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MFOS VCO fault

darren adcock

Sep 26, 2016
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Recieved this reply today from Soundtronics support. Hopefully it may help. However I expect you guys know this already. I'm assuming further diagnostics need to wait until I can connect power now?

"Hi Darren,

The sine is a modified tri wave form so if Tri is not working then the sine will also not be working. The ramp comes from U4a and b and the square uses U7 from U4b so that section must be working okay. The Tri comes from U4c so this is where I would look including the associated components especially D2 and D3 and R7 offset trim. U4a and U4b generate the rise and fall parts of the Tri waveform and D2 / D3 mix and rectify the rise / fall ramps to produce the Tri waveform.

Hope that helps

Regards

Paul

Paul Cooper
Soundtronics Tech Support
soundtronics.co.uk
 

Alec_t

Jul 7, 2015
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How would I check this please?
You could first disconnect at least one leg of each of C7/10 and discharge the caps, With a DMM on it's Ohms function connected across the cap, the measured resistance should quickly increase to a relatively high value (many k). If it doesn't, that would indicate a leaky cap.
 

darren adcock

Sep 26, 2016
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Hi all.

Alec T the capacitors were fine, thanks for sharing how to check them.

I have a proper bench supply now that's set up for -12v/+12v at 20ma, thanks everyone who's been trying to help and being patient with my power supply problems.

I'm hoping you can help further diagnose what has gone wrong with my module.

I have powered up the module and this is what i've found.

Initially I went through the circuit without any ic's to make sure power was going to the right places. All was fine.

I went through one IC at a time to only find U6 is still a problem. Pins 1--2--3 are shorted together, pin 1 and two should be but pin 3 shouldn't. Unplugging the IC checks out as no circuit fault, inserting the IC without power and there's no fault, powering the circuit without IC and there's no fault, powering the circuit with U6 inserted and the fault appears.

I still only get the + to - part of the sine wave, audibly it will not reach low frequencies (i'm not sure how to measure this to be accurate) it cuts out at around mid way.

When the PWM pot is turned at it's maximum anticlockwise setting it still makes a pop sound (i'm guessing this is a voltage sound).

Thanks in advance

Darren
 

darren adcock

Sep 26, 2016
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Here is how I have my power supply set up for ground reference. Best to make sure I have done this correctly.
 

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Alec_t

Jul 7, 2015
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Pins 1--2--3 are shorted together, pin 1 and two should be but pin 3 shouldn't
The way an opamp works in that voltage-follower configuration means that all of pins1,2,3 should be at the same potential (in theory). To check for an actual short, disconnect 1 and 2, temporarily ground 2, then measure the voltage at 3 (with the VCO powered on). It should be non-zero.
 

darren adcock

Sep 26, 2016
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thanks, i will follow your instructions tomorrow at my workshop and report back.

d
 

darren adcock

Sep 26, 2016
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Hi Alec
PIn three measures,

Blck probe on ground red probe on pin 3 =0.034v
Black probe on -12v red probe on pin 3 =12v
 

Alec_t

Jul 7, 2015
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Ok. Now put the black probe on pin 3 and the red probe on +12V. With a 10k (or thereabouts) resistor connected temporarily between the two probes, what is the voltage reading?
 

Alec_t

Jul 7, 2015
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That suggests either (a) there is indeed a short between pins 2 and 3, or (b) C13 is shorted or leaky. Discharge C13 then try an Ohms test on it: the meter should stabilise at a high resistance value.
 

darren adcock

Sep 26, 2016
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I don't know how you've managed to work this out but yes that C13 is leaky. It only rises to 1.3ohms! Sadly I'm can't get back into my workshop until monday now but I can't wait to replace that cap and see what's happening!

Everything crossed

And thanks so much

Darren
 

Alec_t

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I should have said unplug U6 before doing the C13 check.
 

darren adcock

Sep 26, 2016
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Just got round to reading what a voltage follower is. Seems like an absolutely essential building block for circuits, especially anything ran off of batteries. I do a lot of mini projects with motors and batteries so i'm glad inadvertently you pointed me towards this. Thanks.
 

darren adcock

Sep 26, 2016
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Hi guys. Ok so changed the capacitor. Here is the new readings;

To check for an actual short, disconnect 1 and 2, temporarily ground 2, then measure the voltage at 3 (with the VCO powered on). It should be non-zero.

Black probe on ground red on pin 3 =6.89v

Ok. Now put the black probe on pin 3 and the red probe on +12V. With a 10k (or thereabouts) resistor connected temporarily between the two probes, what is the voltage reading?

0volts

Looking at the wave forms from the outputs there is definately an improvement. Below is videos taken from all wave outputs. The saw wave is callorbrated to be centered around ground. However all the other waves seem to be drift further towards positive (is this biasing?).

Hope this is helpful. I have a new board now so can begin again if needs be.

Here is saw: It appears to have me also looking dumb on it as I figure out how to work the camera. Just ignore that bit. Or laugh


Ramp:


Square:


Triangle:


Sine:

 

Alec_t

Jul 7, 2015
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Did you remember to reconnect pins 1 and 2 after installing the new C13?
The waveforms look reasonable shapes (apart from the sine), but static pics of the screen would be far better than videos show to show details of voltage levels, distortions etc.
 

darren adcock

Sep 26, 2016
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Hi. I did remember to reconnect the pins. Just disconnected them again to take measurements as you asked before. I couldn't work out how to take a static image due to flickering of the waveform. I could draw them instead?
 
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