The VCO-555

[Alfredo]

[Alfredo]

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

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[elmegil]

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.

[Alfredo]

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]

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.

[Alfredo]

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

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[elmegil]

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?

[Alfredo]

[elmegil]

Yes, I'm getting there.

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.

[elmegil]

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

[Alfredo]

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

[elmegil]

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.

[elmegil]

"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.

[Alfredo]

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...

[elmegil]

[Alfredo]

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

[elmegil]

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.

[fallenmouse]

Hi all, new to the forum, so I apologize if posting in this older thread is not the correct way to seek some advice.

I've breadboarded the first page of the schematic provided at the beginning of this post, and, after ironing out a handful of silly mistakes I made in laying things out, I've gota great looking triangle output.

However, the ramp output is not working properly.

I have the oscillator mocked up in circuit.js here: https://tinyurl.com/y7836833 and have been using this to help debug the circuit.

I seem to have the proper signal coming out of IC4b (a 0-5V triangle wave that I can adjust the dc offset of with the ramp connect trimmer - as seen in the leftmost scope in the simulation), however when I measure at the inputs of IC4a, I see the same offset triangle signal, rather than the "half-ramp" that we can see in the middle scope on the sim.

This leads me to believe that the issue is somehow related to the discharge pin of the 555, and how it interacts with the IC4a opamp to create the ramp signal.

I am using a TLC555, as outlined in the schematic.

Can anyone lend some insight into how that part of the circuit is supposed to work? Maybe I've just got a bad chip, and its discharge circuit isn't quite doing what it's upposed to?

Any help is greatly appreciated!

[elmegil]

If you're getting a good triangle, odds are against it being the 555 at fault.

What do you see on pin 7 of the 555? That's key to converting the triangle to a ramp. Does it have a good connection to the output op amp?

The other possibility is that IC4 is dead, or not powered or ...? Because if it does nothing, then there is a direct path for the triangle straight out to that output. Would look the same as the triangle output, more or less.

[fallenmouse]

[elmegil]

Breadboards don't always get as solid a connection as you might expect

Glad to hear it's working now.

[fallenmouse]

[JimRead]

Hello there,

After failing with Eddy's 555 stripboard layout I found out how to make single sided PCB's and after various experiments found that the UV film method worked very well for me.

For a couple of months I'd been searching for a single sided 555 and found nothing. I made a VCO-1 board got it working only to find that it would not go into my box, such is life. Searching once again to a 555 board I followed the Google pages a lo and behold a single sided board on page nine: https://modwiggler.com/forum/topic-187192.html

Here is the board artwork for the uv film caper, invert for the presensitised board


I downloaded Kosalos's zip file and looked at the PCB. Altered it slightly so that the BC557's meet face to face and resized it so the i.c. sockets will fit perfectly. This is the 'full' size jpeg @218.521 ppi and the board area is 73 x 104 the black outline is for positioning.

The instructions are second to none e.g. The board working

And one of the instruction sheets showing the layout of the links


Alls well that ends well. Hooray!!

Cheers

[gimble88]

I my finished PCB of the VCO-555 was working perfectly but I needed to rewire the connections from the PCB to the pots and jacks when I mounted it to the panel.

After doing so I found everything working perfectly except the triangle wave. It's positive half-cycle goes to 5V but the negative half-cycle only goes to -1.5V. This, of course, screwed up my sine wave and changed the balance of the PW of the square wave because they are derived from the triangle wave. The ramp is fine and the square wave is fine except that the equal duty cycle position of the PW control is in the wrong place. All other functions work exactly as they did before the rewiring.

I'm at a loss to find the place in the circuit that could limit the extent of the negative half cycle only of the triangle wave. Because of some time pressure I replaced the 13007 and the 555 as well as the U1 TL074 (I'm using SMD parts). I usually don't trouble shoot by throwing parts at a circuit.

Can anyone point me to where a wiring error or component failure could cause this. I really miss my sine wave.

Thanks very much.

Spence

[gimble88]

I should add that all PS voltages at the various IC pins are correct and I don't detect any solder bridges or shorts by visual inspection.

Thanks,
Spence