SVF no sound

robboten · Joined: Nov 26, 2009 Posts: 36 Location: stockholm, sweden

I have managed to etch my own double sided pcb and have soldered the components (hard as hell with the traces under the ics). But I get no sound what so ever from it. I have tried three different VCOs and on all the inputs. I can measure the voltage over the ICs, and I see them change with VC modulation of cutoff. I don't know how they should be but there are some anyway and the chips are getting power as they should. I have checked the traces many times and measured if the currents run thru all the way between the components. But nothing seems to be off. Now I'm a bit out of ideas... how should I proceed? What can be the case here?

Lastly there is some resistors that I have changed because of lack of components (they shouldn't be that sensitive I thought):
300k -> 330k
30k -> 27k
43k -> 47k
49.9k -> 47k

and also I had no ceramic .1 uF caps so I took film caps instead (said on the paga that it was ok).

http://www.musicfromouterspace.com/analogsynth_new/STATEVARVCFFEB2006/STATEVARVCFFEB2006.html

analog_backlash · Posted: Sun Dec 16, 2012 4:48 am Post subject:

Hi,

I built this circuit about a year ago on my own single-sided PCB and it worked fine. Changing those resistor values might affect the tone of the filter slightly, but they shouldn't stop it from working altogether. Your best bet is to trace the signal across the PCB. If you don't have a signal tracer, see Ray Wilson's guitar amp tracer idea:

Worked out how to get the link to work!!

http://www.musicfromouterspace.com/analogsynth_new/HOT_TIPS/ampsignaltracer.html

With an oscillator input at any of AIN1-3, you should be able to hear an output at each op-amp in the signal path, i.e.:

U3-A pin 1
U2-B pin 12
U2-A pin 5
U4-A pin 1 etc...

If there's nothing at the U2 outputs, then there's probably something wrong with the CV coming from U1/Q1/Q2 (or you have a dead LM13700).

Anyway, if you do this methodically, it should help you find the part of the circuit which is at fault. You then have to check the previous part of the circuit to work out why nothing's happening.

I hope that this helps.

Gary

robboten · Joined: Nov 26, 2009 Posts: 36 Location: stockholm, sweden

Thank you for your reply. I have been busy trying to understand this thing a while now. Bought a scope and all. I can see the CV controlling the filter at the outputs you specified, but no sound. The only place I hear a faint sound from is at input 2 of U4A. Not on 1 or after that. The voltage at this (U4A-1) is -10.62V. At U2 I can see the CV at 5 and 12 (however very different on these), but no input wave. I can't for the world understand this. And I don't know if it's the CV-part or anything else. Have switched all IC and is now out of ideas again. Gah! Any more pro-tips?

elmegil · Posted: Fri Mar 08, 2013 9:16 am Post subject:

So you're saying you hear nothing on the input to U3-A (pin 2 of the TL084)??

I assume your VCO out can be heard elsewhere (not using this filter)?

That sounds like something has to be shorted to ground or -12V in that input signal path. I'd test both sides of that 49.9K resistor to see if there is continuity to ground or -12V supply lines (with no power on the circuit, of course). You might also try pulling either U4 or U3 out entirely and seeing if you get a different behavior (hear the signal on that pin for either the pulled chip or the other one).

How does your power supply look? Do you actually have +/- 12V on the pins you expect them on when the board is powered on?

kkissinger · Posted: Fri Mar 08, 2013 9:31 am Post subject:

Suggest that you scrutinize every solder joint to the diodes. I etch my own boards too and, given that the boards are not pre-tinned it is very easy to have an almost-microscopic bad solder joint -- the diodes are susceptible to this because of their narrow leads.

If only one diode on the ladder has a bad solder joint, the filter will produce no output.

robboten · Joined: Nov 26, 2009 Posts: 36 Location: stockholm, sweden

Wow. That post was some time ago. Dug up that old card and fixed the most things just the other day. I'll just say that I'll never ever going to etch double sided pcbs again! Wow, what a mess to get right. Found a couple of places under ICs that where broken and fixed with jumpers.

Only problem now is that the HP out has quite a negative offset. Anyone who could give me a pointer or so about that? Is it "normal" or not?