The VCO-555

[alanwilder81]

thanks Mattias. Priceless !

[alanwilder81]

where can i find the alternating current symbol in my pc keyboard?

[wackelpeter]

@alanwilder

do i understand that correct that your scope's Display isn't scaled within 1cm steps like usual?

P.S. furthermore most scopes do have an build in reference Generator, producing a square wave with set frequency and amplitude to scale the whole thing when connecting your scope probe to it.

[hedefalk]

[fonik]

great progress.

if you are low on budged but have a laptop i would recommend a cheap owon VDS1022I usb scope. really simple, but meets our basic needs.

[hedefalk]

First expo working again, it was a fried Q3

Noticed now when pulling and putting back the transistor (Q3) that they give a whole lot higher frequencies when cold and slowly going down in pitch when I'm at lower frequencies. However, when I'm at high pitch, pitch actually goes up initially and turns back down after a while... Also big diff between different units.

Guess I need to learn about transistor matching and tempco now I have a bag with 100 3906's so… Just using hFe measure on cheap multimeter is no use at all, is it?

[alanwilder81]

to hedefalk and Bastian

nice falling ramp there ! i got the same here
happy to hear about your progresses ! sorry i cant help more

as for my oscilloscope grid, i havent measured the squares witdth, should they be 1 cm wide? need to get a better grasp with it

[fonik]

[Veryfungi]

Hey everyone,

I'm starting to build my first synth. I've decided to start with this vco. I have put together the schematic in Eagle and have read several threads collecting all of the resistor value changes mentioned. I am making this for eurorack so 12v supplies. I have everything in except the actual in/out jacks. I was hoping for a reality check before I get too heavily involved with the pcb layout.

Thanks

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[Veryfungi]

Well I'm making great progress. I'm just about to send off for boards.

It's just the board with pads for off the board mounted jacks and pots. Looking forward to building 3 or 4 of these. I would love to hear any feedback. Thanks

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[Veryfungi]

Hello Mattias,

I found a picture of the 555-VCO with an additional led and switch. Can you tell me where to learn more about this?

Thanks

[fonik]

there are two major options how to get an VCO to dub-audio (LFO).
a) add a negative voltage to the CV summing stage, or
b) change the timing cap (C4) in this case.

b) is the most elegant solution

the LED could be fed directly from the VCO core triangle, which is positive only (IC3/pin8)

[Veryfungi]

Got it, thanks so much.

[Veryfungi]

I sent off for the boards last night. Yippy. I have accomplished my mission, that is to reestablish my familiarity with Eagle like the old days. I definitely fell into the, just one more change then it's done, pitfall. While these boards are being made I will start on my next modular component. Having a great time. Thanks for the help.

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[YashN]

With an oscilloscope, I can see waveforms generated for my VCO-555 at outputs, so I know the core is oscillating. However, I wasn't getting any sound at all.

It turns out that the output has a really high frequency, around 38kHz, therefore putting it out of audio-band!

How can I diagnose and fix this? Scope and DVM are at hand as well as the schematics and layout.

It can probably still be useful for scaring away any stray dogs but I really would like to use it as a normal audio VCO to integrate in a DIY Modular.

[elmegil]

Does the frequency change with your control pots?

Ususally too high means one of:

1) wrong components (too small resistor or too small capacitor)
2) short somewhere that puts more current into the core

If the frequency doesn't change, you might also have a short somewhere around the control pots and just be stuck at the high end of the oscillator's range.

[YashN]

Hey elmegil, how's it going?

I will have to check again about the Coarse Control Pot: I managed to lift of a via with the -12V connection... I'll have to rewire it and tap that from somewhere else with a jumper wire. The mistake I made here was using too large a wire gauge

As for components, I think I got all if not most right as the VCO did work at one point. Maybe one failed in the meantime though.

Am I understanding this correctly?: The core does work at a higher frequency and then this is brought down subsequent to the 555? If so, I can narrow the issue down to the circtuit post 555.

Thanks for chiming in!

[elmegil]

The 555 *is* the core.

If you look and see, the triangle is buffered off of the timing cap (C4)and the ramp is taken directly from pin 7 of the 555. Everything after is just wave shaping.

So your most likely problems not necessarily "in order" but kind of roughly so:

is C4 the right value?
are the resistors connecting to IC2a the right values?
Is IC2 healthy? If you used sockets, try swapping it out.
Are the resistors in the expo converter the right values (both the CV summer and the bits around IC3b and the matched transistors)?
Are the transistors themselves or the op amps themselves OK?
Is there something causing the sync input going to pin 4 of the 555 to be held low? Measure for shorts, or observe pin 4 with a scope.

I would not expect any of the stuff to do with the ramp shaping ( IC4 a & b) Triangle buffering (IC3d and associated resistors) or any of the other wave shaping to be causing this, unless possibly there is a short tying one of the 555's inputs to something it shouldn't be.


Good luck!

[YashN]

[elmegil]

[YashN]

Had to temporarily patch my Coarse Pin 1 as indicated on the layout to the bottom of resistor R46 to get at my -12V again (I'm using -12V, +12V so did the required changes for that).

Here's what I found so far:

1. The waveforms only show when the middle pin of the Coarse Pot crosses above point between -1.4V and 0.7V approximately, and then the frequency remains fixed at around 38kHz and 39kHz approx. So not exactly out-of-audio band, but high enough that I can't hear it directly. Below that threshold, no waveforms appear at all.

2. If I use my DVM and put it on DC measurement, I do get audio out of the Pulse out when I touch the +ve probe of the DVM to the LM13700 Pin 1.

[elmegil]

[YashN]

[elmegil]

So let's talk our way through control voltages.

wiper of the coarse pot should swing from -15V to +15V

Set the exp FM pot all the way CCW and the fine pot as close to center as you reasonably can (you could measure it at the wiper and make sure it was 0V).

Set the linear FM pot all the way CCW as well.

IC3a pin 1 should output something close to -.02 x the voltage at the coarse pot's wiper -- for example if the wiper is set at 10V, the output of the op amp should be -.2V

The output of the wiper of the 100R V/Oct trimmer (connects to base of Q3) should be a little bit less, somewhere above .16V when the coarse pot is at 10V.

When you change the coarse pot, these should all move in the direction you expect.

Verify all that looks right and then we can move on....

[YashN]

Testing if the linked pics work better for the rest of the measurements from the scope here. This is for IC3 Pins 8, 9 and 10:

IC3P8

IC3P9

IC3P10