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



Joined: Nov 20, 2008
Posts: 52
Location: Spain

PostPosted: Sun Feb 07, 2021 2:54 pm    Post subject: Reply with quote  Mark this post and the followings unread

NathanS wrote:
It looks to me as if R2 and R23 should be swapped around. At least according to the original schematic, R2 should go to ground and the trimmer R1, but it's going out to D1 and IC3-pin7 - as R23 should.

Thus in that pcb, R2 should be 22 kOhm while R23 should be 390 Ohm. Does this match up with how it's been put together?


Yes, there was a mistake in the print which I corrected, before R1 was unable to operate after change R1 works but still R36 doesn't.
I think the PCB has a routing error.
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Alfredo



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PostPosted: Mon Feb 08, 2021 4:37 am    Post subject: Turning Trimmers Reply with quote  Mark this post and the followings unread

Turning trimmers, first R1 works as is should, R36 don't do nothing.


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elmegil



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PostPosted: Mon Feb 08, 2021 7:03 am    Post subject: Reply with quote  Mark this post and the followings unread

The main way to find a routing error is to follow the schematic and the PCB with the continuity setting on your meter, and trace all the connections, marking them off on the schematic as you go.
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Alfredo



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PostPosted: Mon Feb 08, 2021 7:37 am    Post subject: Reply with quote  Mark this post and the followings unread

Oh man, how could you make me understand? I think I do not explain myself correctly. I don't have enough knowledge to compare pcb and schematic, I can understand a simple schematic like this CGS DC mixer but the 555 VCO is too hard for me.


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elmegil



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PostPosted: Mon Feb 08, 2021 7:56 am    Post subject: Reply with quote  Mark this post and the followings unread

But for tracing the circuit, understanding how it works is not necessary.

You have component R2 connected to R3, connected to pin 5 of U6 (just picking random numbers). Verify that there is continuity between them all and not between other things that are placed nearby on the PCB. The schematic shows all the connections.... verify that they are there.
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Alfredo



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PostPosted: Mon Feb 08, 2021 1:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi, verified continuity in all the pcb tracks, everything seems correct...
White lines=Checked tracks.


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elmegil



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PostPosted: Mon Feb 08, 2021 2:10 pm    Post subject: Reply with quote  Mark this post and the followings unread

Then whatever is going on is not strictly a PCB layout fault. One might argue that silkscreen problems (wrong designators/values in the wrong places) was a layout fault, but I wouldn't, that's simply a silkscreen fault and is easily enough found by correlating designators to the original schematic.

So: You said you do, or do not see the voltage changing on the wiper of the trimpot? What is the voltage at pin 7 of IC3 when you have no linear FM input? What about pin 1? Pin 1 shoudl swing a bit with the coarse pot, a fair bit less but still visibly with the fine control and the HF trim, do you see those things happen?

To be honest, the effect of the HF trim should not be very significant, it's basically just feeding a little extra bit of CV in at higher frequencies (higher voltages) to compensate for the core's tendency to go flat because of the finite reset time. Bit it should be visible at pin 1 of IC3 regardless, assuming all your values are correct.

You have a meter that can read in the 10s of mV, correct?
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Alfredo



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PostPosted: Mon Feb 08, 2021 2:49 pm    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:

So: You said you do, or do not see the voltage changing on the wiper of the trimpot? What is the voltage at pin 7 of IC3 when you have no linear FM input? What about pin 1? Pin 1 shoudl swing a bit with the coarse pot, a fair bit less but still visibly with the fine control and the HF trim, do you see those things happen?


Yes Voltage changes wen turn the HF trim measuring on the wiper, from 0.000volts to -0.353volts
Voltage at the 7 of IC3 = 0.714V
Voltage at pin 1 of IC3 = Voltage changes from -0.118/0.120volts turning the coarse and fine pots.

The problem is I can't tune the vco correctly, can't play two notes in tune!


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elmegil



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PostPosted: Mon Feb 08, 2021 3:08 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yes, I'm getting there. Very Happy

First thing to do: turn the HF trim pot so that it is at 0V on the wiper.

Can you tune the VCO at all with it in that position? If not, then the HF trim is NOT your problem.

What I would *expect* is that for V/Oct CV inputs from 0V through3V or 4V at least, you should be able to get decent tuning, but above that it gets progressively flatter.
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elmegil



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PostPosted: Mon Feb 08, 2021 3:11 pm    Post subject: Reply with quote  Mark this post and the followings unread

This is a different VCO, but the calibration of V/Oct should be the same, search for "Calibration and Adjustments" section

https://www.birthofasynth.com/Thomas_Henry/Pages/VCO-1.html
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Alfredo



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PostPosted: Mon Feb 08, 2021 4:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi elmegil, thanks for your help!
That Calibration it does not work for My vco, if I play a note C in tune pair with other vco, the next note C# is out of tune! If I turn the R1 it lost the C intonation!
Turning the HF trim does not affects the intonation Sad
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elmegil



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PostPosted: Mon Feb 08, 2021 4:35 pm    Post subject: Reply with quote  Mark this post and the followings unread

the HF trim is NOT your primary V/Octave trimmer.

Put it at zero and leave it there.

You need to adjust R1 to trim for V/Octave.

DO NOT try to do this in comparison to some other oscillator for your initial calibration. You need to either listen and get "close" first, or use a frequency counter. Read the page I linked to, and see what it has to say about how you calibrate for V/Octave.

ONCE you have it mostly calibrated, THEN you can compare to another oscillator, and fine tune it if need be.
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elmegil



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PostPosted: Mon Feb 08, 2021 4:39 pm    Post subject: Reply with quote  Mark this post and the followings unread

"If I turn the R1 it lost the C intonation!"

YES this is EXPECTED.

UNTIL you have R1 set so that you get 1V/Octave, you WILL NOT be able to tune to any other source. WHEN you change R1, you shift the entire range of the oscillator, BUT you wil be adjusting the distance from one note to the next (and you want to do it by Octaves so you can actually hear the difference).

Again: you have to tune the oscillator TO ITSELF before you can start to tune it to any other source. ONCE it responds to a 1V input change with a 1 octave frequency change, THEN you can adjust it to match another oscillator and you should be able to play the two in tune with each other.

Edit: and to be clear, you would adjust the two with the coarse/fine adjustments, NOT TOUCHING R1 & R36 unless the two oscillators do not TRACK -- i.e. you make the C's match with Coarse/Fine, you then hit the C an octave higher and they still differ.
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Alfredo



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PostPosted: Mon Feb 08, 2021 5:56 pm    Post subject: Reply with quote  Mark this post and the followings unread

Ok will try with patience.
Where I must measure volts with the meter?pin 7 of IC3? Pin 1?
Normally that is the Setting up procedure with others VCOs, play octaves or apply 1 volt and measure then adjust the trimmer until the output of the oscillator is exactly one octave higher and then take the input voltage up to 3.00 volts or 4.00 volts, etc...
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elmegil



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PostPosted: Mon Feb 08, 2021 9:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

Alfredo wrote:
Ok will try with patience.
Normally that is the Setting up procedure with others VCOs, play octaves or apply 1 volt and measure then adjust the trimmer until the output of the oscillator is exactly one octave higher and then take the input voltage up to 3.00 volts or 4.00 volts, etc...


Exactly right. As I said, the Thomas Henry site I linked to above discusses the calibration. The top of this entire thread has the schematics, you would simply measure the output of the triangle and take its frequency. Then put 0V, 1V, 2V, 3V into the V/Oct input, and adjust R1 until the frequency doubles with each volt. Then go a little higher and as you notice it going a bit flat, not quite doubling, adjust the HF trim. Since the HF Trim *does* interact with the other things, you'll have to go back to the start and trim R1 some more at that point, but eventually, going back and forth, you will get very good tracking.

At that point if you have another oscillator you plan to play with, you can play them both together and make sure they track together, which is ultimately more important than perfect tracking overall.
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Alfredo



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PostPosted: Tue Feb 09, 2021 5:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi, it is impossible to calibrate.
I set the vco to c then I put 1volt into the V/Oct input and adjust R1 to one octave up, ok to this point and repeat with 2V and tried to adjust R1 but can't, trimmer R1 don't turn next octave up.
Other thing I noticed the sine out is a triangle waveform and trimmers R34 and R38 does not change the round of symmetry of the wave out Sad
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elmegil



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PostPosted: Tue Feb 09, 2021 6:42 pm    Post subject: Reply with quote  Mark this post and the followings unread

No, you are still not understanding the calibration procedure.
You don't seem to have any concept of how this process works.

You do not adjust R1 to increase one octave.

You must have a source of VOLTAGES. Like the output of a MIDI to CV of some sort.

You input 0V or 1V into the V/Octave input.

You set the frequency with the coarse/fine knobs to something reasonably low, like 100Hz.

You then increase the input from 0V to 1V. If your frequency goes to 200Hz, you're done. If not, you need to adjust R1, and do it again.

"Do it again" looks like: input 0V, adjust the frequency with the knobs to be 100Hz, and then increase to 1V, and see if you're closer to 200Hz or farther away. If you're closer, you're adjusting in the right direction. If you're farther away, you need to turn R1 in the opposite direction.

I can't tell you whether to turn it clockwise or counter clockwise because 1) different VCOs work differently in this regard and 2) it depends on whether your interval is ABOVE 200Hz or BELOW 200Hz.

I am basically repeating exactly what the website I linked to above says. PLEASE GO READ IT. Follow the instructions.


AFTER your adjust the first octave, from 0V to 1V or 1V to 2V or whatever is convenient with your source of CV, THEN you do another octave and fine tune it. When all is done you should be able to start at 100Hz, and have each 1V step from there go to 200Hz, 400Hz, 800Hz, 1600Hz.... by the time you get to 1600Hz you might be getting a little flat, at which time you would adjust the HF trim (R36) and then start over at 0V and adjusting R1 again, because R1 and R36 interact.
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