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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
Sub-Commander VCF Problem
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synthmonger



Joined: Nov 16, 2006
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PostPosted: Tue Dec 11, 2007 3:13 am    Post subject: Sub-Commander VCF Problem Reply with quote  Mark this post and the followings unread

Finished wiring it all up and getting barely ANY sound at all with the knobs fully cw. I found out the problem is in the VCF. It just ain't workin'! Everything else works perfectly. I've replaced the tranny's, tl072, and lm13700 with brand new ones (FYI old ones worked fine in other circuits) and I just can't fuggin figure it out. Any suggestions?
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Pehr



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PostPosted: Tue Dec 11, 2007 3:47 am    Post subject: Reply with quote  Mark this post and the followings unread

Are all knobs fully cw? Not all of them should.

Set Resonance, LFO levels and attack/decay-times to 0 and try again. Does it work now?

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synthmonger



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PostPosted: Tue Dec 11, 2007 8:32 am    Post subject: Reply with quote  Mark this post and the followings unread

That was one of the first things I tried. I ran a separate input directly into the VCF to see if it were something pre-VCF and still nothing. FYI The LFO, AR, work fine.
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Pehr



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PostPosted: Tue Dec 11, 2007 2:19 pm    Post subject: Reply with quote  Mark this post and the followings unread

Cold solder joint on PCB? Confused
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synthmonger



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PostPosted: Wed Dec 12, 2007 10:18 pm    Post subject: Reply with quote  Mark this post and the followings unread

Can never be too sure about that.

Pins 16 and 1 on U11 are reading -10.80 volts each. They do not change with the COF or reso knobs turning. I swapped the TL072 and trannies again and still nothing. If anyone has built the guitar synth, would it be possible to post their voltage readings?
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RF



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PostPosted: Thu Dec 13, 2007 10:10 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi Synthmonger -
I don't know all of what's going on in that circuit - but I think those voltages would cut off anything passing through the LM-13700. What are the voltages on Pins 7 & 1 of U10? Do they change when you rotate the cutoff pot?
Good luck

bruce
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synthmonger



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PostPosted: Thu Dec 13, 2007 1:06 pm    Post subject: Reply with quote  Mark this post and the followings unread

Actually I found out the VCF LM13700 was busted. I replaced it with what I believe to be, a working one. The output on pin 8 is loud which should be HP filtering right?


The voltage control circuitry is the problem. The VCA and VCF op-amps are giving me different readings and should be the same given the fact they are identical.

U10
Pin 1 CCW = 0.12volts
pin 1 CW = -0.11volts
pin 7 CCW = -0.51
pin 7 CW = -8.75

U12 (VCA TL072)
pin 1 CCW and CW are nearly idently
pin 7 CCW = -0.57
Pin 7 CW = -4.17


U11 (VCF LM13700)
pin 1 CCW = -10.66
pin 1 CW = -10.85
pin 16 CCW = -10.44
pin 16 CW = -2.08

U13 (VCA LM13700)
pin 1 = same results as U11
pin 16 CCW and CW = 0 volts

VCA works like a charm as does everything else.

The super whacky thing that I don't get is that pin 8 on U11 VCF's works great with the knobs and is at a suitable level. The Lowpass output on pin 9 is hardly even audible.

P.S. When I got outta bed this morning I stepped on a TL072. It was lodged as far in my foot as it could go. I took a video of me pulling it out with needle nose pliers. There is cursing and blood. Anyone wanna see it? Twisted Evil
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Pehr



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PostPosted: Thu Dec 13, 2007 1:43 pm    Post subject: Reply with quote  Mark this post and the followings unread

You can post the video in crazeydazeys gory thread Rolling Eyes Laughing
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RF



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PostPosted: Fri Dec 14, 2007 6:22 am    Post subject: Reply with quote  Mark this post and the followings unread

Ouch.
Better you than the significant other. If that ever happened to mine she would relegate my building to the garage for the rest of my life.
I dunno if I want to see the pic with it still socketed in yer foot.... even that might be enough for my wife to banish me...
Good luck
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synthmonger



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PostPosted: Sat Dec 15, 2007 2:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

I posted the vid in crazeydazey's GORE thread.


Well I sorta got the VCF working. I'm pretty sure there is some zany ghost screwing things up for me there, or part of the LM13700's ota's are busted.

The resonance makes things super chunky when it's more than halfway cw. The clean pre-vcf/vca is unaffected by the filter. So fa-r-e-a-k-i-n-g weird!
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creatorlars



Joined: Nov 26, 2007
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PostPosted: Fri Jan 11, 2008 2:48 pm    Post subject: Reply with quote  Mark this post and the followings unread

I had to replace both of the 13700's while building the Subcommander, so this is not an isolated problem. I know at least once, one was working, then stopped working, during the debugging process. Gonna have to remember to keep a few extras of these chips on hand if I build anything else with them. I've had a lot of issues with my Subcommander, but so far they've all been related to bad connections or bad pots, outside of the bad 13700's. I tried to build this thing into a really small case and was too stubborn to make a new one, so I ended up rewiring it several times different ways and screwed up a few of the pots in the process.

My AR generator in the VCF has stopped working, but I'm sure it's just another pot/wiring issue. I'll open it up soon and see if I can figure it out. I need to mount the board a little better while i'm at it, too... Razz

Good luck, it's a fun thing to play with... especially if you got one guy riffin' and one guy turnin' knobs! Wink Sounds great with pulse all the way up, about 1/3CW on sub-octave, no clean, running through filter AR for a slight auto-wah sound, then through too much fuzz and a really loud stack! I wish I could patch it out... my SUNN Beta-Lead head has two identical channel inputs w/EQ and Reverb.
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synthmonger



Joined: Nov 16, 2006
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PostPosted: Fri Jan 11, 2008 4:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

Well I know what the problem is, I'm getting different voltage levels on the chips so it has to do with the expo circuit for the vcf. EVERYTHING else works dandy except for the dang VCF!

The cut off frequency range is alright but the resonance just isn't working. I wanna hear the "zing" sound! Argh ah well I'll have to look at it again sometime. I added a VC delay and variable shape LFO to mine with true bypass and junk. I'll post pics when it's done.
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creatorlars



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PostPosted: Fri Jan 11, 2008 4:41 pm    Post subject: Reply with quote  Mark this post and the followings unread

Tell me about the VC Delay!! I've been looking for one for a while, but the only one I've found is Thomas Henry's VC Delay -- but the SAD4096 chips used are impossible to find, and also reputably extremely unreliable.

Looking forward to pics. I'll post some of mine soon.
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synthmonger



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PostPosted: Sat Jan 12, 2008 6:03 am    Post subject: Reply with quote  Mark this post and the followings unread

Mine is based similar to Scott Bernardi's except with a little different filtering and vactrols for the vca and delay.

You can find one on this site as well as a pcb layout. Just search for VC Delay PT2399
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