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 Forum index » DIY Hardware and Software » Thomas Henry designs
The VCO-555
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Fabricio pecanha



Joined: Aug 18, 2018
Posts: 1
Location: Brazil

PostPosted: 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: 2111
Location: Chicago
Audio files: 16

PostPosted: 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: 9
Location: France

PostPosted: Thu Jul 11, 2019 3:50 am    Post subject: Reply with quote  Mark this post and the followings unread

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?

Posted Image, might have been reduced in size. Click Image to view fullscreen.

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

PostPosted: Thu Jul 11, 2019 7:46 am    Post subject: Reply with quote  Mark this post and the followings unread

Just a wild guess, but what about swapping the 555s with ones from the 'working' modules? Would this make a difference?
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matthias
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fonik



Joined: Jun 07, 2006
Posts: 3950
Location: Germany
Audio files: 23

PostPosted: Thu Jul 11, 2019 7:47 am    Post subject: Reply with quote  Mark this post and the followings unread

additionally, check C4.
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cheers,
matthias
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Dozemuisc



Joined: Jul 11, 2019
Posts: 9
Location: France

PostPosted: Thu Jul 11, 2019 8:30 am    Post subject: Reply with quote  Mark this post and the followings unread

Hello!

Thanks for the answer Smile

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
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Location: Germany
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PostPosted: Thu Jul 11, 2019 8:37 am    Post subject: Reply with quote  Mark this post and the followings unread

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.

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cheers,
matthias
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fonik



Joined: Jun 07, 2006
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PostPosted: Thu Jul 11, 2019 8:39 am    Post subject: Reply with quote  Mark this post and the followings unread

you could even try to swap the OTAs. IIRC the LM13700 have huge tolerances in behaviour.
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matthias
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Dozemuisc



Joined: Jul 11, 2019
Posts: 9
Location: France

PostPosted: Thu Jul 11, 2019 10:08 am    Post subject: Reply with quote  Mark this post and the followings unread

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
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PostPosted: Thu Jul 11, 2019 11:03 am    Post subject: Reply with quote  Mark this post and the followings unread

hmmm...
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elmegil



Joined: Mar 20, 2012
Posts: 2111
Location: Chicago
Audio files: 16

PostPosted: Thu Jul 11, 2019 11:05 am    Post subject: Reply with quote  Mark this post and the followings unread

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: 123
Location: Poland

PostPosted: Fri Jul 12, 2019 2:17 am    Post subject: Reply with quote  Mark this post and the followings unread

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: 9
Location: France

PostPosted: Sat Jul 13, 2019 5:12 am    Post subject: Reply with quote  Mark this post and the followings unread

What do you mean if IC3 is sane? Why wouldn't it track well? You mean like bad soldering? Thanks Smile

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: 2111
Location: Chicago
Audio files: 16

PostPosted: Sat Jul 13, 2019 7:32 am    Post subject: Reply with quote  Mark this post and the followings unread

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: 2111
Location: Chicago
Audio files: 16

PostPosted: Sat Jul 13, 2019 7:33 am    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:
Re-read:

Quote:
fake TL074. I have encountered bunch of such ICs bought from local supplier.


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: 9
Location: France

PostPosted: Sun Jul 14, 2019 7:18 am    Post subject: Reply with quote  Mark this post and the followings unread

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: 123
Location: Poland

PostPosted: Sun Jul 14, 2019 11:25 pm    Post subject: Reply with quote  Mark this post and the followings unread

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: 9
Location: France

PostPosted: Mon Jul 15, 2019 1:00 am    Post subject: Reply with quote  Mark this post and the followings unread

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: 50
Location: Stockholm, Sweden

PostPosted: Mon Sep 09, 2019 5:59 am    Post subject: Cross-over distortion from 3 to kHz-something Reply with quote  Mark this post and the followings unread

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 Smile )

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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