No resonance: Sidekick + SMR-4

Hello all. I finished building a Sidekick with an SMR-4 filter board and everything works beautifully (so much fun!) EXCEPT the resonance knob doesn’t do anything. I built them from scratch on perf board, but have been round and round the schematics and board layouts and can’t figure it out. Correct me if I’m wrong but the Q pot is only sending +5v (or a portion of it) to the RES input on the filter board, right?

I pulled the Q pot and tested it good, plus I tried bypassing the Q pot and routing +5v and then ground straight through individually and no change in the sound whatsoever, so that isn’t the problem.

I used sockets for all ICs, so none could be solder-fried. Any other specific component failures that could possibly cause this?

I found a post that said, “If no audio source is connected to the sidekick, you need to set the resonance to its maximum value to get at least a sound source,” but I do get sound no matter what when I either a) drone one or both oscillators, or b) route an input signal through… does this mean the resonance is cranked all the way up? When the cutoff sweeps it does sound fairly resonant… there’s just no control! Also the RESO trimmer (I even went up to 10k) does nothing.

Any ideas? Thanks much

The comment “If no audio source is connected to the sidekick, you need to set the resonance to its maximum value to get at least a sound source” means that if nothing is fed to the filter, you need to make it self-oscillate to emit a sound. Settings the oscillators in drone mode or inputing a signal feeds the filter, so the comment about resonance does not apply in those conditions.

Pull IC5, and connect a 10k resistor between 5V and pin1 of the IC socket. Can you observe a voltage change when you sweep resonance? If so> problem around IC4B, R23 or the 1k resistor near R23. If not, problem around IC4A.

Thanks for the fast reply and troubleshooting tips. I finally got to try this last night…
So pulling the IC, I do read a voltage drop across the 10k resistor stuffed in the socket (1.49v with resonance all the way up, 0 with it down). I checked all my soldering and connections around IC4B, the reso trimpot (R23) and its nearby 1k and all looks good.
Maybe I’m not fully understanding the feedback circuit on the schematic.

  1. I took it for granted, but just to be sure, all the FEEDBACK labels should be connected correct?
  2. Pin 6 and 7 of IC4B are both bridged by a 10k AND shorted/jumpered, correct? (a friend told me this is common in feedback circuits though it didn’t make sense to me :slight_smile: . I tried both without a change to the sound.
  3. Since R50 and R51 are jumpers (labeled 0 instead of a value on the schematic), “pin” 3 of R23 is common to the FEEDBACK connections AND ground… so effectively: that side of R23 as well as pins 5, 6, and 7 of IC4B are all grounded, is that right?
    Thanks again for all your help pimping my resonance!

1/ Yes, networks with the same labels are connected

2/ Pins 6 and 7 of IC4B should not be bridged, there should be by default a 10k. All resistors labelled 0 on the schematics should be left unpopulated - they are patch points for mods in the res circuit.

3/ Again, Those are patch points for mods in the res circuit and are not normally populated.

All those resistors are not mentioned anywhere in the BOM, assembly instructions, photos of assembled boards. I am going to rename them as “DNP” (did not know about this term until recently) on the schematics.

Ah ha, that makes much more sense! I made those simple changes (really just the reso trim pot to ground, since I only temporarily connected pins 6 & 7 of IC4B) and it works perfectly. Now that sucker resonates like a mad man, howls like a banshee and makes a rich, juicy sine wave, expanding the already wonderful range of sounds this thing makes.
Thank you for the help good sir! If I had looked at the image of the board instead of just the schematic and board layout I would’ve caught it sooner (I mistook the line between the two solder pads to mean it should be bridged).