HELP(!) SoundLab Ultimate LFO + VCO issues

[cds56]

Okay I'll provide pictures on request.
1st issue is that LFO 1 only works entirely properly on high/low of the ramp waveform.. Square output appears to work but does not cause any significant change to modulation (compared with LFO 2 on similar setting.) Sawtooth simply doesn't do anything, and triangle will cycle from high to low once and then stop.

Vco-3 has a non related tracking issue where the scaling is roughly. 82hz->173hz->350hz->700hz
Ah
Okay. And the others track much much better

SL ultimate (no box yet) W/mfos bipolar PSU
Paia midi->CV converter
DSO nano v2 oscilloscope

[jukingeo]

Hello,

I am surprised that no one has responded to your issue as of yet. Usually someone that has worked with MFOS modules before springs forth to assist.

While I have not yet built an MFOS project, I have been 'hovering' on the MFOS site for years before finally making a purchase of the SDIY Experiment board of which I intend to build this coming Jan/Feb.

Now, the Ultimate was one of the projects I was considering but I have not delved too deep into it as I know it would be a pricy (for me) first project.

Even though I have not built the Ultimate, I am an experienced electronics technician for close to 30 years now. When it comes to proven designs such as the Ultimate, generally issues fall into these categories:

1) Wiring Errors
2) Soldering Errors
3) Defective Components
4) Defect in the circuit board

I put these in the order of most common failures to least common first.

From what you describe on modules working part way, I would assume you have a wiring error. The best thing to do is to go over the documentation step by step as if you were rebuilding that module. Look for swapped wires, wires not grounded properly...see if wires that are supposed to get voltage have the correct voltage in place.

Next turn your attention to the board. Check all your soldering connections by scanning carefully left and right make sure all your solder connections are clean and flow evenly from the component lead to the PCB trace. Look for any solder bridges. Since most boards today are double sided, make sure that solder did flow through to the other (component) side of the board. Also while at it, make sure that you didn't create any solder bridges on this side of the board as well. Concentrate your efforts in the problem section.

If your wiring and soldering look good, turn your attention to the problem area and make sure all the parts were installed correctly. What is very easy to do is accidentally put a resistor in the wrong place. If diodes are the area make sure they are installed properly too. Make sure the chips are in the right sockets as well.

If all else fails, check your grounding. I can't emphasize this enough. I have come across many problems in audio circuits due to poor grounding. Generally when building an audio electronics project you MUST follow the correct way to ground the circuit. If you haphazardly place grounds all over, then you are inviting problems with hum and noise. However a ground wire that wasn't put in place properly could cause issues as you describe, particularly if it is a ground from a pot.

Just go over everything carefully and look the board over section by section. Usually Ray does an excellent job with identifying the module areas on his all in one boards, so that should help in narrowing the problem down.

Finally since you have two LFO's and one is working fine, use the comparison technique and compare everything on the good module with that of the bad one, the problem should crop up very fast this way.

Swap chips out in that area and other suspected components to make sure that you don't have the rare defective component.

That is all I can say for now. Try that out and also go through the troubleshooting techniques on the MFOS site to see if there is anything else that I have not mentioned.

Good luck with solving your issue and Happy Holidays.

Geo