VCO issues

Ali M · Joined: Sep 01, 2009 Posts: 96 Location: uk

Hi Everyone,

My VCO is not producing output on the Pulse and Sine outputs, and my triangle is like a reversed Sawtooth (ramp?). The output is also very weak, less than 1V.
Also when I turn up or down the course tune the waveforms completely disappear. The FM inputs don't seem to work either.

I am suspecting the TL074's or the OPA, but really haven't got much of a clue at this stage.

Can someone give me some hints to what sort of voltages I should be getting at key points of the circuit.

Really not having much luck with the Yu Modules. Crying or Very sad

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

Do you an oscilloscope to trace the signals ?
If so check the signal at pin 7 of U6, there you should observe an ascending ramp with a voltage ranging from 0V up to 4.8V to 5V. All the other waveforms depends on the quality of the signal at pin 7 of U6.

Which version of the VCO have you built ?

Ali M · Joined: Sep 01, 2009 Posts: 96 Location: uk

I am not at my scope right now, but i am 90% sure that i have a ramp but nowhere near 5v. Maybe 0.5v but not 5v.

I have built the sm2210 version.

Ali M · Joined: Sep 01, 2009 Posts: 96 Location: uk

I am not at my scope right now, but i am 90% sure that i have a ramp but nowhere near 5v. Maybe 0.5v but not 5v.

I have built the sm2210 version.

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

What frequency are you measuring ? If it's very high ( > 20 KHz) with a small amplitude then check that the tempco resistor is correctly soldered, also can you post photos of your VCO (component side and track side) ?

Ali M · Joined: Sep 01, 2009 Posts: 96 Location: uk

i did double check the tempco as i was getting a flat part on the waveform every so often, but i think it was just a draft blowing as i get the same effect by blowing on it.

I will have to borrow a camera to take a viewable picture.

Ali M · Joined: Sep 01, 2009 Posts: 96 Location: uk

Ok, pin 7 of U6 (the one on the right I hope!) I am getting 44mV.

When you ask about frequency, do you mean @ output? Because I can vary the frequency I don't know what settings I need.

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

44mV is abnormally low !!!!! Either there is a faulty component or a faulty connection. Can you post photos ?

Ali M · Joined: Sep 01, 2009 Posts: 96 Location: uk

Will post some in the next couple of hours...

Ali M · Joined: Sep 01, 2009 Posts: 96 Location: uk

Here are my pics...

DSCF7346.JPG

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Top, I know a wire is missing from PW, it came off before I took the picture.

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2.25 MB

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192 Time(s)

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DSCF7347.JPG

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Track side

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2.23 MB

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226 Time(s)

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

Components look correct, the track has some suspect points, there is may be a track break on the positive rail for the two TL074 (located just before the right TL074 as seen on track side.

Did you check that you have the right voltages for the ICs (+15V on pin 4 of the TL074 and -15V on pin 11 of the TL074; +14,8V on pin 8 of the OPA2137 and -14,8V on pin 4 of the OPA2137). Check also that you measure between 4.8V and 5V on the long strap wire near by the Silver Mica cap on the upper left of the PCB (as seen on the component side)

Ali M · Joined: Sep 01, 2009 Posts: 96 Location: uk

I did check it for continuity it all seems fine, voltages are:
Both TL074; are +/- 14.8v
The OPA; +13.6 and -14.8
Strap; +4.6

Are you thinking bust IC's?

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

Is the JFET a J112 ?
Measure the voltage on pin 3 of the SSM2210, do the measurment for the min pos of the Frequency knob, the middle position and the maximum position.

Ali M · Joined: Sep 01, 2009 Posts: 96 Location: uk

Yep deffo a J112.
On pin 3 of the SSM I am getting 0.16v min, 0.36v mid, 0.54v max.

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

looks OK until then, can you post scope views of the signal at pin 7 of U6 for various frequency settings

Ali M · Joined: Sep 01, 2009 Posts: 96 Location: uk

All the percentages are aproxomations.
It was one of those cases where you wish you had a third hand!

IMG_2442.JPG

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0% or 100%

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165 Time(s)

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20-30%

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160 Time(s)

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50% (Best it gets!)

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785.47 KB

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173 Time(s)

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Around 50% Distortion

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834.59 KB

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148 Time(s)

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The highest frequency I can trace before it flattens

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780.4 KB

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169 Time(s)

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Ali M · Joined: Sep 01, 2009 Posts: 96 Location: uk

Now it isn't oscillating at all. I think I am going to throw in the towel with this board. I have etched 2 others and I am going to try to get them working....

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

OK the waveshapes are weird and interestingly the amplitude depends upon the frequency. These are clues and I will try to figure out what's wrong there.

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

Looking at your PCB (track side) I would highly recommand that you clean off the resin (solder flux) which can create problems of impedance between tracks. Either use alcohol or flux cleaner spray.