Euro Dual VCO build problem [Solved]

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Pav
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Euro Dual VCO build problem [Solved]

Post by Pav » Sat Jan 25, 2020 12:10 pm

Hi Tony

With the second of two builds of dual vco for eurorack, I have a couple of problems with Osc 2 , and hoping you can steer me.

On first power up, i had U3 (TL072) on the pot board reversed. You think you check everything but there you go. stupid am i.
Fixed that but not sure if my remaining problems are related.

problem 1.

Oscillator 1 is fine. Wave forms appear to be 12v peak to peak.
Oscillator 2 appears to be 2-3 octaves higher than it should be with the tune trimmer not able to bring it down.
All Osc2 waveforms are around 1V peak to peak on oscilloscope
//EDIT CV1 and CV2 from pot board measured @ 3.4v suggesting to me a main board issue.
Where should i start looking?


Problem 2

is that the osc2 pulse wave only appears at 4oclock position when rotating pw knob. Its on then gone.



Problem 3
Get a saw wave on the triangle output. Again a bit of deja vu here .. should be able to trace this myself around U4 / Q5 on main board.

Thanks in advance
Last edited by Pav on Sun Jan 26, 2020 12:41 pm, edited 1 time in total.
rgds Pav

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Synthbuilder
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Re: Euro Dual VCO build problem

Post by Synthbuilder » Sun Jan 26, 2020 5:07 am

I think all three problems are due to the same single problem. The voltage reference, Vref(+6VB), is probably not +6V. The voltage across C19 should be 6.0V. Look firstly at the components connected to the left hand side of U7.

Tony

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Pav
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Re: Euro Dual VCO build problem [solved]

Post by Pav » Sun Jan 26, 2020 12:40 pm

Synthbuilder wrote:
Sun Jan 26, 2020 5:07 am
I think all three problems are due to the same single problem. The voltage reference, Vref(+6VB), is probably not +6V. The voltage across C19 should be 6.0V. Look firstly at the components connected to the left hand side of U7.

Tony

Thanks Tony.
traced it to being U7 had no voltage on U7 pin 8 - I replaced the 100nf capacitor C20 (i think) and all three issues now resolved.
I now have two well behaved DVCOs :tu: :tu:
rgds Pav

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Re: Euro Dual VCO build problem [solved]

Post by Synthbuilder » Mon Jan 27, 2020 4:20 am

Pav wrote:
Sun Jan 26, 2020 12:40 pm
traced it to being U7 had no voltage on U7 pin 8 - I replaced the 100nf capacitor C20...
It probably wasn't the capacitor as it doesn't carry current to U7. But by heating the capacitor's pads when you soldered it in place was possibly enough to reheat and reflow pin 8 of U7 which was nearby.

Tony

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Pav
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Re: Euro Dual VCO build problem [Solved]

Post by Pav » Mon Jan 27, 2020 4:54 am

I probably reflowed pin 8 and changed the cap at the same time , hence the confusion.
rgds Pav

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