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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
MFOS VCO troubleshooting
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codenamerofl



Joined: May 26, 2013
Posts: 9
Location: south carolina

PostPosted: Tue Feb 17, 2015 7:35 pm    Post subject: MFOS VCO troubleshooting Reply with quote  Mark this post and the followings unread

I built an MFOS VCO for my system, and ive got some peculiar issues

Ramp and Triangle wave shapes come out to be around 7 Volts Peak to Peak, whereas Square and Sine are 10 Vpp. The sine is slightly offset however.

I also get some odd behavior with the PWM. past about 50% the volume dies down significantly, but voltage remains 10 Vpp on my oscilloscope. I then have to crank it to about 20% for the volume to return, then i can move it around freely with no change in volume as long as i keep it below 50% PW.

if anyone can help me out with this that would be awesome. I think i mightve used an incorrect cap somewhere in the PW circuit, either C4, C15, or C16. I dont know why the amplitudes for the ramp and triangle are off however.


EDIT: Forgot to add, its running on a +/-15 volt system. I did all the changes outlined on the MFOS site for 15 volt users. The oscillator tracks beautifully across 8 octaves. It goes slightly flat above C8.


EDIT2: My Raw output does not go from +2.2 volts to ground. It goes from+ +2.2 volts to +0.6 volts which then translates down to the waveshapes and might explain all my problems. My R46 should be 1.54K. I used a 1.5K Metal Film resistor. On the board, between the pads, the resistance only measures 1.3k, which is way out of tolerance. Would that be the cause of my problems?
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prgdeltablues



Joined: Sep 25, 2006
Posts: 187
Location: UK
Audio files: 11

PostPosted: Wed Feb 18, 2015 2:39 am    Post subject: Reply with quote  Mark this post and the followings unread

I'd check the circuit feeding Q1, eg C17 - it sounds as if Q1 isn't turned on for long enough to fully discharge C6, preventing the RAW waveform from dropping back to ground. As you say, that problem is probably the cause of the other issues further down the line.

R46 is fine: remember it is in parallel with other resistors (via R47, V+ etc to Ground) On a quick read of the schematic, a network resistance across R46 of 1.3k sounds about right.

Is pin 3 of U2-A properly grounded - a weak solder point there might have the same effect.

Peter
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codenamerofl



Joined: May 26, 2013
Posts: 9
Location: south carolina

PostPosted: Wed Feb 18, 2015 10:37 am    Post subject: Reply with quote  Mark this post and the followings unread

I can reflow the solder joint tonight, but at a quick glance it was good, not cloudy or thin. The pin of the TL072 isnt bent in the socket either
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codenamerofl



Joined: May 26, 2013
Posts: 9
Location: south carolina

PostPosted: Wed Feb 18, 2015 6:31 pm    Post subject: Reply with quote  Mark this post and the followings unread

I used my multimeter to check the continuity of pin 3 of U2-A and got a beep signaling it was good.
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codenamerofl



Joined: May 26, 2013
Posts: 9
Location: south carolina

PostPosted: Thu Feb 19, 2015 2:50 pm    Post subject: Reply with quote  Mark this post and the followings unread

i swapped in a metal can 2n4391 and it works as it should now.
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