the gain carefully, it may also scale the voltages, which would not be too surprising since they are controlling the bases of the 3904 and 3906.

I don't think I could explain the converter, but there's a good analysis available here: http://www.timstinchcombe.co.uk/forums.html?pge=vcs
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wackelpeter



Joined: May 05, 2013
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PostPosted: Thu Jan 29, 2015 11:25 am    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:
wackelpeter wrote:

well it does Trigger, it does self oscillate when Trigger button is on, i can control the rise time and rate with the pots or CV but can't control the fall time or rate...

i checked the Outputs on the tl074... when the Switches for exp control of rise and fall rate are off then there are 0v at each Output 1,7,8,14


If you put a CV into the Fall CV, do you see the same voltage on pin 8? As you switch the CV amount pot from full Clockwise to full Counter Clockwise, does the output on pin 7 go from a positive voltage through zero, to a negative voltage? Compare to the same points on the Rise inputs, or even put your voltage into the both jack and try it. Both sides should behave the same.

wackelpeter wrote:

on the fall Output pin1 it's nearly the same but without changes in frequency which occurs on the rise rate...


Which sounds like you already did that test, and the TL074 and its associated circuits are probably just fine.


wackelpeter wrote:

the only control which wouldn't work is the fall rate pot ... no changes when turning the pot just a slight Change over the frequency...


Now I'm not clear; does the CV on the fall actually work then? In that case, your problem is likely the fall rate pot or the 82K resistor it connects through into the rest of the circuit. Verify you see V+ on the CW side of the pot and ground on the CCW side. Verify that you see the voltage on the wiper change as you adjust the pot.

wackelpeter wrote:

already tried replacing the trannies... but they were not the Problem as it seems... checked my board for bad solderpoints or Shorts but couldn't find anything looking a few times with a magnifier over it... also reheated most of the solderpoints to be sure that there isn't any bad Joint in between i haven't spotted...


Have you verified the orientation of the transistors? Since you built it on stripboard, you wouldn't have the PCB silkscreen to guide you.....

wackelpeter wrote:

can someone explain which part apart from the 074 and it's surrounding parts control the rise and fall rate... this 4 trannie converter is a miracle to me...


The TL074 has an input buffer followed by an attenuverter -- an inverting attenuator. I haven't examined the gain carefully, it may also scale the voltages, which would not be too surprising since they are controlling the bases of the 3904 and 3906.

I don't think I could explain the converter, but there's a good analysis available here: http://www.timstinchcombe.co.uk/forums.html?pge=vcs


Hi Elmegil,

thank you, that was really a quick reply...

well the pots are connected correctly that were these 0 to 350mV i measured when turning both pots... the rise had a bit less but that's likely due to the fact i used only 5% resistors... these are measured directly at the base of both trannies going to the pots... before the 82K resistor it was 0V to 14,6V

The Fall pot has some very little effect on the frequency cycle, but not the full range as expected... it changes the frequency in a small ratio 100:5
Like i wrote before when i have it cycling at 100Hz and i turn the fall pot to the opposite site it goes up to 105HZ, with 1kHz it goes up to 1060 Hz


meanwhile i've just messed around with it a bit... checking some measurements and so on... then resoldered all pins of the TL072 and voila it responds now to a voltage applied to the fall CV Input and the pot works to...
but what makes me wonder is that the pot works on a wide range when a CV is applied and on a very small when no CV is applied...

guess i had one pin of TL072 or so a bad solder Point... the behaviour of the pot i fixed with an 120k resistor parallel to the 82k.... know it works like i think it should work...
really nice circuit and thanks for pointing me to Tim Stinchcombe's site
which lead me to "re-inspect" my Tl072

now i'm happy with my nice VCS and as today a parcel with a lot of other components arrived i will finish my few other modules waiting to be stuffed into my "modular" Razz

maybe i'll make a short demo Clip to Show what it does and maybe someone can tell that's correct and all it's supposed to do....

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