VC-LFO-2 Not working

Aottens · Joined: Mar 14, 2010 Posts: 33 Location: The Netherlands

Hello I've just build the VC-LFO-2. Only problem is, I am not getting it to work. In the first place, the yellow led does nothing, checked the wires, all good. I'm only not sure about the DPDT switch. I really think I've connected it wrong, could it be that I've placed the switch upside down? Does it make any change?

Checked the alignment of the IC's, all good. 1 little question, tje 1uF cap, does it need to be a tantulum? No I've used a polyester film version. It didn't say it had to be a tantulum in the BOM. I will upload some pics asap!

Thanks in advance for the help!

capicoso · Joined: Nov 19, 2012 Posts: 128 Location: Argentina

did you calibrate it?
When I built it I thought it wasn't working, the led wouldn't light up, nothing. I had one error near 7555 and lm741, after fixing that, still the same... Ihad one of the multiturn trimpots way low, and the frequency was loooooow, started calibrating it and it worked.

Aottens · Joined: Mar 14, 2010 Posts: 33 Location: The Netherlands

That could be a logical explanation! I did not calibrate it yet. Tried to, but the scope wasn't showing me a sawtooth only once. This was on the 1x setting on the DPDT switch. So the question also is, does C1a really needs to be a tantalum? Yves doesn't say it in the B.O.M. but he does say it in the calibrating proces. Still a little bit confused over here :S

I am not really familiar with oscilloscopes so can you tell me what settings to use on the scope?

isak · Posted: Sun Jun 15, 2014 4:41 am Post subject:

Hi there.
From the pics of the people who build this unit it dosnt have to be tantalum cap.
My guess is that Yves meant C1b when he said "make sure it's tantalum....."
Look at the pics of the boards:
http://yusynth.net/Modular/Commun/LFO/GALLERY/zarko-VCFLO2-b.jpg
http://yusynth.net/Modular/Commun/LFO/GALLERY/Sebo-LFO2-back.jpg
In the next link the cap is ceramic..
http://yusynth.net/Modular/Commun/LFO/GALLERY/Sbrenner-LFO_b.jpg

Aottens · Joined: Mar 14, 2010 Posts: 33 Location: The Netherlands

Found a small problem in my PCB. I've got the led working right now, and my scope shows a sinewave on the saw output. What could be the problem?

Aottens · Joined: Mar 14, 2010 Posts: 33 Location: The Netherlands

It's strange, but I can't seem to get it calibrated correctly, here are some pictures of the PCB. Can someone spot any errors?

https://www.dropbox.com/s/dytjkg7mg7ixerp/2014-12-22%2010.38.17.jpg?dl=0
https://www.dropbox.com/s/v2cbwcmvhbthbpn/2014-12-22%2010.39.26.jpg?dl=0

Aottens · Joined: Mar 14, 2010 Posts: 33 Location: The Netherlands

No one?

Aottens · Joined: Mar 14, 2010 Posts: 33 Location: The Netherlands

Allright, this thing is still not working.

Did some testing today. I don't have any of the outputs working. Tested if all the chips where getting their power supply, they do. They all get the desired power.

If I look at the schematics I can see that on pin 1 of U2 I need to have a saw, correct? Well, no saw over there. Strange! So no waveshaping happening at all.

Problem is, this is kinda where my troubleshooting capabilities stop. Could you guys give some tips on where to look next? I think I already narrowed it down quite a bit.

Thanks in advance!

gdavis · Posted: Tue Apr 18, 2017 2:13 pm Post subject:

Yes, U2 pin 1 should have a saw. U2 pin 7 should have a ramp (inverted saw).

You can also check:
U2 pin 5 for a ramp
U2 pin 6 is connected to/same voltage as pin 7.
Q1 emitter is +15V
U4 pin 6 voltage moves inversely to rate voltage (when rate voltage goes up, pin 6 should go down).
U4 pin 3 is close to 0V

You can also try removing Q2

Is there an error in the PCB where the black wire is? Where did you get this board? It looks commercially fabricated, not hand etched.

yusynth · Joined: Nov 24, 2005 Posts: 1314 Location: France

Check with a scope that you get a ramp at pin 2 of U1. It is where the ramp is generated. If there is no ramp here it is no use to search other points of the circuit.

Aottens · Joined: Mar 14, 2010 Posts: 33 Location: The Netherlands

Well, this issue has been resolved. Found out that a bad 7555 was the issue. Ordered a new one, everything started working. Got everything trimmed and all is looking good!

Thanks for all the help guys.

Trojanovskx · Joined: Dec 30, 2020 Posts: 1 Location: London, UK

I have built VCLFO-2. I used tantalum capacitors where appropriate and checked all wiring and polarities. PCB was ordered from an online shop and is well soldered.I didn’t find any shorcuts.

Rate knob doesn’t do any change and according to oscilloscope the frequency is constant around 1.2kHz. Note that I am using power supply +-12 V.

Please do you know wherecan be a problem?