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I'm currently troubleshooting a state-variable filter I built. All of the filter types work, but the resonance pot has no effect. Tracing a sine signal through the feedback pathway, I found that it ended at pin 7 of U6 - a LM13700. This is the input to the transistor buffers, and there's no audible signal coming out on pin 8, which is the output from that buffer.

Here's the schematic

enter image description here

I'm working from. I'm referring to U6. It looks like it is hooked up as an emitter follower. DC voltage at pin 7 is -36.7mV and at 8 is -252mV.

Should I be hearing the signal coming out pin 8?

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    \$\begingroup\$ We need more information to answer your question. For starters, what voltage and frequency sine wave are you putting in, and what gain do you expect? (If you could show us a schematic, that would increase the chances of you getting an answer by at least 10x.) \$\endgroup\$ – pingswept Jun 5 '10 at 19:04
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The answer is, yes you should have signal at pin 8. If you have signal at pin 7 and no signal at pin 8. I would disconnect the output from everything but R57, to make sure it isn't loaded somewhere else. If there is still no signal, I would disconnect R56 to leave the 5-7 connection unbiased to ground. Better yet a trim pot at R56 and a trim pot at R57. Adjust till signal re-appears!

That load value(R57) looks low to me but I have only used LM13700's with single sided supplies appx. 9V so that might be why.

But definitely, signal at 7 should lead to signal at 8.

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  • \$\begingroup\$ Thanks, you were right about looking at R57. Needed to change the value. \$\endgroup\$ – Greg Jun 7 '10 at 12:28
  • \$\begingroup\$ Score! That is why this site rocks! Need help find help, have help give help! How does that synth sound btw? \$\endgroup\$ – Scott Murphy Jun 8 '10 at 0:45
  • \$\begingroup\$ Sounds great. I'll probably post some video soon. \$\endgroup\$ – Greg Jun 10 '10 at 2:06
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If the output transistors are connected as an emitter follower, like in this circuit:

alt text

then yes, there should be a signal at pin 8. What's the DC voltage at pins 7 and 8? What else is connected to pin 8? Where is the signal at pin 7 coming from?

Pins 9 and 10 do the same thing, did you compare with those? Are they used as the other half of a stereo pair, or for some other function?

alt text

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  • \$\begingroup\$ Thanks, here's the schematic I'm working from: musicfromouterspace.com/analogsynth/STATEVARVCFFEB2006/… I'm referring to U6, on the second page of the schematic. It looks like it is hooked up as an emitter follower. DC voltage at pin 7 is -36.7mV and at 8 is -252mV. Does that help at all? \$\endgroup\$ – Greg Jun 5 '10 at 22:54

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