Mfos VCO unstable and then 22khz or more

Dr.Macroelasticus · Joined: Sep 04, 2013 Posts: 6 Location: Spain

Hi,

My Mfos VCO suddenly stop working and have no signal in any shape.
My misstake was to install a 1/6w Tempco instead 1/4w. Shocked

I am waiting for the 1/4w tempco.

Perhaps this is the problem?

I replaced all de IC just in case, but nothing.

Thanks a lot

Nacho

wackelpeter · Posted: Fri May 08, 2015 7:55 am Post subject:

If there was too much current flowing through your tempco i would suggest that you would see and smell it...

For checking the functionality you can replace the tempco with an normal resistor of the same resistance...

your oscillator should work then and would only drift in pitch when the room and curcuit temperature changes...

i suppose you don't have a scope, correct?

Dr.Macroelasticus · Joined: Sep 04, 2013 Posts: 6 Location: Spain

Hi,

I have substitute several bad quality caps, the tempco resistor with a carbon resistor and all the IC.
But I have just discover with the oscilloscope that de VCO works in very high frequencies( more than 22khz) and I can set it lower with the v/oct trim.
My other Yusynth Vco works perfect with the same voltage in the input.

Thanks for your answer.

Nacho

wackelpeter · Posted: Sat May 09, 2015 5:12 am Post subject:

i guess it really was trimmed down to 1v/oct then before and you didn't replaced the trimmers too... right?

Then the problem is in the expo converter or in it's associated components...
do you have a link with the schematics?

Dr.Macroelasticus · Joined: Sep 04, 2013 Posts: 6 Location: Spain

I am working with 15v instead 12v, but I have made the changes that the Mfos page recommend.

I haven´t changed the 100 ohm trimer.
Maybe its wrong.

Here are all the schematics.

http://www.musicfromouterspace.com/forums.html?MAINTAB=SYNTHDIY&PROJARG=VCO20120618REV0/VCO20120618REV0.html&VPW=1430&VPH=642

wackelpeter · Posted: Sat May 09, 2015 9:12 am Post subject:

U1 and U3 or Q1 seem to be the suspect ones i would look after...

the trimmer shouldn't be replaced i guess... mostly some Chips or Transistors got fried... when it's a cap or resistor you would normally see and smell it...

but at least someone with more knowledge in electronics can give you better advice...

Dr.Macroelasticus · Joined: Sep 04, 2013 Posts: 6 Location: Spain

Thanks for your answer Very Happy

I forgot to say I am working wit a -+15 volts PSU and my pcb is the last version REV04.

I am doing the recommendations of the MFOS page:

" +/-15V Users Take Note

While helping diyer Doug Slocum improve the tracking on his MFOS VCOs I found a few things +/-15V users should change.

Change R46 to 1.54K metal film (1.5K carbon will be ok too). This sets the bias for the comparator since it changes at +/-15V. This corrects the whole rest of the waveform chain's amplitude.
Change R10 to 150K and R65 to 43K this will give you a better range on the coarse freq control. Use metal film.
Change R28 to 1.5M metal film. Changes the starting current of the expo convertor to 1uA.
Use OP275 for U1. Its performance is superior to other op amps.
Make sure and use a 100K multi-turn for the high frequency adjust.

Using this recomendations I can hear nothing (more than 20khz)
I have just replace R65 with 100k and the range is more audible (like 15k) but the range is still very high.

Thanks a lot

Nacho

Dr.Macroelasticus · Joined: Sep 04, 2013 Posts: 6 Location: Spain

Finally working with the specified components of Mfos...

...but at time the ultrasonic range back!! Sad

I am using the MAT02 matched transistor.
Maybe is the Oct/CV adjustment.?!

I am lost Embarassed

wackelpeter · Posted: Sat May 16, 2015 7:37 am Post subject:

did it ever work correctly before?

you can measure a difference in voltage at the centre of the fine or coarse tune pots?

you have already replaced U1? in case you didn't have one of These pricey's OPA left it should be possible to replace with an TL072... well Performance should be bit different but it should do most of the Thing...

have you already replaced the MAT? if not do so or as someone suggested to not blow These expensive parts because of mistakes away, replace it better with 2 NPN's just to check the functionality... they don't Need to be matched, normally same Batch should be enough... of course the tracking wouldn't be as precise as before but you can check what part of the circuit doesn't work...

otherwise i would suggest to read the notes to the VCO again, there is a much detailed description of how it works and what to check...