VCO not oscillating

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silveur
 
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VCO not oscillating

Post by silveur »

Hello everyone,
I have no oscillation on the triangle vco output (R105) but instead a steady 10.9V. The voltage applied to R90 is 3V.
I checked everything and voltages seems to be ok around the VCO.

The 3 potentiometers TM4 TM5 and VR2 are set to middle positions. I really followed the build manual to the letter.

What should I start checking to fix it?
Thanks a lot for you support :)

Silvere Letellier

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mmm
 
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Re: VCO not oscillating

Post by mmm »

How are you measuring - with a multimeter or an oscilloscope?

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silveur
 
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Re: VCO not oscillating

Post by silveur »

Hey MMM, thanks for your time.
I am measuring the not oscillating voltage with a scope (a good one on DC mode obviously).

I'm not sure where to start looking at
Thanks for your help

Silvere
http://www.weareroli.com

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silveur
 
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Re: VCO not oscillating

Post by silveur »

Also for information Q27 isn't a 512-2SA733P transistor but the generic equivalent 512-MPS6534.

Does it make any difference?

Thanks
Silvere

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mmm
 
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Re: VCO not oscillating

Post by mmm »

silveur wrote:Also for information Q27 isn't a 512-2SA733P transistor but the generic equivalent 512-MPS6534.

Does it make any difference?
I don't know.

Do you have the digital side already assembled? If so please measure the voltage on the Q26 pin closest to the edge when playing the very lowest (transposed down) note and the very highest (transposed up twice) note.

If the digital side is not yet ready please connect your 3v source to R90 and measure the voltage on pin7 of IC11 first.

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silveur
 
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Re: VCO not oscillating

Post by silveur »

Hey MMM,

I apologize for not responding to your previous reply before. I'm am truly dedicated to finish this project :)

The digital side isn't mounted yet so I have measured the voltage on pin 7 of IC11 as you mentioned.
When I apply 3 volt to R90 (from a professional DC regulated source) I see 3.3V on pin7.
Is the 0.3v offset normal in this case ?

When I do not apply the 3V voltage to R90 the voltage at pin7 of IC11 goes up to 10.56V.

When visualising the output of the oscillator from R105 I measure a steady 10.72V DC but no oscillation at all.

Attached a picture of the VCO soldering in case it may help.

I'm looking forward to hearing from you.
Silvere
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The VCO soldering
The VCO soldering
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silveur
 
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Re: VCO not oscillating

Post by silveur »

Please helpppppp :D

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mmm
 
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Re: VCO not oscillating

Post by mmm »

Sorry, I must have overlooked your posting from June 10th. I'll think of something during the next days (I'm a bit busy until the weekend).

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mmm
 
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Re: VCO not oscillating

Post by mmm »

Sorry for the late reply.

A quick search shows that the MPS6534 has a different pinout than the 733.

http://www.sm0vpo.com/_pdf/MP/MPS6534.pdf
http://www.fairchildsemi.com/ds/2S/2SA733.pdf

If I'm seeing this right Q27 is soldered in according to the silkscreen on the PCB; if this is indeed an MPS6534 you need to bend the legs so that the correct pin connects to the correct solder pad. I'd probably use a 733 there.

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silveur
 
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Re: VCO not oscillating

Post by silveur »

Thanks so much MMM,

I will have a look at that on Monday!

Have a great week end

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silveur
 
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Re: VCO not oscillating

Post by silveur »

Thanks MMM that really solved it!

All the best
Silvere

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