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Fabricio pecanha
Joined: Aug 18, 2018 Posts: 1 Location: Brazil
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Posted: Sat Aug 18, 2018 4:55 pm Post subject:
Tune problem vc 555 Subject description: Tune variation |
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Hello, I have a problem in my vco 555, I hope you get some help.
my vco was running well, but for some time he's running the tune alone. Has anyone had this problem?
Thank you for your help |
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elmegil
Joined: Mar 20, 2012 Posts: 2177 Location: Chicago
Audio files: 16
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Posted: Sat Aug 18, 2018 5:41 pm Post subject:
Re: Tune problem vc 555 Subject description: Tune variation |
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Fabricio pecanha wrote: | Hello, I have a problem in my vco 555, I hope you get some help.
my vco was running well, but for some time he's running the tune alone. Has anyone had this problem?
Thank you for your help |
Hi Fabricio. I'm not sure I understand what you mean by running the tune alone? |
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Dozemuisc
Joined: Jul 11, 2019 Posts: 10 Location: France
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Posted: Thu Jul 11, 2019 3:50 am Post subject:
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Hello!!
We are building a biiig4voice-polyphonic semi-modular synth with 8 Thomas Henry 555, 3 quadruple Druid ADSRS, 2 quadruple MFOS VCOs, 1 quadruple Yusynth MiniMoog VCF and one 5tuple LFO. All of wich with global controls and 4-channel patch cables.
For now it's all working well exept i have a problem with two TH555.
They can only go down to around 5Hz while all the others go down to 1Hz...
It's not the V8 Trimmer as the value is the same for all the VCO's after it...
I debugged them for hours and measured some values and i don't understand.
Here is the schematic with measures. I already tried to change the IC's (in green).
Could you help me?
Thanks!
Doze Last edited by Dozemuisc on Thu Jul 11, 2019 8:31 am; edited 1 time in total |
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fonik
Joined: Jun 07, 2006 Posts: 3950 Location: Germany
Audio files: 23
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Posted: Thu Jul 11, 2019 7:46 am Post subject:
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Just a wild guess, but what about swapping the 555s with ones from the 'working' modules? Would this make a difference? _________________
cheers,
matthias
____________
Big Boss at fonitronik
Tech Buddy at Random*Source |
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fonik
Joined: Jun 07, 2006 Posts: 3950 Location: Germany
Audio files: 23
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Posted: Thu Jul 11, 2019 7:47 am Post subject:
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additionally, check C4. _________________
cheers,
matthias
____________
Big Boss at fonitronik
Tech Buddy at Random*Source |
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Dozemuisc
Joined: Jul 11, 2019 Posts: 10 Location: France
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Posted: Thu Jul 11, 2019 8:30 am Post subject:
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Hello!
Thanks for the answer
I already tried swapping 555s it doesn't change anything.
I tought also that it could be C4 but i have exactly the same problem on 2 VCO's that go down to the same frequency... And it is good quality MICA's at 8 euros each... If i don't find answers i'll try to put another condenser but I don't have anymore left! |
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fonik
Joined: Jun 07, 2006 Posts: 3950 Location: Germany
Audio files: 23
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Posted: Thu Jul 11, 2019 8:37 am Post subject:
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Dozemuisc wrote: | I tought also that it could be C4 but i have exactly the same problem on 2 VCO's that go down to the same frequency... And it is good quality MICA's at 8 euros each... If i don't find answers i'll try to put another condenser but I don't have anymore left! |
When mentioned C4 I did not think of the general characteristics of the type used, but more of the absolute value. _________________
cheers,
matthias
____________
Big Boss at fonitronik
Tech Buddy at Random*Source |
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fonik
Joined: Jun 07, 2006 Posts: 3950 Location: Germany
Audio files: 23
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Posted: Thu Jul 11, 2019 8:39 am Post subject:
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you could even try to swap the OTAs. IIRC the LM13700 have huge tolerances in behaviour. _________________
cheers,
matthias
____________
Big Boss at fonitronik
Tech Buddy at Random*Source |
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Dozemuisc
Joined: Jul 11, 2019 Posts: 10 Location: France
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Posted: Thu Jul 11, 2019 10:08 am Post subject:
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Swapped the LM13700, didn't work, measured AND swapped C4 (as i already unsoldered it to measure it), it has the same value and that didn't work as well.
The bastard still oscillates to high! |
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fonik
Joined: Jun 07, 2006 Posts: 3950 Location: Germany
Audio files: 23
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Posted: Thu Jul 11, 2019 11:03 am Post subject:
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hmmm... _________________
cheers,
matthias
____________
Big Boss at fonitronik
Tech Buddy at Random*Source |
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elmegil
Joined: Mar 20, 2012 Posts: 2177 Location: Chicago
Audio files: 16
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Posted: Thu Jul 11, 2019 11:05 am Post subject:
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Have you verified that pin 1 of IC3a actually appears to track the CV inputs from the coarse and fine inputs? And is "sane" ? |
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ixtern
Joined: Jun 25, 2018 Posts: 145 Location: Poland
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Posted: Fri Jul 12, 2019 2:17 am Post subject:
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Dozemuisc wrote: | Hello!!
For now it's all working well exept i have a problem with two TH555.
They can only go down to around 5Hz while all the others go down to 1Hz...
It's not the V8 Trimmer as the value is the same for all the VCO's after it...
I debugged them for hours and measured some values and i don't understand.
Here is the schematic with measures. I already tried to change the IC's (in green).
Could you help me?
Thanks!
Doze |
There is yet one rare but posibble reason for VCO misbehavior - fake TL074. I have encountered bunch of such ICs bought from local supplier. They were all shiny new with TI manufacturer marks but all have big input currents - an order of nA instead of pA. Otherwise they behaved as normal op amps, what suggests they were rebadged bipolar op amps.
Try to swap TL074 for trusted ones. Maybe this helps. |
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Dozemuisc
Joined: Jul 11, 2019 Posts: 10 Location: France
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Posted: Sat Jul 13, 2019 5:12 am Post subject:
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What do you mean if IC3 is sane? Why wouldn't it track well? You mean like bad soldering? Thanks
Ixtern I've already swapped all the IC's including the TL74 for those of the sane VCO's and it stays the same.
It's really a mystery. |
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elmegil
Joined: Mar 20, 2012 Posts: 2177 Location: Chicago
Audio files: 16
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Posted: Sat Jul 13, 2019 7:32 am Post subject:
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Re-read:
Quote: | fake TL074. I have encountered bunch of such ICs bought from local supplier. |
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elmegil
Joined: Mar 20, 2012 Posts: 2177 Location: Chicago
Audio files: 16
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Posted: Sat Jul 13, 2019 7:33 am Post subject:
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elmegil wrote: | Re-read:
Quote: | fake TL074. I have encountered bunch of such ICs bought from local supplier. |
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If all your TL074's are from the same supplier, swapping one won't do anything. |
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Dozemuisc
Joined: Jul 11, 2019 Posts: 10 Location: France
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Posted: Sun Jul 14, 2019 7:18 am Post subject:
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Yeah sure they are not fake, they come from Mouser and they work in all the other TH555's. |
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ixtern
Joined: Jun 25, 2018 Posts: 145 Location: Poland
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Posted: Sun Jul 14, 2019 11:25 pm Post subject:
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Then maybe 2N3906 are not paired and they have different enough characteristics to create bias limiting lower frequency? Have you tried to check their Vbe and beta? |
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Dozemuisc
Joined: Jul 11, 2019 Posts: 10 Location: France
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Posted: Mon Jul 15, 2019 1:00 am Post subject:
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Well I matched them for VBe at 1mV using MFOS circuit, and i don't know how to measure beta. |
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hedefalk
Joined: Aug 29, 2017 Posts: 51 Location: Stockholm, Sweden
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Posted: Mon Sep 09, 2019 5:59 am Post subject:
Cross-over distortion from 3 to kHz-something |
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Okay, so I revisited this design. I have quite a few posts in this thread were I saw this distortion that looks something like cross-over distortion. Well, since my last post I've changed out _everything_ and by everything I mean:
* redesigned entirely new pcb. smd this time so no basically no component from same batch as before.
* new PSU (already tried with two different last time around, but now a third )
So I mean, every single component is new from before. And I see the exact same behaviour. From around 3kHz it's visible and at 5kHz it's very much so.
I think the problem stems from the triangle core and all other output stages are just affected as a consequence. It's most visible in the square wave at high frequencies since the slew basically turns it into something like a triangle.
I've tried adding a 1K load-resistor to ground on all opamp-outputs but couldn't find anything obvious.
I'm thinking of three possibilities:
* I've missed to change some critical value to make this work for 12V instead of 15V.
* The kind of 7555 I'm using is not fit for this design.
* This is all as designed, I'm just picky with my scope.
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dancloud69
Joined: Dec 05, 2016 Posts: 1 Location: dancloud69
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Posted: Tue Nov 05, 2019 12:58 pm Post subject:
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hedefalk, not sure if you're still looking into this. While reading through your posts I noticed that the link for the TL074s you are using was to a Chinese supplier. Nowadays any IC you order from China is a potential fake. I've been there. Have stopped ordering ICs from China altogether a while ago.
My instant guess when seeing the pulse slew was that these op amps could possibly be re-labeled LM324s.
It's curious that when you put in an LM324, the behavior did not change. The typical slew rate of an LM324 is 0.5V/us, 40-50 times slower than a TL074. When I try to do the math, with the output voltage of the pulse comparator having to go from -12V to +10V and back (for a LM324), at that slew rate you'd end up with a triangle at around 11kHz.
Edit 2:The slew of the pulse output is only affected by the output op amp and the components (resistors) around it. Not by the triangle core.
Edit: You mentioned going to all SMT with your latest iteration. Where did you order the SMT TL074s?
You could easily test the slew rate on a breadboard with something like a simple 555 based square oscillator of 10kHz or so, and buffering the output with one of your TL074. With real TL074 the output should still be close to a square.
As a side note, quad through hole op amps are on their way out, more so than duals. I've personally used fonik's PCBs for both of my VCO555s (which both work fine), and I've still been seriously considering doing my own PCB using only dual op amps, which would also enable using an LT1013 or OPA2277 for the expo converter (much lower offset voltage, more precise expo conversion).
Best,
Dan |
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hedefalk
Joined: Aug 29, 2017 Posts: 51 Location: Stockholm, Sweden
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carlo64
Joined: Apr 02, 2020 Posts: 16 Location: Italia
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Posted: Thu Apr 02, 2020 3:58 am Post subject:
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Hello everybody,
I wanted to make this VCO (I state that I'm not very good and I'm starting ..) and I noticed some discrepancies between the value list and the scheme. What are the right values?
Thanks
Carlo
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carlo64
Joined: Apr 02, 2020 Posts: 16 Location: Italia
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Posted: Thu Apr 09, 2020 10:45 am Post subject:
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Wow!
it seems to work!
however, the amplitude of the sine wave seems to be half
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wackelpeter
Joined: May 05, 2013 Posts: 461 Location: germany
Audio files: 10
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Posted: Fri Apr 17, 2020 3:05 pm Post subject:
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There was a fix to boost the sine wave amplitude posted here somewhere... I think it was in this thread somewhere within the first 10 sides or so...
Give it a look/search and enjoy this nice VCO.
edith says: you'll find it on page 3 of this topic _________________ https://soundcloud.com/bastian-j |
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carlo64
Joined: Apr 02, 2020 Posts: 16 Location: Italia
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Posted: Sat Apr 18, 2020 3:39 am Post subject:
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Thank you!
I have seen the suggested change (r27 and R24 trimmer), I will immediately try to do it!
Carlo |
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