My VCOs have been acting up so I’m going to do a little detective work to figure out what is up. I’ve tried a few things with no luck. Tonight, I’m going to look at current to see if anything sticks out. Feel free to join me. I could use all the help I can get.
Okay so I did a few measurements. I still need to do more, I think.
I measured the amp draw from the top case and it’s only ~.38 amps with all the modules plugged in. That’s around what I expected.
I also measured the voltage for a few of the modules across a 10r resistor:
VCO with 220r resistor: .69V
VCO with 1k resistor: .47V
Midimuso: .65V
LFO: .22V
Dialup: .146V
I’m not sure the measurements I’m taking are correct yet, except for the case amp draw. I’m actually going to modify the stripboard tool to make it easy to directly measure the amps of each module. Maybe tonight as the oscillators got wacky again last night.
Alrighty. My next steps are going to be a few more measurements. I have an idea to modify the tester board a bit more.
Perhaps I’ll try a similar setup in the bottom case to see if it’s a case-specific issue. The bottom case has the same power setup.
I’m also considering it may be an issue with the pair of FC microbuses powering the case. They are wired together so I just plug in one socket. I don’t really want to mess with them because it means a bit of disassembly, but I might as well take a closer look when I’m taking everything apart.
After that, I’m not sure what else to do. I’ve tried various AC adapters, etc. I’m running out of ideas.
Start with what you know, or think you know. Is there anything that reliably makes the problem appear or disappear? For instance, does it ever occur without the Midimuso plugged in? Does it ever occur with only one VCO plugged in and not the other?
If you can discover anything that repeatably makes the problem go away and come back you’re halfway there.
Good advice. When I unplug other modules, the amount of wobble lowers. The midimuso was the most obvious because change because I think it’s one of the more hungry modules I think. However, when I removed LFOs one at a time, I could hear the wobble go down with each module unplugged.
The wobble is isolated to the top case likely because it has more modules, but it effects every oscillator on the top case. I’ll try to see if I can get it to show up for the single oscillator on the bottom row by moving a modules down.
I can’t reliably get the wobble to show up, but once it is there, it remains even through a power cycle. That said, I haven’t tried seeing if it goes away after a five or ten minute power cycle (giving the caps time to drain). Yet.
It does sound power related and then the question is, is it caused by the VCO or the power system? So trying to make it appear on a different power system is a good idea. Move a known-to-wobble VCO into the bottom row too, to see if it appears on one VCO or both on the bottom.
Saving over 30% of the module’s power consumption by changing one resistor!
(With no detrimental effect.)
If there wasn’t already reason enough to change that resistor, that would push it over the top.
Update: I left things on for a bit and then did some measurements. The results are in amps (if I’m reading the multimeter correctly). I noticed the 220R resistor was really hot on the three I did not replace. I then ran out of time to do further testing. I plan to replace the 220R resistors with 1k resistors tonight and will continue on tomorrow.
Edit: I also noticed the 1k resistor on the one VCO I used replaced the 220R with was slightly warmer than the rest of the resistors. I wouldn’t call it warm, but it wasn’t as cold as the rest.
Started testing again this afternoon now that the 220R resistors have all been replaced with 1k resistors. The system been running for 70 minutes now with no wobble so far. It’s a bit soon to call it fixed, but I have high hopes. I suspect that even if one of them still had a 220R and was connected to the same power bus as the others, they would all wobble. I’m waiting another hour or two before celebrating.
Edit: I’ve also been checking the temperature of said resistors and so far all the 1k resistors remain relatively cool.
A little over two hours in, I’m hearing the slightest wobble in the higher octaves. Hmm. Barely noticeable, but I’m wondering if I use a higher value resistor than a 1k if it will go away or at least be reduced to imperceptible levels. It might have been there earlier, but I didn’t notice it yet. Going to keep letting it run a bit more.
Update: I swapped out the LM4040AIZs on all my oscillators after upping the resistors to 4K7. After testing for a few hours, the wobble is gone for me.
Now I need to get some gremlins out of my rat module. I likely wired a pot backwards or something. We shall seeeeee.
Rat gremlin found: Everything was wired correctly. It just seems the module prefers A100K pots instead of B100K. Most of the change happens in the first quarter turn or so. Might swap them out the next time I put together a potentiometer order.
Next gremlin: ground loop for the fancier Eurorack modules. I’m thinking of making a filtered bus board mounted to the back of the case with a dedicated off switch. The switch would would also resolve my Rample Gremlin.
Huge breakthrough today! On each of my two cases I had two fcuk microbuses were connected (properly) and powered with a 12vAC/AC 2amp brick. On a whim I decided to drill more holes in my case and power each microbus separately. This was a huge win. Any weirdness with my oscillators was gone. Also, my noise floor dropped significantly. This thing is solid now!!!
